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E i g h t h E d i t i o n

Estelle LevetinThe University of Tulsa

Karen McMahonThe University of Tulsa

lev80044_fm_i-xvi.indd 1 2/26/19 11:53 AM

PLANTS & SOCIETY, EIGHTH EDITION

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Names: Levetin, Estelle, author. | McMahon, Karen, editor.Title: Plants & society / Estelle Levetin, The University of Tulsa, Karen McMahon, The University of Tulsa.Other titles: Plants and societyDescription: Eighth edition. | New York, NY : McGraw-Hill Education, [2020] | Includes index.Identifiers: LCCN 2019001477| ISBN 9781259880049 (alk. paper) | ISBN 9781260812602 (loose-leaf edition)Subjects: LCSH: Botany. | Botany, Economic. | Plants—Social aspects.Classification: LCC QK47 .L48 2020 | DDC 580—dc23LC record available at https://lccn.loc.gov/2019001477

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In loving memory of our mothers

Pauline Levetin

& Dorothy Sink McMahon


iv

UNIT I Plants and Society: The Botanical Connections to Our Lives

1 Plants in Our Lives 1

UNIT II Introduction to Plant Life: Botanical Principles

2 The Plant Cell 16 3 The Plant Body 28 4 Plant Physiology 47 5 Plant Life Cycle: Flowers 69 6 Plant Life Cycle: Fruits and Seeds 85 7 Genetics 101 8 Plant Systematics and Evolution 120 9 Diversity of Plant Life 136

11 Origins of Agriculture 17512 The Grasses 18713 Legumes 21014 Starchy Staples 22415 Feeding a Hungry World 241

UNIT IV Commercial Products Derived from Plants

16 Stimulating Beverages 27117 Herbs and Spices 28918 Materials: Cloth, Wood, and Paper 308

UNIT V Plants and Human Health

19 Medicinal Plants 33720 Psychoactive Plants 35821 Poisonous and Allergy Plants 378

UNIT VI Algae and Fungi: The Impact of Algae and Fungi on Human Affairs

22 The Algae 40023 Fungi in the Natural Environment 41924 Beverages and Foods from Fungi 44625 Fungi That Affect Human Health 469

UNIT VII Plants and the Environment

26 Plant Ecology 486

Contents in Brief


UNIT III Plants as a Source of Food

10 Human Nutrition 152

v

Preface xiiList of Boxed Readings xv

UNIT I Plants and Society: The Botanical Connections to Our Lives

1 Plants in Our Lives 1Plants and Human Society 2

The Flowering Plants 2The Non- Flowering Plants 4The Algae 4The Fungi 4Plant Sciences 4Scientific Method 5

A CLOSER LOOK 1.1 Biological Mimics 6

Fundamental Properties of Life 6Molecules of Life 8

Carbohydrates 8Proteins 9Lipids 11Nucleic Acids 12

A CLOSER LOOK 1.2 Perfumes to Poisons: Plants as Chemical Factories 14

UNIT II Introduction to Plant Life: Botanical Principles

2 The Plant Cell 16Early Studies of Cells 17The Cell Wall 19The Protoplast 19

Membranes 19Moving Into and Out of Cells 19Organelles 20

A CLOSER LOOK 2.1 Origin of Chloroplasts and Mitochondria 22

The Nucleus 23Cell Division 24

The Cell Cycle 24Prophase 24Metaphase 25Anaphase 27Telophase 27Cytokinesis 27

3 The Plant Body 28Plant Tissues 29

Meristems 29Dermal Tissue 29Ground Tissue 31Vascular Tissue 32

Plant Organs 33Stems 33Roots 35Leaves 37

A CLOSER LOOK 3.1 Studying Ancient Tree Rings 37

Vegetables: Edible Plant Organs 41Carrots 41

A CLOSER LOOK 3.2 Plants That Trap Animals 42Lettuce 44Radishes 44

A CLOSER LOOK 3.3 Supermarket Botany 44Asparagus 45

4 Plant Physiology 47Plant Transport Systems 48

Transpiration 49Absorption of Water from the Soil 49

A CLOSER LOOK 4.1 Mineral Nutrition and the Green Clean 50

Water Movement in Plants 51Translocation of Sugar 51

A CLOSER LOOK 4.2 Sugar and Slavery 53

Metabolism 55Energy 55Redox Reactions 55Phosphorylation 55Enzymes 56

Photosynthesis 56Energy from the Sun 56Chloroplasts and Light- Absorbing Pigments 57Overview 59The Light Reactions 59The Calvin Cycle 61Variation to Carbon Fixation 63

Cellular Respiration 63Glycolysis 64The Krebs Cycle 64The Electron Transport System 64Aerobic vs. Anaerobic Respiration 67

Contents


vi C o n t e n t s

5 Plant Life Cycle: Flowers 69The Flower 70

Floral Organs 70

A CLOSER LOOK 5.1 Mad about Tulips 71Modified Flowers 72

Meiosis 75Stages of Meiosis 75Meiosis in Flowering Plants 77Male Gametophyte Developm

A CLOSER LOOK 5.2 Pollen Is Moto Sneeze At 79

Female Gametophyte Development 7Pollination and Fertilization 79

Animal Pollination 80

A CLOSER LOOK 5.3 Alluring Scents 82Wind Pollination 83Double Fertilization 83

6 Plant Life Cycle: Fruits and Seeds 85Fruit Types 86

Simple Fleshy Fruits 86Dry Dehiscent Fruits 86Dry Indehiscent Fruits 86Aggregate and Multiple Fruits 86

Seed Structure and Germination 88Dicot Seeds 88Monocot Seeds 88Seed Germination and Development

Representative Edible Fruits 88Tomatoes 90

A CLOSER LOOK 6.1 Thon Plant Reproductive Cy

e Influence of Horcles 92

mones

ent 77

re Than Something

9

88

Apples 94Oranges and Grapefruits 95Chestnuts 97Exotic Fruits 98

7 Genetics 101Mendelian Genetics 102

Gregor Mendel and the Garden Pea 102

Monohybrid Cross 103Dihybrid Cross 105

Beyond Mendelian Genetics 106Incomplete Dominance 106

A CLOSER LOOK 7.1 Solving Genetics Problems 107

Multiple Alleles 109Polygenic Inheritance 109Linkage 109

Molecular Genetics 110DNA— The Genetic Material 111

A CLOSER LOOK 7.2 Try These Genes On for Size 112Genes Control Proteins 114Transcription and the Genetic Code 114Translation 116Other Roles of RNA 116Mutations 117Epigenetics 118Recombinant DNA 118

8 Plant Systematics and Evolution 120Early History of Classification 121

Carolus Linnaeus 121How Plants Are Named 123

Common Names 123

A CLOSER LOOK 8.1 The LanguageScientific Names 126

Taxonomic Hierarchy 127Higher Taxa 127What Is a Species? 129

Phylocode 130A CLOSER LOOK 8.2 Saving Species throSystematics 131

Barcoding Species 132

ugh

The Influence of Darwin’s Theory of Evolution 132

The Voyage of the HMS Beagle 133

of Flowers 125

Natural Selection 134

9 Diversity of Plant Life 136The Three- Domain System 137Survey of the Plant Kingdom 137

A CLOSER LOOK 9.1 Alternation of Generations 139

Liverworts, Mosses, and Hornworts 141

Lycophytes and Ferns 143Gymnosperms 146

A CLOSER LOOK 9.2 Amber: A Glimpse into the Past 148

Angiosperms 151

UNIT III Plants as a Source of Food

10 Human Nutrition 152Macronutrients 153

Sugars and Complex Carbohydrates 153

A CLOSER LOOK 10.1 Famine or Feast 154


C o n t e n t s vii

Fiber in the Diet 156Proteins and Essential Amino Acids 156Gluten and Celiac Disease 158Fats and Cholesterol 159

Micronutrients 162Vitamins 162Minerals 167

Dietary Guidelines 169Balancing Nutritional Requirements 169Healthier Dietary Guidelines 170Glycemic Index 171Meatless Alternatives 172

A CLOSER LOOK 10.2 Eat Broccoli for Cancer Prevention 172

11 Origins of Agriculture 175Foraging Societies and Their Diets 176

Early Foragers 176Modern Foragers 177The Paleo Diet 177

A CLOSER LOOK 11.1 Forensic Botany 178

Agriculture: Revolution or Evolution? 179

Latitudinal Spread 180Early Sites of Agriculture 180

The Near East 181The Far East 182The New World 183

Characteristics of Domesticated Plants 184Centers of Plant Domestication 184

12 The Grasses 187Characteristics of the Grass Family 188

Vegetative Characteristics 188The Flower 188The Grain 188

Wheat: The Staff of Life 189Origin and Evolution of Wheat 190Modern Cultivars 191Wheat Genome 191

A CLOSER LOOK 12.1 The Rise of Bread 192Nutrition 193

Corn: Indian Maize 193An Unusual Cereal 194Types of Corn 194

A CLOSER LOOK 12.2 Barbara McClintock and Jumping Genes in Corn 196

Hybrid Corn 197Ancestry of Corn 197

Corn Genome 199Value of Corn 199

Rice: Food for Billions 200A Plant for Flooded Fields 200Varieties 201Rice Genome 202Flood- Tolerant Rice 202

Other Important Grains 202Rye and Triticale 202Oats 204Barley and Tritordeum 204Sorghum and Millets 204

Other Grasses 205Forage Grasses 205Lawn Grasses 205

Bioethanol: Grass to Gas 206Corn 206Sugarcane 207Cellulosic Ethanol 207

13 Legumes 210Characteristics of the Legume Family 211

A CLOSER LOOK 13.1 The Nitrogen Cycle 212

Important Legume Food Crops 214Beans and Peas 214Peanuts 215Soybeans 217

A CLOSER LOOK 13.2 Harvesting Oil 219

Other Legumes of Interest 221A Supertree for Forestry 221Forage Crops 221Beans of the Future 222

14 Starchy Staples 224Storage Organs 225

Modified Stems 225Storage Roots 226

A CLOSER LOOK 14.1 Banana Republics: The Story of the Starchy Fruit 226

White Potato 228South American Origins 228The Irish Famine 228Continental Europe 229The Potato in the United States 229Solanum tuberosum 230Pathogens and Pests 231Potato Genome 232Modern Cultivars 232

Sweet Potato 232Origin and Spread 233Cultivation 233


viii C o n t e n t s

Cassava 234Origin and Spread 234Botany and Cultivation 234Processing 235

A CLOSER LOOK 14.2 Starch: In Our Collars and in Our Colas 237

Other Underground Crops 238Yams 238Taro 238Jerusalem Artichoke 238

15 Feeding a Hungry World 241

Breeding for Crop Improvement 242The Green Revolution 243

High-Yield Varieties 243Disease-Resistant Varieties 244History of the Green Revolution 245Problems with the Green Revolution: What Went

Wrong? 245Solutions? 246

Genetic Diversity 247Monoculture 247Sustainable Agriculture 248Genetic Erosion 249Seed Banks 249Heirloom Varieties 250Germplasm Treaty 250Crops and Global Warming 251

Alternative Crops: The Search for New Foods 252

Quinoa 252Amaranth and Chia 252Tarwi 253Tamarillo and Naranjilla 254Oca 254

Biotechnology 254A CLOSER LOOK 15.1 Mutiny on the HMS Bounty: The Story of Breadfruit 255

Cell and Tissue Culture 256Molecular Plant Breeding 257

Genetic Engineering and Transgenic Plants 257

Herbicide Resistance 258Against the Grain 260Insect Resistance 260Bt Corn and Controversy 261The Promise of Golden Rice 262Other Genetically Engineered Foods 262Disease Resistance 263Farming Pharmaceuticals 264Nonfood Crops 265Genetically Modified Trees 265

Regulatory Issues 266Environmental and Safety Considerations 266

Gene Editing: CRISPR/Cas9 268

UNIT IV Commercial Products Derived from Plants

16 Stimulating Beverages 271Physiological Effects of Caffeine 272

Medical Benefits of Caffeine 273Coffee 273

An Arabian Drink 273Plantations, Cultivation,

and Processing 274

A CLOSER LOOK 16.1 Climate Change and the Future of Coffee 276

From Bean to Brew 277Varieties 277Decaffeination 278Variations on a Bean 279Shade Coffee vs. Sun Coffee 279Fair Trade Coffee 279

Tea 279Oriental Origins 279Cultivation and Processing 280

A CLOSER LOOK 16.2 Tea Time: Ceremonies and Customs around the World 280

The Flavor and Health Effects of Tea 281History 282

Chocolate 283Food of the Gods 283Quakers and Cocoa 283

A CLOSER LOOK 16.3 Candy Bars: For the Love of Chocolate 284

Cultivation and Processing 285The High Price of Chocolate 286

Coca- Cola: An “All- American” Drink 287Other Caffeine Beverages 287

17 Herbs and Spices 289Essential Oils 290History of Spices 290

Ancient Trade 290

A CLOSER LOOK 17.1 Aromatherapy: The Healing Power of Scents 291

Marco Polo 291Age of Exploration 292Imperialism 293New World Discoveries 293

Spices 293Cinnamon: The Fragrant Bark 293Black and White Pepper 294


C o n t e n t s ix

Cloves 295Nutmeg and Mace 295Ginger and Turmeric 296Saffron 297Hot Chilies and Other Capsicum Peppers 297Vanilla 299Allspice 300

Herbs 300The Aromatic Mint 301The Parsley Family 303The Mustard Family 303The Pungent Alliums 304

A CLOSER LOOK 17.2 Sweet Talk 306

18 Materials: Cloth, Wood, and Paper 308Fibers 309

Types 309

A CLOSER LOOK 18.1 A Tisket, a Tasket: There Are Many Types of Baskets 310

King Cotton 312Linen: An Ancient Fabric 315Other Bast Fibers 316Miscellaneous Fibers 317Rayon: “Artificial Silk” 319Bark Cloth 319

A CLOSER LOOK 18.2 Herbs to Dye For 321

Wood and Wood Products 323A CLOSER LOOK 18.3 Good Vibrations 324

Hardwoods and Softwoods 326Lumber, Veneer, and Plywood 327Fuel 329Other Products from Trees 329Wood Pulp 330

Paper 331Early Writing Surfaces 331The Art of Papermaking 333Alternatives to Wood Pulp 333

Bamboo 334

UNIT V Plants and Human Health

19 Medicinal Plants 337History of Plants in Medicine 338

Early Greeks and Romans 338Age of Herbals 338

A CLOSER LOOK 19.1 Native American Medicine 340

Modern Prescription Drugs 340Herbal Medicine Today 342

Active Principles in Plants 343Alkaloids 343Glycosides 343

Medicinal Plants 343Foxglove and the Control of Heart Disease 343Aspirin: From Willow Bark to Bayer 345Malaria, the Fever Bark Tree, and Sweet

Wormwood 346Diabetes, French Lilac, and Metformin 349Snakeroot, Schizophrenia, and Hypertension 350The Burn Plant 351Ephedrine 351Cancer Therapy 352Herbal Remedies: Promise and Problems 354

20 Psychoactive Plants 358Psychoactive Drugs 359The Opium Poppy 360

An Ancient Curse 360The Opium Wars 360Opium Alkaloids 361Heroin 361Withdrawal 362

Marijuana 362Early History in China and India 362Spread to the West 363THC and Its Psychoactive Effects 364Medical Marijuana 365Legal Issues 365

Cocaine 366South American Origins 366Freud, Holmes, and Coca- Cola 366

A CLOSER LOOK 20.1 The Tropane Alkaloids and Witchcraft 367

Coke and Crack 369Medical Uses 369A Deadly Drug 369

Tobacco 370A New World Habit 370Cultivation Practices 371Health Risks 371

Peyote 374Mescal Buttons 374Native American Church 375

Kava— The Drug of Choice in the Pacific 375Preparation of the Beverage 375Active Components in Kava 375

Lesser Known Psychoactive Plants 376

21 Poisonous and Allergy Plants 378Notable Poisonous Plants 379

Poisonous Plants in the Wild 379Poisonous Plants in the

Backyard 382

A CLOSER LOOK 21.1 Allelopathy—Chemical Warfare in Plants 383


x C o n t e n t s

Poisonous Plants in the Home 386Plants Poisonous to Livestock 386

Plants That Cause Mechanical Injury 387Insecticides from Plants 388Allergy Plants 389

Allergy and the Immune System 390Respiratory Allergies 390Hay Fever Plants 391Climate Change and Allergy Plants 394Allergy Control 394Contact Dermatitis 395Food Allergies 397Latex Allergy 398

UNIT VI Algae and Fungi: The Impact of Algae and Fungi on Human Affairs

22 The Algae 400Characteristics of the Algae 401Classification of the Algae 401

Cyanobacteria 402Euglenoids 403Dinoflagellates 403Diatoms 404Brown Algae 404Red Algae 405Green Algae 405

Algae in Our Diet 409Seaweeds 409

Biofuels from Algae 410Other Economic Uses of Algae 411Toxic and Harmful Algae 411

Toxic Cyanobacteria 412

A CLOSER LOOK 22.1 Drugs from the Sea 413Red Tides 414Pfiesteria 414

A CLOSER LOOK 22.2 Killer Alga— Story of a Deadly Invader 415

Other Toxic Algae 417

23 Fungi in the Natural Environment 419Fungi and Fungal- Like Organisms 420Fungal-Like Protists 420

Slime Molds 421The Kingdom Fungi 422

Characteristics of Fungi 422Classification of Fungi 424

Role of Fungi in the Environment 433Decomposers—Nature’s Recyclers 433

A CLOSER LOOK 23.1 Lichens: Algal-Fungal Partnership 434

Mycorrhizae: Root- Fungus Partnership 435

A CLOSER LOOK 23.2 Dry Rot and Other Wood Decay Fungi 436

Plant Diseases with Major Impact on Humans 437

Late Blight of Potato 438Rusts— Threat to the World’s

Breadbasket 439Corn Smut— Blight or Delight 441Dutch Elm Disease— Destruction in the Urban

Landscape 441Sudden Oak Death— Destruction in the

Forest 443

24 Beverages and Foods from Fungi 446Making Wine 447

The Wine Grape 448Harvest 449Red or White 449Fermentation 449

A CLOSER LOOK 24.1 Disaster in the French Vineyards 450

Clarification 452Aging and Bottling 452“Drinking Stars” 453Fortified and Dessert Wines 454Climate Change and the Wine Grape 454

The Brewing of Beer 454Barley Malt 455Mash, Hops, and Wort 455Fermentation and Lagering 455Sake, a Rice “Beer” 457

Distillation 457The Still 458Distilled Spirits 458The Whiskey Rebellion 458Tequila from Agave 459Hard Cider 460

A Victorian Drink Revisited 461Absinthe 461The Green Hour 461Active Principles 461Absinthism 462

A CLOSER LOOK 24.2 Alcohol and Health 463

Fungi as Food 465Edible Mushrooms 465Fermented Foods 466Quorn Mycoprotein 467


C o n t e n t s xi

25 Fungi That Affect Human Health 469Antibiotics and Other Wonder Drugs 470

Fleming’s Discovery of Penicillin 470Manufacture of Penicillin 471Other Antibiotics and Antifungals 471

Fungal Poisons and Toxins 472Mycotoxins 472

A CLOSER LOOK 25.1 The New Wonder Drugs 473Ergot of Rye and Ergotism 475The Destroying Angel and Toadstools 477Soma and Hallucinogenic Fungi 478

Human Pathogens and Allergies 480Dermatophytes 480Systemic Mycoses 481Allergies 482Indoor Fungi and Toxic Molds 482

UNIT VII Plants and the Environment

26 Plant Ecology 486The Ecosystem 487

Ecological Niche 487Food Chains and Food Webs 488Energy Flow and Ecological

Pyramids 489Biological Magnification 491

Biogeochemical Cycling 492The Carbon Cycle 492Source or Sink? 492The Greenhouse Effect 494Species Shift 496Global Warming Pact 496

Ecological Succession 497The Anthropocene Epoch 498The Green World: Biomes 500

Deserts 501Crops from the Desert 501Chaparral 504Grasslands 504Forests 505Harvesting Trees 506

A CLOSER LOOK 26.1 Buying Time for the Rain Forest 508

Appendix A Atoms, Molecules, and Chemical Bonds 513

Appendix B Classification of Plants and Those Organisms Traditionally Classified as Plants Discussed in Plants and Society 517

Appendix C Metric System: Scientific Measurement 519

Glossary 521Index 533


xii

Preface


I n the twenty- first century, plant science is once again assuming a prominent role in research. Renewed empha-sis on developing medicinal products from native plants

has encouraged ethnobotanical endeavors. The destruction of the rain forests has made the timing for this research impera-tive and has spurred efforts to catalog the plant biodiversity in these environments. Efforts to feed the growing populations in developing nations have also positioned plant scientists at the cutting edge of genetic engineering with the creation of trans-genic crops. However, in recent decades botany courses have seen a decline in enrollment, and some courses have even dis-appeared from the curriculum in many universities. We have written Plants and Society in an effort to offset this trend. By taking a multidisciplinary approach to studying the relation-ship between plants and people, we hope to stimulate inter-est in plant science and encourage students to further study. Also, by exposing students to society’s historical connection to plants, we hope to instill a greater appreciation for the botan-ical world.

AUDIENCERecently, general botany courses have emphasized the impact of plants on society. In addition, many institutions have developed plants and society courses devoted exclusively to this topic. This emphasis has transformed the traditional economic botany from a dry statistical treatment of “bushels per acre” to an exciting discussion of “botanical marvels” that have influenced our past and will change our future. Plants and Society is intended for use in this type of course, which is usually one semester or one quarter in length. There are no prerequisites because it is an introductory course. The course covers basic principles of botany and places a strong emphasis on the economic aspects and social implications of plants and fungi.

Students usually take a course of this nature in their fresh-man or sophomore year to satisfy a science requirement in the general education curriculum. Typically, they are not biology majors. Although most students enroll to satisfy the science requirement, many become enthusiastic about the subject mat-ter. Students, even those with a limited science background, should not encounter any problems with the level of scientific detail in this text.

As indicated, the primary market for this text is a plants and society course; however, it would certainly be suitable for an introductory general botany course as well.

ORGANIZATIONWe feel that Plants and Society is a textbook with a great deal of flexibility for course design. It offers a unique balanced approach between basic botany and the applied or economic aspects of plant science. Other texts emphasize either the basic or applied material, making it difficult for instructors who wish to provide better balance in an introductory course. Another distinctive feature is the unit on algae and fungi. While other texts cover certain aspects of this topic, we have an expanded coverage of algae and fungi and their impact on society.

Plants and Society is organized into 26 chapters that are grouped into seven units. The first nine chapters cover the basic botany found in an introductory course. However, even in these chapters we have included many applied topics, some in the boxed essays but others directly in the chapter text.

UNIT I Plants and Society: The Botanical Connections to Our Lives. Chapter 1 stresses the overall importance of plants in everyday life. The properties of life, molecules of life, flowering and non flowering plants, algae, and fungi are introduced. The scientific method is explained as the pro-cess used by scientists to study and expand our knowledge of the natural world. The diversity and applications of phytochemicals are also presented.

UNIT II Introduction to Plant Life: Botanical Principles. This unit addresses basic botany. Chapters cover plant struc-ture from the cellular level to the mature plant. Repro-duction, including mitosis and meiosis and the life cycle of flowering plants, is discussed in two chapters. Other chapters cover genetics, evolution, plant physiology, plant systematics, and plant diversity. Some of the economic aspects of plants discussed in this unit are the importance of vegetables and fruits, the connection between sugar and slavery, plant essential oils and perfumes, phytore-mediation, the applications of palynology, and species conservation.

UNIT III Plants as a Source of Food. This unit describes the major food crops. It begins with a chapter on the require-ments for human nutrition and continues with a chapter on the origins of agriculture. Other chapters cover the grasses, the legumes, and starchy staples. The unit ends with a chapter on the Green Revolution, the loss of genetic diversity, the search for alternative crops, and the controversial development of transgenic crops.

P r e f a c e xiii

UNIT IV Commercial Products Derived from Plants. This unit covers other crops that provide us with consumable prod-ucts, such as beverages, herbs and spices, and materials such as cloth, wood, and paper. The historical origin and societal impact of these crops are explored.

UNIT V Plants and Human Health. This unit introduces stu-dents to the historical foundations of Western medicine, the practice of herbal medicine, and the chemistry of sec-ondary plant products. Descriptions of the plants that pro-vide us with medicinal products and psychoactive drugs are discussed. The unit also covers the common poisonous and allergy plants that are found in the environment.

UNIT VI Algae and Fungi: The Impact of Algae and Fungi on Human Affairs. This unit describes the economic importance of the algae and fungi, including their biol-ogy and crucial roles in the environment. The algae are recognized as key producers in aquatic environments and as sources of human food, devastating blooms, and industrial products. Fermented beverages and foods from fungi are discussed, as is the medical importance of fungi as sources of antibiotics, toxins, and diseases affecting crops and people.

UNIT VII Plants and the Environment. Chapter 26 is an introduction to the principles of ecology: the ecosystem, niches, food chains, biogeochemical cycles, and ecological succession. The major biomes of the world are discussed, with an emphasis on the economic value of certain desert plants and the strategy of extractive reserves in the rain forest. The problems associated with rising levels of the greenhouse gas CO2 and the environmental consequences of global warming are addressed.

APPROACHThis textbook is written at the introductory level suitable for students with little or no background in biology. Like any introductory book, this book uses a broad- brush treatment. The nature of the course dictates an applied approach, with the impact of plants on society as the integrating theme, but the theoretical aspects of basic botany are thoroughly covered.

LEARNING AIDSIn addition to the textual material, each chapter begins with a chapter outline and key concepts. Important terms are in boldface type throughout the text, and each chapter ends with a summary and review questions. Thinking Critically questions are inserted in the text to draw the attention of the students as they read the chapter. The questions begin with either a summary of the preceding text or an introduction to new information that is complementary to the chapter. The questions that follow are designed not only to test comprehension but also, in many instances, to promote critical thinking by asking students to apply

their knowledge to real-life situations. Thinking Critically questions may also be assigned by instructors or used to ini-tiate in-class discussions. Three appendices and a glossary conclude the text. The classification of plants and other organisms discussed in the text and a review of metric units are located for quick access on the inside front and back covers, respectively.

NEW TO THE EIGHTH EDITIONThe eighth edition of Plants and Society been updated to spot-light exciting discoveries and update major advancements in the science of plants, algae, and fungi, with special emphasis on how these organisms impact humanity. These include:

• Expanded information on the regulation of cell division (Chapter 2)

• Subsidiary cells in grasses enable the greater inflation of guard cells to bring in more carbon dioxide for photosynthesis as well as facilitating guard cells to respond rapidly to changing environmental conditions (Chapter 3).

• Sun pitchers (Heliamphora), carnivorous pitcher plants native to South America, exhibit remarkable adaptations: a nectar spoon to lure prey and a drainage pipe to siphon off excess water from the pitcher (Chapter 3).

• The section on orchids has been updated to include significantly more detail on the unique modifications of orchid flowers (Chapter 5).

• There are updates on the latest research surrounding colony collapse disorder and the search for alternative pollinators (Chapter 5).

• There is additional coverage of the ways in which researchers are using genomic analysis to discover how to enhance the flavors of mass- market tomatoes to better resemble heirloom varieties (Chapter 6).

• Information on CRISPR, a powerful technique in the gene editing of organisms, is presented (Chapters 7 and 15).

• Fiber in the diet may promote health as the food to support microbiota beneficial to the digestive tract (Chapter 10).

• The paleo diet, the so- called original diet of ancient humans, is evaluated (Chapter 11).

• A new update on the International Wheat Genome Sequencing Consortium’s work to sequence the bread wheat genome is featured (Chapter 12).

• A Closer Look 13.1 has been updated to include new information on biofertilizers and A Closer Look 13.2 on Harvesting Oil also has data updates (Chapter 13).

• A Closer Look 14.1 features new information on the recent resurgence of Panama disease and its effect on bananas (Chapter 14).

• New evidence on the disjunct distribution of sweet potatoes is detailed (Chapter 14).

• A section on the effects of global warming on crops has been included (Chapter 15).


xiv P r e f a c e

•

Information on genetically modified rice and corn has been added (Chapter 15).

• A new Closer Look on Climate Change and the Future of Coffee has been added (Chapter 16).

• Update on the legal status of hemp cultivation (Chapter 18).

• There is a new content on the increased use of plant biomass (such as wood pellets) to supplement or replace coal (Chapter 18).

• A new section highlights the remarkable history of French lilac in the development of the medicine Metformin, which is used to treat both diabetes and polycystic ovarian syndrome (Chapter 19).

• Updated content is included on the antimalarial properties of certain plants used in Chinese medicine (Chapter 19).

• Updates on the legal status of marijuana has been included (Chapter 20).

• Giant Hogweed, an oversized invasive weed from Asia, produces a toxic sap that causes debilitating blisters and burns to the skin upon exposure to sunlight (Chapter 21).

• There are updates to gluten and nonceliac gluten sensitivity (Chapter 21).

• There is information on new research suggesting a potential connection between algae blooms and global warming (Chapter 22).

• A 2018 research on Batrachochytrium dendrobatidis fungus highlights how the pathogen traveled from the Korean peninsula and has been responsible for the extinction or decline of frog and salamander populations around the world (Chapter 23).

• A new section covers the history and cultivation of apple ciders, the alcoholic beverages made by fermenting apple juice (Chapter 24).

• With the rise of MRSA infections worldwide, there has been renewed interest in the therapeutic use of fusidic acid, covered now in additional detail (Chapter 25).

• The global, indelible impact of human activity on Earth has initiated a demand for the creation of a new geologic epoch, the Anthropocene (Chapter 26).

• The Paris Climate Agreement has revised its long- term goal of limiting global warming from an increase of 2.0°C above preindustrial temperatures down to 1.5°C (Chapter 26).

• Over 50 new photographs and several new or revised figures and tables have been added to the eighth edition.

ACKNOWLEDGMENTSFrom our first introduction to botany as college students, we became irrevocably fixated on the lives of plants. We can remember the fascination we felt when we read about the plant explorers who discovered extinct ginkgo trees alive in China and how a trichome of the stinging nettle was the inspiration for the invention of the hypodermic needle. It is our hope that Plants and Society will present the world of plants and how they sustain humanity in a way that will inspire students to have a lifelong appreciation of plants.

We wish to thank the editorial staff at McGraw-Hill Higher Education for their editorial expertise and their endless patience during the publication of the eighth edition of Plants and Society. We especially want to acknowledge our Portfolio Manager, Michael Ivanov; Senior Product Developer, Chipper Scheid; Freelance Product Developer, Jen Thomas; and Con-tent Project Manager, Becca Gill.

REVIEWERSWe are indebted to our colleagues who have taken time from demanding schedules to meticulously review Plants and Society for errors, inconsistencies, or omissions and to offer constructive feedback and suggestions. We thank you for mak-ing the eighth edition of Plants and Society the best edition.

Karen AmisiGrand Valley State University

Elise C. HollisterGrand Valley State University

Meshagae Hunte-BrownDrexel University

Arthur KneelandUniversity of Wisconsin— Stout

Diane M. LahaiseGeorgia State University— Perimeter College

Elizabeth MartinLewis- Clark State College

Michael SundueUniversity of Vermont

Jacob ThompsonCollege of Coastal Georgia

Alexander WaitMissouri State University


xv

List of Boxed Readings


1. Biological Mimics (Chapter 1)2. Perfumes to Poisons: Plants as Chemical Factories

(Chapter 1)3. Origin of Chloroplasts and Mitochondria (Chapter 2)4. Studying Ancient Tree Rings (Chapter 3)5. Plants That Trap Animals (Chapter 3)6. Supermarket Botany (Chapter 3)7. Mineral Nutrition and the Green Clean (Chapter 4)8. Sugar and Slavery (Chapter 4)9. Mad about Tulips (Chapter 5)

10. Pollen Is More Than Something to Sneeze At (Chapter 5)

11. Alluring Scents (Chapter 5)12. The Influence of Hormones on Plant Reproductive

Cycles (Chapter 6)13. Solving Genetics Problems (Chapter 7)14. Try These Genes On for Size (Chapter 7)15. The Language of Flowers (Chapter 8)16. Saving Species through Systematics (Chapter 8)17. Alternation of Generations (Chapter 9)18. Amber: A Glimpse into the Past (Chapter 9)19. Famine or Feast (Chapter 10)20. Eat Broccoli for Cancer Prevention (Chapter 10)21. Forensic Botany (Chapter 11)22. The Rise of Bread (Chapter 12)23. Barbara McClintock and Jumping Genes in Corn

(Chapter 12)24. The Nitrogen Cycle (Chapter 13)

25. Harvesting Oil (Chapter 13)26. Banana Republics: The Story of the Starchy Fruit

(Chapter 14)27. Starch: In Our Collars and in Our Colas (Chapter 14)28. Mutiny on the HMS Bounty: The Story of Breadfruit

(Chapter 15)29. Climate Change and the Future of Coffee (Chapter 16)30. Tea Time: Ceremonies and Customs around the World

(Chapter 16)31. Candy Bars: For the Love of Chocolate (Chapter 16)32. Aromatherapy: The Healing Power of Scents

(Chapter 17)33. Sweet Talk (Chapter 17)34. A Tisket, a Tasket: There Are Many Types of Baskets

(Chapter 18)35. Herbs to Dye For (Chapter 18)36. Good Vibrations (Chapter 18)37. Native American Medicine (Chapter 19)38. The Tropane Alkaloids and Witchcraft (Chapter 20)39. Allelopathy— Chemical Warfare in Plants (Chapter 21)40. Drugs from the Sea (Chapter 22)41. Killer Alga— Story of a Deadly Invader (Chapter 22)42. Lichens: Algal- Fungal Partnership (Chapter 23)43. Dry Rot and Other Wood Decay Fungi (Chapter 23)44. Disaster in the French Vineyards (Chapter 24)45. Alcohol and Health (Chapter 24)46. The New Wonder Drugs (Chapter 25)47. Buying Time for the Rain Forest (Chapter 26)

xvi

Supplements


PLANTS AND SOCIETY COMPANION WEBSITEThe companion website to accompany Plants and Society offers a variety of additional resources for instructors and stu-dents. Instructors will appreciate full- color PowerPoint image slides that contain illustrations and photos from the text, along with suggested activities. A comprehensive bank of test ques-tions, aligned with each chapter of the text, is also available along with access to TestGen. TestGen. allows instructors to create paper and online tests or quizzes in one easy- to- use pro-gram. Students will find multiple-choice quizzes, short- answer concepts, and further resources to aid in their study. Also included is a listing of useful and poisonous plants, as well as tips for growing houseplants and home gardening.www.mhhe.com/levetin8e

Craft your teaching resources to match the way you teach! With McGraw-Hill

CreateTM, www.mcgrawhillcreate.com, you can easily rear-range chapters, combine material from other content sources, and quickly upload content you have written like your course syllabus or teaching notes. Find the content you need in Create by searching through thousands of leading McGraw- Hill textbooks. Arrange your book to fit your teaching style. Create even allows you to personalize your book’s appearance by selecting the cover and adding your name, school, and

course information. Order a Create book, and you’ll receive a complimentary print review copy in 3–5 business days or a complimentary electronic review copy (eComp) via email in minutes. Go to www.mcgrawhillcreate.com today and register to experience how McGraw-Hill Create TM empowers you to teach your students your way.

THE LABORATORY MANUAL FOR APPLIED BOTANY BY LEVETIN, MCMAHON, AND REINSVOLD

The lab manual features 18 exer-cises that focus on examining plants and plant products that have sustained or affected human society. Although the manual includes standard information on plant cells and tissues, there is a practical approach to the inves-tigations. Students extract plant dyes, make paper from plant fibers, and study starch grains used in archeology. Several labo-

ratory topics are devoted exclusively to economically import-ant crops— grasses, legumes, starchy staples, and spices. Four additional appendixes— titled Science as a Process, A Field Trip to a Health Food Store, A Taster’s Sampler of Caffeine Beverages and Foods, and Notes for Instructors— provide addi-tional information for each of the labs.

152

UNIT III

C H A P T E R

10Human

Nutrition

Nutrition Facts label imparts important nutritional information for the consumer.

KEY CONCEPTS1. Human nutritional needs are supplied by

macronutrients (carbohydrates, proteins, and fats) and micronutrients (vitamins and minerals).

2. If nutritional requirements are not satisfied,deficiency diseases can result that have widespread effects on the bodily systems.

3. Plants can supply the majority of human nutritional requirements, and there is evidence that increasing the proportion of plant foods in the diet can have positive health benefits.

CHAPTER OUTLINEMacronutrients 153

Sugars and Complex Carbohydrates 153

A CLOSER LOOK 10.1 Famineor Feast 154

Fiber in the Diet 156Proteins and Essential Amino Acids 156Gluten and Celiac Disease 158Fats and Cholesterol 159

Micronutrients 162Vitamins 162Minerals 167

Dietary Guidelines 169Balancing Nutritional Requirements 169Healthier Dietary Guidelines 170Glycemic Index 171

A CLOSER LOOK 10.2 Eat Broccolifor Cancer Prevention 172

Meatless Alternatives 172Chapter Summary 173Review Questions 174

©Karen McMahon

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C H A P T E R 10 Human Nutrition 153

N ew ideas in nutrition are quickly incorporated by the health-conscious segment of society and advertisers looking for a new marketing gimmick. The benefits of

several nutritional concepts, such as fiber, monounsaturated oils, and low-carbohydrate diets, have made headlines and influ-enced lifestyle changes. All these concepts promise better health and many are, in fact, dependent on a greater consumption of plants in the human diet. This chapter will examine human nutritional needs and how plants can satisfy these needs.

MACRONUTRIENTSThe basic nutritional needs of humans are to supply energy and raw materials for all the various activities and processes that occur in the body. In addition to the need for water, humans require five types of nutrients from their food sup-ply; three of these are required in relatively large amounts and are called macronutrients, consisting of carbohydrates, proteins, and fats. The other two types of nutrients, vitamins and minerals, are required in small amounts and are known as micronutrients. If water were removed, the macronutrients would make up almost all the dry weight of foods.*

Human energy requirements vary with the age, sex, andactivity level of the individual, within a wide range of 1,200 to3,200 kilocalories per day. The current recommendation forAmericans is an average daily intake of 2,000 kilocalories or1,600 kilocalories for women and 2,200 kilocalories for men.(A calorie is a measure of energy—technically, the amount ofenergy needed to raise the temperature of 1 gram of waterby 1 degree Celsius.) Food energy is normally measured in kilocalories (1,000 calories = 1 kilocalorie), which can be abbre-viated as kcal, or Calories with a capital C. Most dietary guidessimply use the term calories, but this book will use the moreaccurate Calories or kilocalories. Each gram of c arbohydrateor protein can supply 4 kilocalories; for each gram of fat con-sumed, the amount of energy supplied is more than double,9 kilocalories. Although all the macronutrients can be used asa source of energy, normally only carbohydrates and fats do so,while proteins provide the raw materials, or building blocks,required for the synthesis of essential metabolites, growth, andtissue maintenance. The consequences of undernutrition, mal-nutrition, and overnutrition for the world’s population are thetopics of A Closer Look 10.1: Famine or Feast.

Sugars and Complex CarbohydratesAlthough carbohydrates are commonly grouped into sugars and starches, recall (see Chapter 1) that these compounds can be chemically classified into monosaccharides, disaccharides, and polysaccharides, based on the number of sugar units in the molecule.

*All organisms have nutrient needs requiring some compounds in large amounts (mac-ronutrients) and other compounds in smaller amounts (micronutrients). The elements that plants require, macronutrients and micronutrients refer to the mineral require-ments. When referring to human nutritional needs, these terms take on a different meaning, as discussed here.

MonosaccharidesMonosaccharides are the basic building blocks of all carbo-hydrates, and glucose is the most abundant of these sugars. During the process of digestion, many carbohydrates are bro-ken down or converted into glucose, which is then transported by the blood to all the cells in the body. Within cells, the process of cellular respiration metabolizes glucose to produce the energy necessary to sustain life. Other common mono-saccharides are fructose and galactose, which have the same chemical makeup as glucose, C6H12O6, differing only in the arrangement of the atoms within the molecules. In the body, most of the fructose and galactose is converted into glucose and metabolized as such. In the United States, an inexpensive sweetener for many types of processed foods is high-fructose corn syrup, often preferred because fructose is sweeter-tasting than table sugar. Fructose is commonly found in many fruits, unlike galactose, which does not normally occur free in nature.

DisaccharidesDisaccharides are composed of two monosaccharides chemi-cally joined together. The most common disaccharide is sucrose, or table sugar, formed from a molecule of glucose and a molecule of fructose. Other disaccharides are the milk sugar lactose (a combination of glucose and galactose) and maltose (formed by two glucose molecules), which are largely found in germinating grains. Table sugar, which primarily comes from sugarcane and sugar beet, is at least 97% pure sucrose with little nutritional value, thereby supplying only kilocalories. (See A Closer Look 4.2: Sugar and Slavery.) During digestion, these disaccharides are broken down to yield their component monosaccharides.

PolysaccharidesPolysaccharides, also known as complex carbohydrates, con-tain hundreds to thousands of individual sugar units, and forthe most part, glucose is the only monosaccharide present.The different polysaccharides are distinguished by the way inwhich the glucose units are joined together, their arrangement,and their number. Starch is the storage form of glucose foundin plants; it occurs abundantly in seeds, some fruits, tubers,and taproots. The presence of starch in foods can be traceddirectly to its plant origin; the starch in white bread and pastawas originally stored in the grain of a wheat plant. The majorgrain crops (wheat, rice, and corn), the major undergroundcrops (potato, sweet potato, and cassava), and the majorlegumes (beans and peas) supply the majority of starch in thehuman diet. In the body, starch is broken down into glucoseby enzymes in saliva and the small intestine and is transportedby the bloodstream to body cells.

Glycogen is the body’s storage form of glucose, found in the liver and skeletal muscles. When the levels of glucose in the blood are higher than the demands of the cells, the excess is used for the synthesis of glycogen in liver and muscle cells. Only a limited amount of glycogen can be stored as a reserve—no more than a day’s worth of energy needs. Excess glucose beyond this amount is generally converted to fat. During

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154

A CLOSER LOOK 10.1Famine or Feast

In 2018, estimates of the world population size were placed at 7.7 billion, with a projection of 9.8 billion by the year 2050 (box fig. 10.1a). This unprecedented popu-lation growth presents many problems related to the pro-duction and distribution of sufficient food to meet human nutritional needs. Although global food production has increased during recent decades, chronic hunger and mal-nutrition are ever-present problems in many developing nations, especially in sub-Saharan Africa and Asia. It is estimated that 815 billion people suffer from undernutri-tion. In addition, many are malnourished and deficient in vitamin A, iron, iodine, or zinc. Undernutrition is defined as an insufficient number of kilocalories to maintain daily energy requirements, while malnutrition is a quality defi-ciency in which one or more essential nutrients is lacking, even though caloric intake may be sufficient. The majority of starvation-related deaths are among children, with about half of those dying under the age of five. Starvation is usu-ally the underlying cause of death, but most die of diseases such as diarrhea or measles, which would not be fatal in a properly nourished individual.

Two devastating conditions specifically related to under-nourishment and malnourishment are kwashiorkor and marasmus (box figs. 10.1b and c). Kwashiorkor occurs when the diet is deficient in protein but has sufficient kilocalories. It is particularly prevalent after weaning, when a child no longer receives the protein-rich breast milk and is switched to a starchy diet low in protein content or quality or both. Symptoms of kwashiorkor include puffy skin and swollen belly due to edema, a fatty liver, a reddish orange cast to the hair, dermatitis, and listlessness.

Marasmus results from starvationwhen the diet is low in both kilocalo-ries and protein; other nutrients areprobably deficient as well. Sufferersfrom marasmus are extremely thin andshriveled—literally, skin and bones asthe muscles of the body, even the heartmuscle, are wasted away becausemuscle protein is digested to supplyenergy needs. The overt symptoms ofboth marasmus and kwashiorkor canbe reversed if treated in time, but espe-cially in infants and young children, mildmental retardation and stunted growthmay be permanent results.

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The counterpart to the conditions of undernutrition and malnutrition is overnutrition, in which an excessive intake of food can result in obesity and chronic disease. Currently, a global epidemic of obesity is the chief concern of many public health officials.

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Box Figure 10.1 (a) The human population has continued to grow rapidly, up to 7.3 billion in 2015, as indicated by the J-shaped curve. (b) A child suffering from kwashiorkor, a protein-deficiency disease. (c) Marasmus victims have a skeletal appearance as the body wastes from starvation.

155

The WHO (World Health Organization), which first listed obesity as a disease in 1979, has asked the UN member nations to adopt programs in their home countries to reduce the intake of foods high in fats and sugar. It is ironic that developed nations, which have nearly eradicated many nutritional deficiencies, are now faced with a new nutri-tional threat with the prevalence of overweight and obesity. Developing nations are also experiencing a rise in obesity in certain populations—often at a faster rate than that seen in industrialized nations—at the same time that they are trying to combat the serious problem of undernutrition.

Almost 40% of U.S. adults (20–74 years) in 2015–2016 were overweight or obese, up from 14% in 1971. This increase has been seen for both men and women and across all eth-nic, socioeconomic, and age groups. The United States is not alone; obesity has been identified as a growing health problem in the United Kingdom, Australasia, Eastern Europe, the Middle East, and the Pacific Islands—all these areas have seen the incidence of obesity more than triple since 1980. The WHO estimates that globally more than 1.9 billion adults are overweight, with 650 million of them obese. Estimates of obesity vary widely, from below 5% of the population in China and Japan and some African nations to greater than 75% in urban Samoa.

Body mass index (BMI) is the most current method health care professionals and fitness experts use to determine if a person’s weight poses a health risk. BMI calculates a per-son’s body weight in kilograms per square meter (BMI = body wt in kg/[ht in m]2 or body wt in lb/[ht in in.]2 × 703). Most people are considered overweight if their BMI ranges from 25 up to 30. A BMI of 30 or more indicates obesity; a rating of 40 or greater is indicative of severe obesity. An optimal BMI for most adults is below 25, and a BMI below 18.5 is considered underweight. The average BMI for adults in Africa and Asia ranges from 22 to 23, while a range of 25 to 27 is the norm in North America, Europe, and some Latin American, North African, and Pacific Island nations.

Obesity is associated with a higher risk of many diseases and chronic conditions: type 2 diabetes, hypertension (high blood pressure), cardiovascular disease, stroke, asthma,gallstones, cancers (prostate, breast, and colon), and osteo-arthritis. Eighty percent of obese adults suffer from oneof these diseases; 40% have two or more. Recently, it has been shown that women who are overweight in their 70s have a higher risk of developing Alzheimer’s disease in their late 80s.

The alarming rise in childhood obesity over the past twodecades has been documented in many countries: Haiti,Costa Rica, Chile, Brazil, England, Scotland, China, Egypt,Australia, Ghana, Morocco, and others. In 2015–2016, 18.5%of children and adolescents (ages 2–19) in the United Statesare classified as overweight or obese). In the world, it is esti-mated that 41 million children under the age of 5 were over-weight or obese. Developing countries have a 30% higherrate of increase in childhood overweight and obesity levelsthan that of developed nations.

The repercussions of childhood obesity can be devas-tating. As with adult obesity, childhood obesity is linked to numerous complications: hypertension, sleep apnea, asthma, negative self-image, and gallstones, among others. Type 2 diabetes, formerly rare in adolescence, is now on the rise in children, mainly as a consequence of childhood obesity. In some populations, 50% of the newly diagnosed are adoles-cents. In January 2004, the American Academy of Pediatrics issued the statement that “overweight is the most common medical condition of childhood.”

The global epidemic of obesity appears to be directlyrelated to an energy imbalance. In most cases, obesity iscaused by an excessive consumption of energy-rich foods,especially those high in sugar and/or saturated fats, coupledwith inadequate expenditure of energy due to the limitedphysical activity associated with a modern, urban lifestyle. Inthe United States, some researchers suggest that the prev-alence of fast foods in the American diet should take muchof the blame for the fattening of Americans. Others haveadvocated a sin tax on sugary snack foods and soft drinks todiscourage their consumption.

Responding to the nation’s rising obesity rate, the SurgeonGeneral released The Surgeon General’s Call to Action toPrevent and Decrease Overweight and Obesity in 2001.The report warns that health problems resulting from thecurrent epidemic of obesity could reverse many of the healthgains achieved in the United States in recent decades. Itdetails specific steps to educate the American people about obesity-linked health issues and what actions should be takento reduce the incidence of obesity in the American popula-tion. Released in 2002, Healthier U.S. Initiative is encourag-ing Americans to prevent obesity by being physically activeevery day and making healthier choices in their diets.

Critics have countered that the so-called obesity epidemic was exaggerated by flawed statistical studies and the media.The research behind the obesity epidemic headlines washeavily funded by the weight-loss industry, which may haveinfluenced the results. Newer studies report that there is only a small increase in mortality rates of the mildly obese and,in fact, the underweight, even when smokers are excluded,have a higher death rate when compared with those of nor-mal weight. Critics note that the researchers who identifiedobesity as a major cause of death in the United States didnot consider the impact of recent medical advancements,which have significantly improved the outlook for peoplewith heart disease, diabetes, and high blood pressure, dis-eases associated with obesity. Another observation ignoredis that obesity can have a protective effect in the elderly.Elderly patients who are mildly obese usually outlive theirnormal-weight cohorts when hospitalized for an extendedtime. Apparently, a nutritional reserve can be helpful in therecovery process. Colon cancer and postmenopausal breastcancer are slightly elevated in the obese, but lung cancerrates are surprisingly lower. The effects of obesity on healthare probably more complex than once thought, and furtherresearch is needed.

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156 U N I T I I I Plants as a Source of Food

strenuous exercise, the body’s glycogen reserves are called upon; therefore, athletes training for a competition practice a regimen of carbohydrate loading by eating lots of starchy foods to build up muscle glycogen reserves.

Fiber in the DietAnother important dietary component is fiber, which isderived from plant sources. Although not digestible, it doesprovide bulk and other benefits. There are many types ofdietary fiber: cellulose, lignin, hemicellulose, pectin, gums,mucilages, and others. Cellulose, a principal component ofplant cell walls, is another polysaccharide composed of glu-cose; however, humans do not have the enzymatic ability tobreak the bonds connecting the glucose molecules in celluloseas they do for starch and glycogen, and thus cellulose passesthrough the digestive tract as roughage, largely unaltered.Other cell wall components considered dietary fiber are lignin,pectins, and hemicelluloses. Lignin, a cell-wall component inplant cells that have secondary walls, is not a polysaccharidebut a complex polymer. Pectins and hemicelluloses, which arecell wall polysaccharides, form the matrix in which cellulosefibrils are embedded. Pectins also occur in the middle lamellabetween adjacent cells. Gums and mucilages are exudatesfrom various plants that are used commercially as thickeningagents in prepared foods. Cell wall polysaccharides, refinedfrom some species of red and brown algae, can also be consid-ered dietary fiber. Although not digestible by human enzymes,some fiber, especially some hemicelluloses, can be brokendown by intestinal bacteria and the nutrients made availableto the body.

Dietary fiber can be conveniently grouped into two types,soluble and insoluble, relating to their solubility in water.Insoluble fiber includes cellulose, lignin, and some hemicel-luloses, while soluble fiber includes other hemicelluloses,pectins, gums, mucilages, and the algal polysaccharides.Soluble fiber is resistant to digestion and absorption in thesmall intestine, but as soluble fiber enters the large intestine, itis acted upon by naturally occurring bacteria. The bacteria fer-ment soluble fiber into gases and products beneficial to health.Butyric acid is one of these products. It is known to stabilizeblood glucose levels, which decreases the risk of type 2 diabe-tes. Butyric acid also has been shown to reduce blood levels ofcholesterol, reducing the risk of cardiovascular disease, and toraise the acidity of the colon, which prevents cancerous polyps(small, tumorlike growths on the lining of the large intestine)from forming.

Fruits, vegetables, seeds, and whole grains supply mostof the fiber in the human diet. Some plants are higher in oneor more of these types of fiber, and the beneficial effects ofhigh fiber foods differ depending on which fiber is abundant.For example, the soluble fiber present as gum in oat branand as pectin in apples is believed to lower cholesterol levelsin the blood. Wheat bran, which is largely cellulose, an insol-uble fiber, has no particular cholesterol-lowering ability butseems to be most effective in speeding passage through thecolon, which may reduce the risk of colon cancer. Psyllium

husk from the seed coat of several species of plantain(Plantago ovata, Plantago arenaria) is an especially valuabledietary fiber because it is a good source of both soluble andinsoluble fiber.

A study, which tracked the diets of more than 700,000people for over 20 years, found that those eating the leastamount of fiber (less than 10 grams per day) were at anincreased risk of colorectal cancer. The study also suggestedthat fiber from cereals and whole grains, but not fruits andvegetables, is best for slightly lowering the risk of rectalcancer. Physicians and nutritionists still advocate the valueof fiber in lowering the risk of obesity, heart disease, anddiabetes.

The value of fiber in the diet in promoting health hasbeen widely reported, but the exact mechanism by which ahigh-fiber diet brings about these benefits has been unknownuntil recently. Studies indicate that fiber plays a critical rolein maintaining a healthy microbiota in the gut. Humans havea limited number of enzymes in the digestive tract and cannotmake the enzymes needed to breakdown dietary fiber, butcertain bacteria in the microbiota that inhabits the humanintestines can. In other words, dietary fiber is the food or fuelfor these bacteria, and if a diet is high in dietary fiber, thepopulation of these bacteria rises and health benefits accom-pany this rise. For example, when the bacteria digest fiber,they often produce short-chain fatty acids as waste, which arein turn then consumed by the intestinal cells as an importantsource of fuel.

Experiments in which mice were switched from feedingon a high-fiber diet to a fiber-free, high fat, and high pro-tein diet show that the microbiota in their guts underwentrapid changes in species composition. Some populations ofthe microbiota crashed or disappeared completely; on theother hand, the bacterial species that fed on protein and fatincreased significantly. Deprived of fiber for fuel, some bacte-rial species start to feed on the mucus layer of the intestines.Mucus is the layer lining the gut produced by intestinal cellsthat coats the intestinal lining and protects intestinal cellsfrom digestive enzymes and invasive bacteria. If intestinalcells are deprived of the short-chained fatty acids provided byfiber-eating bacteria, the health of the intestinal cells declinesand the result is a thinner mucus layer. With a thinner mucuslayer, bacteria can begin to penetrate the intestinal wall trig-gering the body’s immune response to destroy the invaders.The end result is chronic inflammation and may result inconditions such as ulcerative colitis, a painful chronic boweldisease.

Proteins and Essential Amino AcidsProteins are a group of large, complex molecules that serve as structural components and regulate a large variety of bodily functions (table 10.1). Recall that the constituents of proteins are amino acids; there are 20 naturally occurring amino acids, which can be assembled in various combinations and numbers to make thousands of different types of proteins. During diges-tion, proteins are broken down into their component amino

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157

Table 10.1Functions of Proteins

Type of Protein Function Examples

Structural Support Collagen, keratinEnzymes Catalysts Digestive enzymesHormones Regulation InsulinTransport Transport

substanceHemoglobin

Storage Storage of amino acids

Ovalbumin in eggwhite, casein in milk

Contractile Movement Actin and myosin in muscles

Defensive Protection Antibodies (immunoglobins)

acids by enzymes in the digestive tract and transported in the bloodstream to the liver and body tissues.

Essential Amino AcidsThe necessary role of dietary proteins is to supply aminoacids, so that the body can construct human proteins. All20 amino acids are necessary for protein synthesis, and cellsin the human body have the ability to synthesize 11 aminoacids from raw materials; the other nine cannot be madeby the body. These nine are called the essential amino acids(table 10.2 and see fig. 1.6) and must come from the diet. It isimportant to note that these essential amino acids cannot bestored by the body. For this reason it is critical that the bodyreceive all the essential amino acids in a single day. Persistentlack of these essential amino acids prevents synthesis of neces-sary proteins and results in protein-deficiency diseases.

Complete ProteinsComplete proteins contain all the essential amino acids andin the right proportions. Almost all proteins derived fromanimals are complete proteins, whereas proteins derived fromplants are usually incomplete, deficient in one or more essen-tial amino acids. Although plant proteins are incomplete, theessential amino acid requirements can be met by combiningcomplementary plant proteins. For example, the traditionaldiet of the native peoples of Mexico, beans and corn, containscomplementary protein sources. The beans are low in methi-onine but adequate in tryptophan and lysine, but corn, whichis poor in tryptophan and lysine, contains adequate amountsof methionine.

Although we have an absolute requirement for theessential amino acids, the actual amount of protein requiredby humans is a small percentage of our nutrient needs. It isrecommended that approximately 10% (a range from 8% to10%) of our total caloric intake be provided by proteins. Onthe basis of this percentage, individuals on a 2,000-kilocalorie

Table 10.2Essential and Nonessential Amino Acids

Essential Nonessential

Histidine AlanineIsoleucineLeucineLysineMethioninePhenylalanineThreonine

AsparagineAspartic acidArginineCysteineGlutamic acidGlutamine

TryptophanValine

GlycineProlineSerineTyrosine

C H A P T E R 10 Human Nutrition

Table 10.3 Protein Content (in grams) of Some Common Food Items

1 ounce meat (beef, chicken, turkey) . . . . . 71 ounce cheese . . . . . . . . . . . . . . . . . . . . . 71 glass milk . . . . . . . . . . . . . . . . . . . . . . . . 81/2 cup beans . . . . . . . . . . . . . . . . . . . . . . 61 slice whole-wheat bread . . . . . . . . . . . . . 41 egg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 tbsp. peanut butter . . . . . . . . . . . . . . . . 81 serving oatmeal . . . . . . . . . . . . . . . . . . . 5

diet should have 50 grams of protein per day; those on a1,600-kilocalorie diet, 40 grams of protein per day; and thoseon a 2,200-kilocalorie diet, 55 grams of protein. The followingdaily protein intakes have been recommended for specificage groups: infants under 1 year—14 grams; children 1 to 4years old—16 grams; pregnant women—60 grams; and nursingmothers—65 grams. The protein we require can be obtainedfrom many foods. Table 10.3 contains examples of the proteincontent of some common foods.

Proteins can be assigned a numerical value that reflectshow well they supply the essential amino acids. The proteinin eggs has been assigned a biological value of 100, and allother foods are given values using egg protein as the referencestandard. Another factor that needs to be considered is thedigestibility of a particular protein. Some proteins cannot bebroken down completely; that is, the amino acids are not fullyreleased during digestion. This incomplete breakdown reducesthe dietary value of the protein. For example, when digestibilityis taken into account, even egg protein, considered the perfectprotein source, drops to a value of 94. High-quality proteins con-tain all the essential amino acids in the right proportions andare fully digestible, freeing their amino acids, which are thenabsorbed into the blood and transported to the body’s cells.

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Thinking CriticallyHumans have a dietary requirement for proteins, although proteins are needed in smaller quantities than carbohy-drates and fats.

On the basis of caloric intake, determine your protein requirement.

Gluten and Celiac DiseaseEating gluten, the major protein in wheat, or the relatedproteins secalin and hordein, in rye and barley, respectively,(see Chapter 12), can trigger celiac disease in individuals whohave an inherited sensitivity to these proteins. Celiac diseaseis characterized by destruction of the intestinal lining andrecurrent abdominal pain and diarrhea. In the most seriouscases, the body is unable to absorb nutrients. Aretaeus, aGreek physician who practiced during the first century a.d.,was the first to describe children who, while apparently wellfed, were nonetheless exhibiting signs of starvation. In 1887,the English physician Samuel Gee observed that the conditionof chronic indigestion could affect people of all ages but wasespecially prevalent in children of 1–5 years old. Willem-Karel Dicke, a Dutch pediatrician, was the first to link thecondition of chronic indigestion to wheat gluten during WorldWar II. He observed that, during times of bread shortages inthe Netherlands, children with celiac disease were no longerdying, but the mortality rate returned to the pre-war level of35% when bread was once more plentiful.

A newly available blood test has enabled physicians tomore clearly identify sufferers of celiac disease. When intes-tinal cells are damaged, an enzyme called transglutaminaseis released. Evidence of elevated levels of transglutaminase inthe blood is indicative of celiac disease. In 2003, a widespreadscreening trial revealed that the incidence of celiac disease inNorth America is about 1 in 133, a ratio that indicates that theaffliction is 100 times more common than thought previously.It may seem puzzling that the human body would react in suchan abnormal way to the protein in wheat, a staple food formuch of humanity, but it is important to realize that wheat andother cereal grains were not part of the original human diet.Our ancestors subsisted primarily by foraging for sweet fruits,starchy tubers, seeds of legumes, and eggs and by huntinganimals for the occasional meat meal until agriculture aroseapproximately 10,000 years ago (see Chapter 11).

Celiac disease is an autoimmune disorder in which theimmune system of genetically predisposed individuals reactsabnormally to the presence of gluten. Gluten is made up oflarge amounts of glutamine and proline, amino acids for whichthe body lacks the enzymes to digest completely. Instead, thegluten is broken down into peptides, small protein fragments,containing these amino acids. Normally, these peptides arenot absorbed by the small intestine but remain inside the

gastrointestinal tract and are eventually eliminated in thefeces. In patients with celiac disease, gluten penetrates thesmall intestine, eliciting an immune response, which damagesthe cells in the lining.

It has been discovered that patients with celiac diseasehave abnormally high permeability of the intestinal wall. Thecells of the intestinal lining are joined together by tight junc-tions, complexes of proteins that join adjacent cells to forma permeability barrier. Tight junctions prevent most materi-als from moving through the spaces between the cells andpenetrating the lining of the small intestine to reach deepertissues. However, in the normal functioning of the body, thereare circumstances in which this barrier must be breached toallow for the passage of large molecules or cells. Zonulin isthe body’s protein that, when released, increases intestinal wallpermeability. Apparently, the amount of zonulin is abnormallyhigh in patients with celiac disease, and this allows gluten topass through the gut wall.

The immune system is programmed to react to f oreignproteins that may signal the arrival of disease-causing micro-organisms. Eating food is one way foreign proteins are intro-duced into the body. Immune cells are situated beneath thelining of the small intestine, primed to attack any foreignproteins that pass through the intestinal lining. Ninety-five per-cent of those suffering from celiac disease have certain geneticmarkers on their cells, either HLA-DQ8 or HLA-DQ2 orboth. These protein molecules pick up the peptide fragmentsof gluten and present them to T cells, a type of immune cell.These T cells are now trained to attack and destroy the glutenfragments whenever they are encountered. As the T cellsdestroy the gluten fragments, cells in the intestinal lining aredamaged, causing the gastrointestinal symptoms of celiacdisease. Additionally, it has been discovered that sufferers ofceliac disease also release immune-stimulating chemicals thatintensify the immune response.

Patients who suffer from celiac disease apparently alsohave certain abnormalities in their small intestine. Repeatedexposure to gluten in patients with celiac disease causes thevilli of the small intestine to become chronically inflamed.Villi, fingerlike projections in the lining of the small intestine,absorb digested nutrients from the gut and pass them into thecirculatory system for distribution. In celiac disease, the villibecome damaged and flattened, which decreases the surfacearea of the small intestine and its ability to absorb nutrients.Undiagnosed patients are often thought to be suffering fromother conditions, such as osteoporosis or chronic fatigue, butthe underlying cause is impaired nutrient absorption becauseof celiac disease. When patients with celiac disease eliminategluten from their diet, the villi resume their original shape andfunction normally.

The primary treatment for celiac disease is to remove thegluten trigger by eating a gluten-free diet. Another promisingstrategy is to administer enzymes that would break downcompletely any gluten present in the small intestine. Otherpotential strategies are to depress the immune response ordesensitize the immune system by introducing small, repeated

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exposures to gluten. Some researchers recommend that infantsborn to a family with an incidence of celiac disease have agluten-free diet during their first year. By preventing exposureduring the period of rapid development of the immune system,gluten sensitivity might be avoided. Another promising tacticis to develop drugs that are zonulin inhibitors to reduce thepermeability of the intestinal lining in celiac patients.

Researchers are also working to develop gluten-free vari-eties of wheat that would be safe to eat for people who suffer from celiac disease. Recently a mutant form of barley wasisolated that completely lacked the gliadin-like protein found in gluten but was high in lysine, one of the essential amino acid which is usually deficient in wheat flour. Gliadin is the pri-mary culprit in celiac disease and researchers are hopeful that the gliadin-free barley will be the start of developing gluten-free wheat varieties that will retain all of the flavor and bakingqualities associated with wheat flour. Research has shownpreviously that eliminating gliadins from gluten does not affect the bread-making qualities of wheat flour. It is also known that heirloom varieties of wheat such as einkorn, emmer, and spelt (see Chapter 12) are much lower in gluten content and may also be used to develop gluten-free wheat varieties.

Thinking CriticallySome people apparently develop celiac disease not asinfants but much later in life. It has been suggested that the delay of celiac disease in genetically predisposed individuals is due to changes in the population of bacteria that normally inhabit the gut.

What might the bacteria population be supplying to the host that has forestalled the activation of the immune system to attack gluten? Does this suggest another possible treatment for sufferers of celiac disease?

Fats and CholesterolFats are usually considered culprits in the diet because theyare associated with cardiovascular disease, but some fat isnecessary because it serves several vital functions and somefats are heart-healthy. Fats and related compounds belong to a larger category of organic molecules called lipids. Although adiverse group of compounds, all lipids share the characteristic of insolubility in water (table 10.4).

TriglyceridesNinety-five percent of the lipids in foods are fats and oils; boththese compounds are chemically classified as triglycerides,which are formed from glycerol and three fatty acids (seefig. 1.12). Fatty acids themselves are the simplest type of lipidand serve as building blocks for triglycerides and phospholipids.A glycerol backbone is common to all triglycerides, but many

Table 10.4Functions of Lipids

Type of Lipid Function Examples

Triglyceride Energy, storage Animal fat, vegetable oils

Insulation Subcutaneous fat

Steroid Structure Cholesterol inmembranes

Hormonal regulation

Cortisol, estrogen,testosterone

Phospholipid Structure Phosphatidylcholine in cell membranes

types of fatty acids can occur. It is the nature of the fatty acids that determines the chemical and physical properties of the tri-glyceride. Each fatty acid contains a carbon chain with hydro-gen attached to the carbon atoms; different fatty acids vary in the number of carbon and hydrogen atoms.

During digestion in the small intestine, triglycerides are first acted on by bile, which is made by the liver and stored and released by the gallbladder. Bile contains a complex mixture of lipids, bile salts, and pigments; prominent among the lipid components are cholesterol and lecithin. Bile acts as an emul-sifier, breaking up the triglycerides into smaller droplets that can be acted on by enzymes. Enzymes from the pancreas and intestinal cells split these smaller droplets of triglycerides into monoglycerides and two fatty acids or into glycerol and three fatty acids. These end products are absorbed into the intestinal cells, where they are resynthesized into new triglycerides that enter the lymphatic system and eventually the bloodstream. High blood triglyceride level is a risk factor for coronary heart disease.

Essential Fatty AcidsThe body is capable of synthesizing most fatty acids, but three must be supplied in the diet. Linoleic, linolenic, and arachidonic acids are designated essential fatty acids, but few adults suffer deficiency symptoms because these three fatty acids are widely found in foods, especially vegetable oils. Even if an adult were consuming a totally fat-free diet, as little as 1 teaspoon of corn oil, as an ingredient in foods, would supply the essential fatty acids. Deficiency symptoms, such as poor growth and skin irritation, have been seen in infants fed a formula lacking these essential nutrients.

Saturated and Unsaturated FatsFatty acids can be separated into two types, saturated and unsaturated. Saturated fatty acids contain all single bondsbetween the carbon atoms and have the maximum number of hydrogen atoms (it is said to be saturated with hydrogen). Unsaturated fatty acids have one or more double bondsbetween carbon atoms and consequently fewer hydrogen

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HH

HH

HH

HH

H

H

CC

CC

CH

H

C

Stearic acid, a saturated fatty acid.

Oleic acid, a monounsaturated fatty acidwith double bond in cis configuration.

Linoleic acid, a polyunsaturated fatty acid. Both double bonds are in the cis configuration.

OC

OHH

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

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C

H

H

C

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C

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H

C

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C

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C

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H

C

H

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C

H

H

C

H

H

C

H

H

C

H

HH

CH

HC

HH

HC

H

HC

HC

H

HC

H OC

OHH

HC

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HC

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HC

HC C

H H

H

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C CC

H

HC

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H

HC H

CH

HC

H OC

OH

H

HC

HC C

Figure 10.1 Structure of a saturated, monounsaturated, and polyunsaturated fatty acid.

atoms. Each carbon atom can form only four bonds, so if a double bond occurs between two carbons, then less than the full complement of hydrogen atoms can be attached. A fatty acid with one double bond is called monounsaturated and lacks two hydrogen atoms; a polyunsaturated fatty acid has two or more double bonds and lacks four or more hydrogen atoms (fig. 10.1).

All food fats contain a mixture of both saturated andunsaturated fatty acids (fig. 10.2). Saturated fats containmostly saturated fatty acids and are solid at room tempera-ture; animal fats, such as lard, butter, and beef fat, are familiarexamples. Vegetable oils are generally composed of unsaturatedfatty acids and are liquid at room temperature. Oils containingmostly monounsaturated fatty acids are olive oil, peanut oil,and canola oil; other vegetable oils, such as corn oil, soybeanoil, and walnut oil, contain mostly polyunsaturated fatty acids.Coconut oil, palm and palm kernel oils, and cocoa butter areexceptions to the rule. Although they are of plant origin, theyconsist mostly of saturated fatty acids. On the other hand, cer-tain fish oils are actually unsaturated. The oils from fish suchas salmon, tuna, and herring are polyunsaturated and containomega-3 fatty acids. Omega-3 fatty acids lower the tendency ofblood platelets to stick together and form blood clots. Loweringthe risk of clot formation reduces the incidence of blockedblood flow to the heart and the onset of coronary heart disease.

0 10 20 30 40 50 60 70 80 90 100

Fatty acid composition (%)*

Coconut

Corn

Walnut

Soybean

Peanut

Canola

Almond

Fat o

r oil

Avocado

Olive

Sunflower**

Lard

Palm

Beef tallow

Cocoa butter

Table cream

Butter

Palm kernel

SaturatedMonounsaturated

PolyunsaturatedOther lipids

Figure 10.2 Fatty acid composition in common fats and oils.*Calculated as a percent of total lipids.**Newer varieties of sunflower and safflower produce oils which are 70% oleic acid, a monounsaturated fatty acid.

Source: Data from USDA National Nutrient Database for Standard Reference.

The health implications of saturated versus unsaturatedfats have been intensely studied by the scientific community.Because saturated fats increase blood cholesterol levels, theyare linked to cardiovascular diseases. Unsaturated fats, on theother hand, lower the risk of cardiovascular disease by lower-ing blood cholesterol levels.

CholesterolCholesterol belongs to a subcategory of lipids known as ste-roids, which are compounds containing four carbon rings (seeChapter 1). Several steroids, including cholesterol, also havea hydrocarbon tail and an —OH group, making them sterols.Cholesterol is a vital constituent of cells; it is part of the lipidcomponent of cell membranes and is used in the synthesis ofsex hormones and several other hormones.

Cholesterol is synthesized in the liver from saturated fattyacids and is absorbed by intestinal cells from animal foods,especially eggs, butter, cheese, and meat. If the diet is high insaturated fats, even if it is low in cholesterol, the liver responds

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by increased cholesterol synthesis. Because cholesterol, likeall lipids, is insoluble in the watery medium of the blood, it istransported by a special complex that consists of a cholesterolcenter with a coating of lipids and water-soluble proteins.These transport molecules, known as lipoproteins, exist in sev-eral forms. Two of the most significant are the low-density lipo-proteins (LDLs) and high-density lipoproteins (HDLs). LDLstransport cholesterol to all the body cells, while HDLs removeexcess cholesterol from the body’s tissues and carry it to theliver for degradation and elimination. In the popular press, theLDLs are considered the “bad” cholesterol because they canbe taken up by the cells that line the arteries. The resultingdeposition of cholesterol blocks the arteries, restricting theblood flow and is known as atherosclerosis. This condition canlead to heart attacks if the coronary arteries are blocked andstrokes if the arteries delivering blood to the brain are blocked.HDLs are considered “good” cholesterol because they can pre-vent atherosclerosis by preventing the buildup of cholesteroldeposits on the lining of the arteries.

Diets high in cholesterol, saturated fat, or both contrib-ute to high blood cholesterol levels, especially in the formof LDLs. Plant sources do not contribute dietary cholesteroldirectly, although, as pointed out previously, some are high insaturated fats. On the other hand, plant oils are generally richin unsaturated fats, which are known to lower blood choles-terol levels. However, monounsaturated and polyunsaturatedfats act differently. Polyunsaturated fats tend to lower all cho-lesterol levels, including the protective HDLs, whereas mono-unsaturated fats raise the HDLs while lowering total and LDLlevels. LDL blood cholesterol level is a better gauge of therisk of heart disease than is total blood cholesterol; the lowerthe LDL level, the lower the risk of heart disease. Cholesterolratio, the ratio of total cholesterol to HDL, is another measurefor the risk of heart disease. The ratio should be less than 5to 1; the recommended ratio is 3.5 to 1 or lower. Althoughhereditary factors, cigarette smoking, and exercise play a rolein the LDL-HDL balance, for the majority of people diet is thesingle most important factor in controlling cholesterol levelsand the inherent disease risks.

The HDL hypothesis suggests that raising HDL and low-ering LDL levels should lower the risk of cardiovascular dis-ease, but recent findings contest this idea. For example, peoplewith certain genetic mutations that raise HDL levels werestudied and were found not to have the expected reduced riskof cardiovascular disease. Also, people who have a differentmutation that lowered the level of HDL to about half of whatis seen in the general population were found to have a lower,not higher, risk of cardiovascular disease as compared to thegeneral population. In addition, during testing of a proposeddrug that raised HDL levels from 30% to 100%, the risk ofcardiovascular disease was not lessened but actually increasedin some individuals.

The problem may be that not all HDL function similarly.It has been suggested that only young HDL can remove cho-lesterol that has built up in artery walls and deliver it to theliver for elimination, a process known as reverse cholesterol

transport, and thereby reduce the risk of cardiovascular dis-ease. Older HDL particles may no longer be capable of reverse cholesterol transport. Newer strategies aim to develop medi-cations that raise only the young, functional HDL particles.

Trans Fatty AcidsTrans fatty acids that occur naturally in foods such as beef,lamb, whole milk, cream, and butter are derived from rumi-nant (cud-chewing) animals. Conjugated linoleic acid, a nat-ural trans fatty acid, may be beneficial as an anticarcinogenand in strengthening the immune system. However, mosttrans fatty acids are made when manufacturers convert liquidoils into solid fats. In this practice, known as hydrogenation,vegetable oils are partially or fully hydrogenated to make mar-garine, vegetable shortening, peanut butter, and salad dressing.Trans fats are present in many processed foods, such as crack-ers, cookies, baked goods, snack foods, and practically anyfood made with or fried in partially hydrogenated oils.

Trans fats act somewhat like saturated fats, but chemi-cally, trans fats are unsaturated. The carbon-carbon doublebonds found in unsaturated vegetable oils are in a cis configu-ration in which the molecular groups attached to each carbon in the double bonds bend in the same direction, either both up or both down. During the hydrogenation process, someof the double bonds are broken and hydrogen is added to the carbons. More significantly, most of the double bonds change configuration from cis to the trans position. In the trans config-uration, the molecules attached to each carbon of the double bonds bend in opposite directions or in a zigzag fashion. Thus, trans fatty acids can stack closer together, and the transformed oil readily takes on a solid, spreadable form that many consum-ers prefer over the natural liquid state.

A study of elderly men in the Netherlands indicated that adiet high in trans fat raised blood LDL cholesterol levels whilelowering HDL levels. Coronary heart disease was higher inthose men who ate more trans fats, and the risk of developingcoronary heart disease within 10 years increased by 25% foreach additional 2% of trans fats consumed in their diet. After10 years, the percentage of trans fats in the diets of these mendropped from a high of over 4% to just less than 2%. It isestimated that trans fats make up about 2%–4% of the typicalU.S. diet.

In the Nurses’ Health Study, conducted by the HarvardSchool of Public Health, nearly 90,000 women filled outdetailed questionnaires about their diet every few years for14 years. The results found no correlation between the onsetof type 2 diabetes and the total fat, saturated fat, or mono-unsaturated fat consumed. There was, however, a significantoccurrence of this most common type of diabetes when thewomen had diets high in trans fats. It was also discovered thata diet high in polyunsaturated oils lowered the risk of type 2diabetes.

Since 1983, the FDA has required that saturated fatand dietary cholesterol be listed on food labels. Due to theapparent association between high trans fats in the diet and

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the higher risk of cardiovascular disease and, in females, type2 diabetes, the content of trans fats must be listed on theNutrition Facts panel of food labels in the United States.

Manufacturers have responded to health concerns andconsumer demand by developing new methods of hydroge-nating vegetable oils. Using supercritical carbon dioxide asa solvent and under conditions of higher pressure and lowertemperatures, USDA researchers produced a hydrogenatedsoybean oil with only 10% trans fats. This percentage is lowerthan the usual 10%–30% trans fats found in most hydrogenatedvegetable oils.

MICRONUTRIENTSLike macronutrients, micronutrients are essential for propernutrition, but they are required in much smaller amounts.While macronutrients make up the bulk dry weight of food,micronutrients constitute only 1%–2% of the dry weight. There

are two categories of micronutrients, the organic compoundsknown as vitamins and the inorganic compounds, the minerals.

VitaminsMany vitamins play roles as coenzymes (molecules that arerequired for the proper functioning of certain enzymes) inmany metabolic pathways in the body; others are directlyinvolved in the synthesis of indispensable compounds.Vitamins are classified according to their solubility, with four

fat-soluble vitamins (A, D, E, and K) and nine water-solublevitamins (eight B-complex vitamins and C). The water-solublevitamins are not readily stored in the body, and any excess iseliminated in the urine; therefore, these are unlikely to becometoxic. On the other hand, the fat-soluble vitamins are easilystored in the fatty tissues of the body, and excessive intake canlead to toxicity symptoms. The dietary sources and deficien-cies of these vitamins are described in Tables 10.5 and 10.6.The following discussion is limited to vitamins A, D, and Cand the B vitamins thiamine, niacin, and B12.

Vitamin AVitamin A has many roles in the body. One of the best known involves the formation of vision pigments (rhodopsin and oth-ers) present in the retina of the eye. Each pigment is composed

of a molecule of retinal (a form of vitamin A) and a proteinmolecule, called an opsin, that differs from pigment to pig-ment. The pigments are contained in two types of photorecep-tor cells, rods and cones, located deep in the retina of the eye.All light stimulates the rods, providing only black-and-whitevision, while the cones are selectively stimulated by differentcolors, providing the full spectrum of color vision. In dim lightor at night, only the rods are used for vision; therefore, a short-age of retinal has especially pronounced effects, leading to oneof the earliest signs of vitamin A deficiency, night blindness.

Vitamin A is also necessary for the maintenance of epithe-lial tissues that line both internal and external body surfaces, an

Table 10.5Fat-Soluble Vitamins

Vitamin Dietary Source Results of Deficiency

A Yellow, orange, and dark green vegetablesand fruits; dairy products

Night blindness,xerophthalmia

D Eggs and enriched dairy products

Rickets

E Seeds, leafy green vegetables

Unknown

K Leafy green vegetables Poor blood clotting

Table 10.6Water-Soluble Vitamins

Vitamin Dietary SourceResults of Deficiency

B1 (thiamine) Whole grains, legumes, seeds, nuts

Beriberi

B2 (riboflavin) Dairy products, wholegrains, leafy green vegetables, poultry

Mouth sores, lesions of eyes

Niacin Meat, eggs, seeds, legumes

Pellagra

B6 (pyridoxine) Dried fruits, seeds, poultry, leafy greenvegetables

Irritability, muscle weakness, skindisorders

Pantothenic acid Dried fruits, seeds,poultry, leafy greenvegetables, nuts

Insomnia, weakness

Folic acid (folate) Legumes, wholegrains, green vegetables

Anemia, diarrhea, neuraltube defects

Biotin Legumes, vegetables, meat, egg yolks

Fatigue, dermatitis

B12 (cobalamin) Meat, eggs, dairy products

Perniciousanemia

C Fresh fruits andvegetables

Scurvy

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area roughly equivalent to one-fourth of a football field. Whenvitamin A is lacking, these tissues fail to secrete their protec-tive mucus covering, producing instead a protein called keratin(normally found in hair and nails), which results in the tissues’becoming dried and hardened. This aspect of vitamin A defi-ciency can therefore affect many different areas in the body. Oneof the most tragic consequences is a type of blindness known asxerophthalmia, in which severe vitamin A deficiency results inirreversible drying and degeneration of the cornea. This perma-nent blindness, easily preventable with proper nutrition, is found


most frequently among malnourished children in developingnations. In the skin, keratinization results in rough, dry, scaly,and cracked skin, often with an accumulation of hard materialaround a hair follicle that looks like a permanent goose bump.Other epithelial tissues, such as those in the mouth, gastrointesti-nal tract, and respiratory system, are also affected by the decreasein mucus production, becoming progressively drier and subject toinfection. Vitamin A is additionally involved in dozens of otherroles in the body, including normal bone and tooth developmentand hormone production in the adrenal and thyroid glands.

In food derived from animal sources, especially liver, vitamin A occurs primarily as retinol, which is readily absorbedby the body and converted to retinal. In plant sources, no ret-inol is present, but a vitamin A precursor, beta-carotene (firstisolated from carrots) occurs abundantly in many yellow,orange, and dark green fruits and vegetables (see Chapter 3)and can be split into two molecules of retinol in the body.

AntioxidantsThe importance of beta-carotene as an antioxidant has beeninvestigated. Antioxidants may protect the body againstthe destructive action of reactive ions called free radicals.Destructive free radicals form when electrons escape from theElectron Transport System during ATP production and com-bine with oxygen available in mitochondria (see Chapter 4).These highly reactive ions can damage the metabolic machin-ery of the cell, impairing energy production. They may alsodamage DNA itself, inducing mutations, and they have beenimplicated in cancer, heart disease, and even the signs of aging.

Several recent animal studies are challenging the prevail-ing concept that the build-up of free radicals, destructive ionsthat produce oxidative damage in cells, accounts for manyof the deleterious effects associated with aging. There is alsorecent evidence that the practice of taking antioxidant supple-ments to neutralize the effects of free radicals can be harmful.Researchers bred roundworms (Caenorhabditis elegans) to over-produce a specific free radical (superoxide) with the expectationthat the worms would die prematurely compared to worms withlower levels of this free radical. Surprisingly, the worms whichhad higher levels of the free radical, lived 32% longer. Moreastonishing, treating the worms with high levels of free radicalswith the antioxidant vitamin C reversed the longer life span.

In another group of experiments with roundworms,normal worms were exposed to an herbicide that is knownto induce free radical production in animals. The herbicidetreated worms, with the higher level of free radicals, livedalmost 60 percent longer than untreated worms; and onceagain feeding antioxidants to the worms prevented this lon-gevity. Researchers then genetically engineered roundwormswithout the ability to produce certain natural enzymes thatact as antioxidants. As anticipated, the genetically engineeredworms had higher levels of free radicals and more potentiallydamaging oxidative reactions throughout their bodies butunexpectedly the worms did not age prematurely but insteadlived just as long as worms with normal levels of antioxidants.

Other model organisms have showed similar results. Somestrains of mice were genetically engineered to produce higherthan normal levels of antioxidant enzymes, whereas otherstrains were created that produced lower than normal levelsof antioxidant enzymes. It was expected that the mice withhigher levels of antioxidant enzymes in their bodies would livelonger that those with lower levels but there was no differencein life spans between the strains. Observations from naturealso support the experimental data. The naked mole rat withlower levels of natural antioxidants and evidence of more oxi-dative damage to their bodies lives 25–30 years, about eighttimes longer, than another rodent, the common mouse.

As antioxidants have been shown to not always be benefi-cial, new research is indicating that free radicals are not alwaysharmful. At lower levels, free radicals appear to signal thebody to turn on genes to repair oxidative cellular damage andprevent further injury. Free radicals have been shown to acti-vate gene HIF-1 which itself turns on genes that are involvedin cellular repair. Current research is suggesting that at lowerlevels free radicals are not necessarily destructive agents inthemselves but are instead signals to the body to turn on genesto repair oxidative cellular damage and prevent further injury.Only, in larger amounts, free radicals destructive.

Vitamin DThe primary function of vitamin D is the regulation of calciumand phosphorus levels, especially for normal bone devel-opment. Vitamin D helps control the blood levels of these minerals in three ways:

1. the absorption of calcium and phosphorus from food in thegastrointestinal tract,

2. the removal of these minerals from bones to maintain theconcentration in the blood, and

3. the retention of calcium by the kidneys.

Vitamin D is unique in that it can be synthesized by thehuman body on exposure to sunlight; in fact, it has long beencalled the sunshine vitamin. The precursor (a cholesterolderivative) is manufactured by the liver and transported to theskin, where exposure to the sun’s ultraviolet rays converts itto provitamin D; the final steps in the manufacture of activevitamin D occur in the liver and kidneys. The amount ofpigmentation in the skin affects the synthesis of vitamin Dbecause pigment blocks ultraviolet absorption. Darker skinrequires longer exposure to sunlight to produce adequateamounts of vitamin D. Thirty minutes of sunlight is adequatefor light skin, while darker skin may require up to 3 hours. Thisexposure to sunlight must be achieved with caution, since over-exposure to the ultraviolet rays in sunlight is linked to higherrisk of skin cancer. The Environmental Protection Agency hasestimated that the average American is indoors 93% of thetime, in transit 5% (cars, buses, trains), and outdoors only 2%of the time daily. This limited exposure to sunlight makes itessential that the diet contain vitamin D to avoid deficiencysymptoms. Vitamin D is not naturally abundant in any food.

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164 U N I T I I I Plants as a Source of FoodSo

urce

: Cen

ter

for

Dis

ease

Con

trol

and

Pre

vent

ion

Figure 10.3 Characteristic bowing of the legs and knees in rickets, a disease caused by insufficient vitamin D.

None occurs in plant sources, but vitamin D does occur in lim-ited amounts in animal sources, such as egg yolks, liver, cream,some fish, and butter. Because there is concern about meetingthe nutritional needs for vitamin D, especially for children,milk, which does not contain adequate vitamin D, is routinelyfortified with this vitamin.

Because of the role of vitamin D in calcium regulation,the deficiency symptoms are most evident in bone formation.The effects of vitamin D deficiency are most pronouncedin children and result in a characteristic malformation ofthe skeleton known as rickets. The bowing of the legs socommonly associated with the condition is just one of manyabnormalities of the skeletal system (fig. 10.3). Dark-skinnedchildren who live in northern, smoggy cities are especiallyvulnerable to developing this deficiency. Once believed to havebeen eradicated in developed countries, rickets has reemergedin the United States and the United Kingdom. The problemseems to be that breastfed children are not receiving enoughvitamin D to absorb the amount of calcium necessary to buildstrong bones and teeth. Pediatricians are now recommendingvitamin D supplements for breastfed infants. Adult rickets,also known as osteomalacia, is rare but does occur in womenwho have undergone repeated pregnancies, have low calciumintake, and have inadequate exposure to sunlight. However,a large scale study in 2018 found no benefit in the taking ofhigh—dose vitamin D supplements in either increasing bonedensity or in the prevention of fractures from falls.

Currently, some researchers in nutrition are advocatingthat the daily recommended dose of vitamin D be raised to1,000 IU* (International Units) per day for adults not only topromote healthy bones but also to prevent certain forms ofcancer. Previous studies have shown that colon cancers aremore prevalent in the northern United States where exposure

*International Unit is a unit of measurement for biologically active substances (such asvitamins, hormones, and vaccines) that produces a measured biological effect agreedupon as an international standard.

to sunlight is the lowest. Recent studies have correlated bloodlevels of vitamin D in women to colon cancer. Those with thelowest blood levels of the vitamin had double the risk of coloncancer compared with those women who had the highest lev-els of vitamin D. Preliminary research shows that this trendmay also be repeated for type 2 diabetes and other diseases.

Excess vitamin D causes abnormally high levels ofcalcium in the blood; this condition often leads to calciumdeposits in soft tissues, such as kidneys and blood vessels.If not caught in time, this may result in irreversible damageto the cardiovascular system, kidney failure, and even death.In excess, vitamin D is the most toxic of all the vitamins.However, this toxicity cannot occur from sunlight or food; itresults only when megadoses of the vitamin are administeredwithout medical supervision.

Vitamin CFresh fruits and vegetables are the richest sources of vitamin C,ascorbic acid, with organ meats the only significant animalsources. The most important role of vitamin C in the bodyis in the synthesis of collagen, a connective tissue proteinthat serves as a “cellular cement,” holding cells and tissuestogether. Collagen, the most abundant protein in the body, isfound in the matrix of bones, teeth, and cartilage and providesthe elasticity of blood vessels and skin. Vitamin C also func-tions as an antioxidant in the body, preventing other moleculesfrom being oxidized (losing electrons). Because vitamin Cprevents oxidation, it is sometimes added to packaged foodsto extend the shelf life; in a similar manner, orange juice andlemon juice, with their high vitamin C content, prevent theoxidation (browning) of sliced apples and bananas. Vitamin Cis additionally involved in promoting iron absorption throughthe intestines. When foods containing vitamin C are con-sumed with foods containing iron, absorption is enhanced.Finally, vitamin C is involved in a number of other metabolicreactions, including the production of various hormones.

For centuries, sailors on long ocean voyages faced thepossibility of developing scurvy, a disease that could causebleeding of the gums, pinpoint hemorrhages under the skin,severe fatigue, poor healing of wounds, brittle bones, and evensudden death due to massive internal bleeding. It was notuncommon for half to two-thirds of the ship’s company to dieof scurvy on a long voyage. The first cure for this disease wasidentified in 1747 by Dr. James Lind, who experimented with12 sailors afflicted with scurvy. Lind tried various dietary sup-plements and found that sailors who were given either orangesor lemons for 6 days improved rapidly. Interestingly, it wasalmost 50 years after these findings that the British admiraltytook measures to prevent scurvy by dictating that all sailorsreceive lemon or lime juice daily. (British sailors were soonnicknamed “limeys” because of this practice, a name that isstill heard today.) Now it is well known that the vitamin C incitrus juice prevents scurvy. The typical symptoms of scurvycan be traced directly to the inability of the body to makecollagen. The scurvy–vitamin C connection is reflected in thename ascorbic acid, which literally means “without scurvy.”

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Since the 1970s, the daily intake of vitamin C has beenthe focal point of a heated debate, after the publication ofVitamin C and the Common Cold by Linus Pauling, a NobelPrize–winning chemist. The RDI (reference daily intake)** forvitamin C is 60 milligrams (approximately 10 milligrams dailycan prevent scurvy), but Pauling recommended megadosesas high as 2,000 to 10,000 milligrams for optimum health.Pauling maintained that large doses of vitamin C can preventcolds and other viral infections and that vitamin C is bacteri-cidal and even cures cancer. Recent research has shown thathigh doses of vitamin C is effective in killing certain types ofcolon cancer cells. However, some authorities note that tissuesbecome saturated with vitamin C at levels of 80 to 100 milli-grams per day and that intakes above these levels are generallyexcreted in the urine. Controversy also exists regarding thetoxicity of vitamin C. Some scientists maintain that high dosesare nontoxic; others feel that toxicity can occur. The mostcommon toxicity symptoms are nausea, abdominal cramps,and diarrhea. Another possible complication of megadosingis rebound scurvy, a condition that may occur after an abruptcessation of these high doses. Symptoms mimic scurvy, eventhough vitamin C intake is not deficient; these symptoms donot occur if the megadoses are decreased gradually.

Vitamin B ComplexThe vitamin B complex includes a group of eight vitamins thatare often found in foods together and have similar roles in thebody; that is, they function as coenzymes, involved in thou-sands of metabolic reactions. They are found in each cell ofthe body and must be present for normal cell functioning. Foreach of these vitamins, specific deficiency symptoms occurwhen the vitamin is lacking in the diet, but in general, no toxic-ity symptoms have been reported because excess is excreted inthe urine. As is vitamin C, the B vitamins are water soluble andcan be leached out during food preparation when excess wateris used and discarded. In addition, some of the B vitamins maybe destroyed by high temperatures during cooking. Thiamine,niacin, and B12 will be considered because these vitamins maybe deficient in plant sources.

Thiamine Thiamine, also known as vitamin B1, is part of thecoenzyme thiamine pyrophosphate, which is involved in the met-abolic breakdown of carbohydrates just before the Citric AcidCycle. Because of thiamine’s central role in metabolism, thesymptoms of thiamine deficiency are profound: fatigue; depres-sion; mental confusion; cramping, burning, and numbness in thelegs; edema; enlarged heart; and eventually death from cardiacfailure. This thiamine deficiency is known as beriberi and was

**Daily values are determined by the U.S. FDA (Food and Drug Administration) andindicate the percentage amount of a nutrient that is provided by a single serving of aparticular food, based on the current recommendations for a 2,000-kilocalorie diet.There are two categories of daily values: daily reference values (DRVs) and referencedaily intakes (RDIs). DRVs have been determined for total fat, saturated fat, cholesterol,

total carbohydrate, dietary fiber, sodium, potassium, and protein. RDIs are establishedfor 19 vitamins and minerals and have replaced the older term recommended dailyallowances (RDAs).

found mainly in Asia, where diets were based mainly on polishedor white rice rather than on the whole-grain brown rice that stillhas the outer bran (husk) intact. The thiamine that occurs in theouter layer of the rice is removed during the polishing process.Beriberi became more prevalent when improved techniques forpolishing rice were developed that removed more of the branand, inadvertently, more of the thiamine. In the 1880s, thiaminedeficiency in the Japanese navy was particularly widespread,with 25%–40% of the sailors developing beriberi. A Japanesephysician, Dr. K. Takaki, observed that few sailors developedthe disease when milk, meat, and eggs were added to the nor-mal staple of white rice. Unfortunately, he did not realize thatthe enriched diet was supplying a nutrient missing in the whiterice diet. A few years later, a Dutch physician, Dr. ChristiaanEijkman, studied beriberi in the East Indies; he showed that, indiets based almost exclusively on rice, the consumption of brownrice instead of white rice prevents the appearance of beriberi.It was not until the twentieth century that thiamine deficiencywas actually identified as the cause of the disease. Good dietarysources of thiamine include meat, especially pork and liver,whole grains, seeds and nuts, and legumes.

Niacin Niacin is the collective term for two compounds,nicotinic acid and nicotinamide, either one of which is usedto form the coenzymes NAD+ and NADP+. Recall the impor-tance of these coenzymes for oxidation-reduction reactionsin many energy-yielding metabolic pathways (see Chapter 4).Without these coenzymes, the release of energy from thebreakdown of foods cannot occur and cellular death results.Niacin can be supplied directly through foods rich in niacinitself or foods rich in the essential amino acid tryptophanbecause the body can synthesize niacin from the amino acid.Niacin deficiency, therefore, is coupled with a low-protein diet.

A lack of niacin severely affects every organ of the body,and a severe deficiency disease, pellagra, develops. The symp-toms of pellagra are referred to as the 4 Ds: dermatitis (skindisorders), dementia (mental confusion), diarrhea, and eventu-ally death if niacin is not supplied. The dermatitis (fig. 10.4) ischaracterized by rough, reddened skin with lesions developing

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Figure 10.4 Pellagra, caused by a lack of niacin, ischaracterized by dermatitis of the hands.

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in exposed areas; in fact, pellagra means “rough skin.” The cen-tral nervous system is affected, and confusion, memory loss,dizziness, and hallucinations occur. The gastrointestinal systemis also involved, and diarrhea, along with abdominal discom-fort, nausea, and vomiting, is common. Another characteristicof the condition is a bright red, or strawberry, tongue.

Pellagra is especially common in areas where corn isthe dietary staple. Outbreaks have occurred in southernEurope, particularly Italy, parts of southern India, and therural South in the United States, where the disease was epi-demic early in the twentieth century. It was estimated that10,000 people died and another 200,000 were afflicted eachyear. During this period, about half the patients in mentalhospitals in the South were suffering from the dementiacaused by pellagra.

Although corn does contain some niacin, it is in a formthat makes it unavailable; furthermore, corn is also deficientin tryptophan. However, pellagra was not a problem in thetraditional diet of the natives of Mexico, Central America,and parts of South America for two reasons. Lime (calciumoxide from wood ash or shells), used in the preparation ofcorn meal, is able to release the bound niacin. In addition, thebeans, squash, tomatoes, and peppers commonly eaten withthe corn also supplied niacin.

The addition of milk and meat to the diet was recom-mended to prevent pellagra long before the vitamin was identi-fied. Although meat contains niacin, milk is low in the vitamin

but does supply tryptophan. Sources rich in niacin includemeat, poultry, fish, eggs, nuts, seeds, and legumes. One waythe pellagra in the South could have been prevented was by theconsumption of a handful of peanuts every other day, becausepeanuts are an excellent source of niacin.

There has been an increased interest in the therapeu-tic value of megadoses of niacin (the nicotinic acid form)for reducing blood cholesterol levels. Unfortunately, themegadoses can cause some toxicity symptoms, the mostcommon of which is a niacin flush. It produces a temporarywarm flush of the skin, with a tingling or stinging sensation.Intestinal irritation and liver damage have also been reported.The other form, nicotinamide, does not produce those tox-icity symptoms but is not at all effective in lowering bloodcholesterol levels.

Vitamin B12 Vitamin B12 (cobalamin) is unique in thatit does not occur naturally in any foods of plant origin butoccurs only in animal sources, where it is widely available.Those who completely eliminate meat, dairy products, andeggs from their diets are at risk of developing a B12 deficiencyunless they take vitamin supplements or eat fortified foods.Soy milk, breakfast cereals, and meat substitutes are oftenfortified with B12.

The absorption of vitamin B12 in the small intestinerequires the presence of a substance secreted by the stomachcalled an intrinsic factor. Poor absorption of B12 has beenreported in people who, because of a genetic defect, do notproduce the intrinsic factor. This defect most often shows up

after the age of 60, when intrinsic factor production becomesimpaired. In this case, B12 deficiency symptoms show up eventhough there are sufficient quantities in the diet, and the vitamin must be received by injection.

The most common result of B12 deficiency is perniciousanemia, characterized by the production of improperly formedred blood cells. The associated symptoms include fatigue andweakness because the delivery of oxygen to the body’s tissuesis impaired. A more serious consequence of B12 deficiency isnerve damage that begins as a creeping numbness of the lowerextremities.

In general, vitamin B12 is involved in nucleic acid syn-thesis and interacts with folic acid (another B vitamin) in thisfunction. Because blood cells are constantly being formed inthe bone marrow, this site of rapid cell division is one of thefirst affected by impaired synthesis of DNA due to a deficiencyof either vitamin. These vitamins are, therefore, involved in thenormal development of red blood cells, and a deficiency ofeither vitamin causes anemia. The anemia can be treated witheither B12 or folic acid supplements. However, folic acid sup-plements have no effect on the nerve damage caused by a B12

deficiency, and the administration of folic acid for the anemiacan mask a true B12 deficiency and result in permanent neu-rological degeneration. In this regard, vitamin B12 functionsin maintaining the sheath surrounding nerve fibers, which isnecessary for the transmission of nerve impulses.

Under directions from the FDA and the Department ofHealth and Human Services, U.S. manufacturers fortify mostenriched breads, flours, corn meals, rice, and other grain prod-ucts with folic acid. This action was taken because it was deter-mined that insufficient levels of folic acid contribute to spinabifida (the backbone does not form properly, leaving the spinalcord exposed) and other neural tube birth defects. Becausemore than half of all pregnancies are unplanned and thesedefects of the spine and brain occur in the developing fetusbefore most women realize they are pregnant, it is importantthat all women of childbearing age consume 0.4 milligram offolic acid daily. Since 1998, when folic acid fortification offoods began, the number of neural tube birth defects in theUnited States has dropped by one-third.

Adequate levels of folic acid may also afford protectionfrom early heart disease. In patients with atherosclerosis,

Thinking CriticallyVitamins are classified according to their solubility. Thewater-soluble vitamins include C and the B complex. A, D, E,and K are the fat-soluble vitamins.

Many people subscribe to the notion of vitamin megadosing to ensure better health and consume manytimes the RDI for particular vitamins. What are the practical effects of vitamin solubility on this practice?

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high levels of plasma homocysteine are commonly found.Homocysteine is an intermediate in amino acid metabolism. Itis toxic to the lining of blood vessels, bringing about changesthat lead to cardiovascular disease. It may also promoteclotting factors in the blood. Normally, homocysteine is notfound in high levels in the bloodstream because it is brokendown by enzymatic activity. Certain vitamins—folic acid isone—are cofactors for these enzymes. Many patients withearly coronary artery disease have low levels of folic acid andcorrespondingly high levels of homocysteine. Increasing thenutritional intake of folic acid will decrease homocysteinelevels as well as the risk of heart attack or stroke.

MineralsMinerals are inorganic compounds that exist in the body asions (charged atoms) or as part of complex molecules. Atleast 18 minerals are required for normal metabolic activi-ties (table 10.7). In 2014, bromine was identified as anotheressential element necessary for proper human nutrition.

Table 10.7Dietary Mineral Requirements

Mineral Function

Major Minerals

Calcium Bone and tooth formation, blood clotting, nerve impulse transmission, muscle contraction

Phosphorus Nucleic acids, bone and tooth formation, cell membranes, ATP formation

Sulfur Protein formationPotassium Muscle contraction, nerve impulse

transmission, electrolyte balanceChlorine Gastric juiceSodium Nerve impulse transmission, body

water balanceMagnesium Protein formation, enzyme cofactor

Trace Minerals

Iron HemoglobinZinc Component of many enzymes and

insulin, wound healingIodine Component of thyroid hormonesFluorine Bone and tooth formationCopper Enzyme component, red blood cell

formationSelenium AntioxidantCobalt Component of vitamin B12

Chromium Normal glucose metabolismManganese Enzyme cofactorMolybdenum Enzyme cofactorBromine Enzyme cofactor

Animal studies have shown that if bromine, active in the bodyin the form of the bromide ion, is eliminated from the diet,abnormal basement membranes form. Basement membranesare extracellular structures found in connective tissue whichform scaffolds that support tissues. Bromide ion is a necessarycofactor for the enzyme peroxidasin; this enzyme catalyzes theformation of sulfilimine (sulfur-nitrogen) bonds between pro-tein ropes of collagen that make up the basement membrane.This research is of tremendous interest because it had beenshown previously that cells in contact with abnormal base-ment membranes can become cancerous. Also it is known thatthiocyanate, one of the chemicals in tobacco smoke, inhibitsthe enzyme peroxidasin, which leads to the breakdown ofbasement membranes in the lungs of smokers.

Minerals are subdivided into two categories, the majorminerals, needed in amounts greater than 100 milligrams perday, and the trace minerals, needed in amounts no more thana few milligrams per day. The following discussion will be lim-ited to calcium, a major mineral whose RDI has been recentlyrevised, and two trace minerals that have been extensivelystudied, iron and iodine.

CalciumCalcium is the most abundant mineral in the body, with theaverage adult containing 800 to 1,300 grams of the element.Ninety-nine percent of the body’s calcium is found in thebones and teeth; the other 1% is in the blood and tissues.The concentration of calcium is under the control of severalhormones and vitamin D. If the amount in the blood gets toolow, calcium reserves in the bone are drawn upon to restorelevels to the normal range. If the amount of calcium in theblood is too high, more calcium is deposited in the boneand more is excreted by the kidneys. Excess calcium intakes(12,000 milligrams per day and above are considered toxic)have been associated with increased risk of kidney stone for-mation. In addition to forming the matrix of bones and teeth,calcium in the body fluids is involved in many important func-tions: nerve impulse transmission, muscle action (includingheartbeat), blood clotting, cell membrane integrity, intracellu-lar communication, and as a cofactor for enzymes (cofactorsare mineral ions and, like coenzymes, are necessary for theproper functioning of certain enzymes).

Calcium deficiency may lead to osteoporosis, a degener-ative bone disease that may strike older individuals withoutwarning. In osteoporosis, the bone density is greatly reduced(osteoporosis literally means porous bone), resulting in bonesthat fracture readily. This condition can result from years oflow dietary calcium intake or poor absorption of calcium fromthe intestines (caused by lack of vitamin D or other factors).To maintain blood calcium levels, the reserves in the bone aredangerously depleted. Postmenopausal women are particularlyat risk of developing osteoporosis because bone loss is accel-erated at this time. Estrogen replacement therapy appears toretard this bone loss, but is associated with a higher risk ofbreast cancer, heart attacks, and strokes. Adequate dietary cal-cium and regular exercise also prevent osteoporosis; however,

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there are many interacting factors (both genetic and environ-mental), and much more research is needed in this area.

Milk and milk products are among the best sources ofcalcium, but the element is also present in dark green leafy veg-etables, many seeds, and other foods. Unfortunately, in somevegetables, the presence of oxalic acid inhibits the absorptionof calcium. Recently, the RDI for dietary calcium for adultsover 50 has been increased to 1,200 milligrams to prevent thedevelopment of osteoporosis. New evidence has also shownthat calcium, together with vitamin D, may provide protectionagainst colon cancer; the amount of calcium required for thisbeneficial action is 1,500 milligrams per day.

IronAlthough most of the trace minerals are usually found in ade-quate amounts in a well-balanced diet, iron and iodine presentspecial problems. Iron deficiency is common in women andchildren, and care must be taken to ensure that the diet suppliessufficient quantities of the element. Meat, especially liver andother organ meats, shellfish, fish, and poultry are excellent ironsources. Many foods from plants are also rich in iron, includingdark green leafy vegetables, dried fruits, legumes, whole grains,and enriched breads and cereal products. Overall, only about10% of dietary iron is actually absorbed by the body, with theabsorption dependent on the type of iron compound present;the rest is eliminated in the feces. The iron from animal sourcesmay be present as heme iron (40%) or nonheme iron (60%),whereas the iron in plant sources is nonheme. Heme ironis more readily absorbed by the body, but the absorption ofnonheme iron can be improved by the presence of vitamin C.

The most important role of iron is as a component ofhemoglobin, the molecule that carries oxygen in red bloodcells; in fact, it is the iron that imparts the red color to thesecells. In addition, iron occurs in myoglobin, a molecule similar

to hemoglobin, the oxygen carrier in muscle cells; in severalstorage proteins in the liver, bone marrow, and spleen; and inenzymes present in each cell.

Because the majority of iron is found in hemoglobin, irondeficiency has its greatest impact on red blood cells. Wheniron reserves in the body are low, not enough hemoglobincan be synthesized for newly formed red blood cells. Thesecells are smaller, paler, and less efficient in oxygen transportthan are normal red blood cells and are characteristic ofiron-deficiency anemia, the most common dietary deficiencydisease in the world. The symptoms of iron-deficiency anemiainclude fatigue, inability to concentrate, pale coloration, weak-ness, and listlessness.

The greatest risk of iron toxicity comes from overdosingon iron supplements, which can result in damage to the liverand pancreas and even sudden death in young children.

IodineThe presence of iodine in food is dependent on the availability

of iodine in the natural environment where the plant or ani-mal developed. Foods from the ocean are reliable sources ofiodine because this element is plentiful in seawater. In general,

inland areas, especially mountainous regions, are likely tohave iodine-deficient soils. It is in these areas where peoplemay develop the iodine-deficiency disease endemic, or simple,goiter. In the United States, the area around the Great Lakeswas formerly known as the “goiter belt” because of the highincidence of goiter. The most obvious symptom of goiter is aswelling of the neck caused by an enlargement of the thyroidgland, which straddles the trachea.

Iodine is required for the formation of thyroid hormones,which regulate metabolism in all cells of the body and controlbody temperature, growth, development, and reproduction.When the amount of iodine is low, hormone production isimpaired. The thyroid enlarges in an attempt to produce moreof the needed hormones; this attempt is futile without the nec-essary iodine. A person suffering from simple goiter exhibitsa lack of energy, decreased blood pressure, sensitivity to coldtemperatures, and weight gain.

Goiter has been a recognized ailment since ancient times,and various treatments have been suggested as a cure. Theearliest known treatment is recorded in a Chinese source from5,000 years ago that recommended eating seaweed and burnedmarine sponge. Today it is known, of course, that organismsfrom the ocean are naturally rich in iodine. The relationshipbetween iodine and goiter was confirmed in 1820 by theFrench physician Jean-Francois Coindet, who reported thetreatment of goiter using doses of iodine salts. Today goiter israre in the United States and Europe because of iodized tablesalt, first introduced in 1924. However, in other areas of theworld, almost 200 million people still suffer from goiter, aneasily preventable disorder.

More than half of the salt sold in the United States isiodized, and a single teaspoon of this salt supplies almosttwice the RDI of 0.15 milligram. However, overconsumptionof iodine-containing substances can also be a problem becauseiodine can be toxic. As little as 2.0 milligrams per day is consid-ered toxic, resulting also in an enlargement of the thyroid gland.

In addition to a lack (or even an excess) of dietary iodine,goiter can result from the overconsumption of goitrogeniccompounds. Certain medications, including some of the sulfadrugs, and vegetables in the cabbage family are known to con-tain compounds that block the utilization of iodine in the thy-roid. In a varied diet, these compounds are harmless, but theymay be a problem in diets restricted solely to those vegetables.

Thinking CriticallyThe macronutrients (carbohydrates, proteins, and fats) arerequired in relatively large amounts for proper nutrition,whereas the micronutrients, the vitamins and minerals, areneeded in smaller quantities.

Since micronutrients are required in much smaller amounts than macronutrients, are they any less important to the human diet? Explain.

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DIETARY GUIDELINESResearch has shown that many significant diseases are influ-enced by nutrition, and beneficial changes in diet can, there-fore, reduce the risk of developing these conditions. Diseaseslinked to nutrition are some of the major causes of deathin the United States: cardiovascular diseases, hypertension,some forms of cancer, and type 2 diabetes. These diseasesmay arise in part from excess consumption of fat (especiallysaturated fat), cholesterol, refined sugar, and salt. In light ofthese findings, government agencies and health professionalshave recommended dietary guidelines for better health and theprevention of disease.

Balancing Nutritional RequirementsThe U.S. Senate Select Committee on Nutrition and HumanNeeds issued the first Dietary Goals for the United Statesin 1977. This was followed in 1980 by the publication ofNutrition and Your Health: Dietary Guidelines for Americans,issued jointly by the U.S. Department of Health and HumanServices (HHS) and the U.S. Department of Agriculture(USDA). Revisions of the Dietary Guidelines have fol-lowed every 5 years. The latest revision of the USDA rec-ommendations, Dietary Guidelines forAmericans, 2010, featured MyPlate (fig.10.5), which replaced the Food Pyramid.Some of its recommendations are asfollows:

1. Make half your plate fruits and vegeta-bles; especially red, orange, and darkgreen-vegetables. Eat fruit, vegetables,and unsalted nuts as snacks.

2. Drink fat-free or low-fat (1%) milk. Trycalcium-fortified soy products as a sub-stitute for dairy foods.

3. Make at least half of your grains wholeby choosing whole-grain cereals,breads, and pasta.

4. Vary protein choices by eating seafoodtwice a week and eating beans. Eatsmaller and lean portions of meat andpoultry.

5. Cut back on foods and drinks high insaturated fats, added sugars, and salt.Replace sugary drinks such as sodawith water. Choose low-sodium ver-sions of soups, bread, and frozen meals.

6. Balance Calories by eating less andavoiding oversized portions. Be physi-cally active.

The dietary guidelines originally sug-gested that complex carbohydrates should make up about 60% of the daily caloric

intake, proteins about 8%–10%, and fats no more than 30%,with 10% each from saturated, monounsaturated, and poly-unsaturated fat sources. Since the first dietary guidelineswere issued, some healthy trends have developed among theAmerican people. Fresh fruits, vegetables, and whole-grainproducts have undergone a resurgence in popularity, owingto expanded selections available in the supermarkets andincreased nutritional awareness of the value of fiber.

Although the guidelines call for a reduction in the con-sumption of refined and processed sugars, consumption hascontinued to rise (a trend that began early in the twentieth cen-tury). At least part of the rise is attributable to the increasedconsumption of soft drinks, which contain high-fructose cornsyrup.

Americans are eating less red meat, eggs, and whole dairy products than ever before, which is a good trend for reduc-ing saturated fats and cholesterol. Consumption of poultryand fish has increased as consumption of beef has declined,and low-fat dairy products have become the preferred choicefor many people. Although the intake of saturated fat hasdecreased (dramatically for butter and lard), more plant oilsare being consumed. This trend has had a significant agricul-tural impact, as farmers have increased the acreage devotedto growing oil crops, such as sunflower, safflower, rapeseed

Figure 10.5 MyPlate illustrates the 2010 Dietary Guidelines for Americans, which offer the best scientific advice on how to eat for health issued by the USDA.Source: http://www.choosemyplate.gov/food-groups/downloads/MyPlate/MyPlateGraphicsStandards.pdf. Source: USDA

CD

C

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(canola oil), corn, and soybean. Margarine, shortening, salad oils, and cooking oils account for the expanded use of plant oils, but remember, if the plant oils are hydrogenated, theyhave reduced health value.

Since the early 1980s, there has been a heightenedawareness about the dangers of high blood cholesterol levels,and Americans have responded with changes in diet and exer-cise. A report on several thousand men and women showeda decrease in cholesterol levels: for men, the 1980–1982average was 205 milligrams, lowered to 200 milligrams inthe 1985–1987 study; likewise, women showed a drop from201 milligrams to 195 milligrams in the same period. Thistrend indicates that many Americans are attempting to keepblood cholesterol at or below the recommended level of200 milligrams.

Although sodium is one of the major minerals, mostAmericans ingest more sodium (in the form of sodium chloride—table salt) than required; high sodium intake isrelated to hypertension (high blood pressure). Excessive saltions in the bloodstream draw water from the tissues, therebyraising the fluid pressure in the blood vessels. A low-saltdiet may be effective in lowering blood pressure in sodium--sensitive individuals. Some foods naturally contain sodium,but much of the sodium in our diet comes from the table saltadded during cooking, during the meal, or in prepared foods.In fact, two-thirds of dietary salt is actually “hidden” in com-mercially prepared food and beverages.

Healthier Dietary GuidelinesIn February 2004, the CDC (Centers for Disease Control andPrevention) released the results of a study on the prevalenceof obesity and the changes to the American diet over the past30 years. The incidence of obese individuals in the total U.S.adult population increased from 14.5% in 1971 to nearly 31%in 2000 as the average amount of kilocalories per day rose.For men, the average energy intake increased from 2,450in 1971 to 2,618. The rise in energy intake for women wasnearly double that at 335 kilocalories. As Americans ate moreand got fatter, what they ate changed, too. Carbohydratesmade up a greater percentage of the American diet. For men,carbohydrate intake increased from about 42% to 49%; forwomen, carbohydrates rose from approximately 45% to 52% ofthe daily diet. At the same time, the percentage of both totalfat and saturated fat in the diet declined. For men, total fatdecreased from 37% to 33%, and saturated fat fell from 14%to 11%; for women, total fat and saturated fat fell from 36%to 33% and 13% to 11%, respectively. Protein levels for bothmen and women declined slightly from an average of approx-imately 17% to just over 15%. As many low-carbohydratedieters suggest, the rise in obesity in the United States appearsto be correlated with a rise in carbohydrates, especially refinedstarches and sugars.

This trend is especially disheartening because earlier ver-sions of the USDA Dietary Guidelines advised Americans tominimize consumption of fats to just 30% per day and increase

complex carbohydrates, such as breads, cereals, rice, andpasta, to 60% of the daily diet. Other directives were to limitdairy products and protein sources (meat, eggs, fish, poultry,and beans) to two to three servings for each group per dayand increase the number of servings of fruits and vegetableseaten daily.

The most controversial aspect of previous DietaryGuidelines is the advice about fats and carbohydrates. Manyresearchers are coming to the conclusion that it is just too sim-plistic to advocate eating all types of complex carbohydratesand avoid all types of fats. Increasingly, it has been shown thatthere is no nutritional evidence that a diet high in complexcarbohydrates and low in all fats is beneficial to health. In fact,the reverse may be true.

Although it is known that a diet high in saturated fatsraises total blood cholesterol levels and that high cholesterolis associated with an increased risk of coronary heart disease,certain fats, as stated previously, are beneficial to health.Monounsaturated fats, such as olive oil, can reduce the risk ofcardiovascular disease by lowering LDL cholesterol levels andraising HDL cholesterol. It has also been shown that a diet ofpolyunsaturated fats can reduce total blood cholesterol levels.

For example, the so-called Mediterranean diet commonin Italy, France, and Greece, in which fats make up 40% of thetotal kilocalories, is associated with a low rate of heart disease.Apparently, the type of fats—mainly monounsaturated olive oiland polyunsaturated fish oils with omega-3 fatty acids—not thepercentage, is the determining factor in lowering the incidenceof heart disease.

The original intent of the earlier USDA Dietary Guidelineswas to influence the American public to decrease consumptionof saturated fats. At the time, it was thought to be too difficultfor the public to distinguish saturated fats from other types offats, so the message was simplified to decrease consumptionof all fats to 30% of total kilocalories, down from the 40% typ-ical of the American diet. To compensate for the kilocalorieslost by decreased fat consumption, the percentage of complexcarbohydrates was raised from 45% to 60%. The recommendeddaily percentage of protein stayed about the same at 10%–15%because there was concern that, if increased protein consump-tion was recommended, people would eat more red meat as aprotein source without realizing that red meat is usually asso-ciated with saturated fat. “Fats are bad” was the rallying cry ofthe earlier Dietary Guidelines.

Unfortunately, this message was the wrong one. Nutritionalstudies from the early 1990s have shown that, if people replacekilocalories from saturated fat with an equal amount from car-bohydrates, their LDL and total cholesterol levels do fall, butso does their level of HDL. Similarly, if people eat a diet highin monounsaturated or polyunsaturated fats but then switch toan equivalent amount of kilocalories from carbohydrates, theirLDL levels rise and HDL levels decline.

Trans fatty acids found in many dietary substitutes forfoods rich in saturated fats are uniquely bad because theyraise LDL and triglycerides while reducing HDL. Eating transfatty acids greatly increases the risk of cardiovascular disease.

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In contrast, consuming saturated fats increases the risk onlyslightly because consumption of saturated fats increases LDLbut also HDL.

Another common misconception is that eating a greaterpercentage of fats in the daily diet is linked to obesity becausefats have more than double the kilocalories of equal portionsof proteins or carbohydrates. The standard advice has been toavoid obesity by eating a low-fat diet. The only way to avoidobesity is to lower total kilocalories, not just fat kilocalories,and increase energy expenditure through physical activity.

No nutritional studies have definitively linked the con-sumption of fat with a higher risk of breast or colon cancer.A diet high in red meat has been associated with a higher risk

of colon cancer, but this link is probably due to carcinogensproduced during cooking and the type of chemicals found inprocessed meat. A low-fat diet has not been shown to reducethe risk of cancer. A 2006 study on women’s health comparedwomen who reduced fat consumption from 8% to 10% over aperiod of 8 years with a control group. In that study, the ratesfor colon and heart disease were not statistically different, butthe rate of breast cancer showed a small decline.

Incorporating weight control and physical activity appearto be the key factors in decreasing the risk of many cancers.Unfortunately, recent data did find that sedentary, overweight

women who ate diets high in refined carbohydrates had a highincidence of both pancreatic and breast cancer. Increasingly,evidence points out that a diet high in certain carbohydrates isnot synonymous with healthy eating.

Glycemic IndexSome carbohydrates are readily digested and quickly metab-olized by the body into glucose. A rapid increase in bloodglucose stimulates the release of insulin from the pancreas.Insulin facilitates the uptake of glucose into body cells, suchas muscle and fat cells, and quickly lowers blood sugar levels.Glycemic index (GI) measures the effect that a particular foodhas on blood glucose levels. The faster the food is convertedto glucose, the higher the GI number. White bread, white rice,and white potatoes are examples of carbohydrates with rela-tively high GI values. Whole grains, and high-fiber fruits andvegetables have lower GI ratings. A more practical measurethat is often used is glycemic load, which takes the GI value for

a food and multiplies it by the number of carbohydrate gramsthe food contains. Thus, many fruits and vegetables that havea relatively high GI value have a much lower glycemic load.Fiber slows the rate of digestion, and high-carbohydrate foods,such as many beans, that also are high in fiber have a lower GI.

There are difficulties in accurately determining the GI fora particular food because the calculation is based on the aver-age blood glucose response of 10 subjects after they have eatena particular food. Different people have different responsesto the same food, and the same person may have a differentresponse on a different day. Eating different combinations offoods and changing the method of cooking can also alter theGI value.

A high GI diet is associated with a greater risk of heartdisease, type 2 diabetes, and obesity. A diet high in GI car-bohydrates raises blood triglyceride levels and lowers HDLcholesterol, increasing the risk of coronary heart disease.Another consequence of a high GI diet is insulin resistance,common in many overweight, sedentary people. One of themain indicators of insulin resistance, also known as SyndromeX, is a sustained high blood glucose level after the ingestionof high GI foods. Although insulin is produced in affectedindividuals, it is not as effective in moving glucose from thebloodstream into body cells. Because the insulin is not aseffective, more and more insulin is released to do the job ofmoving glucose out of the blood and into cells. Eventually,the insulin-producing cells of the pancreas become overtaxedby higher production levels and give out. People who exhibitinsulin resistance have a higher risk of hypertension, heartdisease, and most significantly type 2 diabetes. Foods with thehighest GI numbers correspond with greater insulin release,and the higher the insulin spike, the lower the blood glucosesinks. Low blood glucose stimulates hunger and a craving formore high GI foods, which may lead to overeating and weightgain. In contrast, low GI foods are digested more slowly, glu-cose levels in the blood drop gradually, and it takes longer forhunger to return.

Another problem with the previous USDA DietaryGuidelines was that they did not distinguish between dif-ferent sources of protein. They lumped red meat, poultry,fish, legumes, nuts, and eggs into a single group, despite theevidence that a diet high in red meat is high in saturatedfats and cholesterol and is associated with an increased riskof coronary heart disease and type 2 diabetes. Poultry andfish possess fewer saturated fats, and fish are a source ofbeneficial omega-3 fatty acids. Certain nuts, such as walnuts,are also rich in beneficial omega-3 fatty acids but regrettablyhigh in Calories. What’s more, the pyramid promoted over-consumption of dairy products, a recommendation that wasinitially made to ensure significant calcium content in thediet to prevent osteoporosis. The problem is that most dairyproducts are associated with saturated fat and high caloricvalues. The best advice for those concerned about obtainingsufficient calcium for preventing osteoporosis would be totake calcium supplements. The Dietary Guidelines do advo-cate eating plenty of fruits and vegetables, but consumptionof the white potato and other starchy vegetables should belimited.

An improved version of the Dietary Guidelines, as sug-gested by Walter Willett of the Harvard School of PublicHealth, emphasizes keeping one’s weight under controlthrough daily exercising and avoiding an excessive total intakeof kilocalories. It recommends that the majority of one’s dailydiet should be the consumption of the healthy monounsat-urated and polyunsaturated fats and healthy carbohydrates,whole-grain and unrefined. A variety of fruits and nonstarchyvegetables with protective phytochemicals should be eaten inabundance. Healthy protein sources, such as nuts, legumes,fish, poultry, and eggs, should be eaten in moderation. Dairy

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A CLOSER LOOK 10.2Eat Broccoli for Cancer Prevention

Cancer has been a dreaded disease for centuries, and it con-tinues to plague humanity today. The term cancer actuallyrefers to over 100 forms of a disease that can strike justabout every tissue and organ in the body that shares several

basic processes. Cancer cells are abnormal cells that prolif-erate uncontrollably, forming masses called tumors. Cancercells also possess the ability to migrate, or metastasize, fromthe original site, forming tumors in other parts of the body.It is the interference of malignant tumors with normal bodyfunctioning that makes cancer lethal.

A number of agents have been identified as carcinogens(cancer-causing agents)—certain microbes, ultraviolet radia-tion, chemicals (such as PCBs, arsenic, and benzene), radon,and various gene mutations—but more than half of all can-cers in the United States are related to tobacco smoke anddiet. In addition to tars, a number of known carcinogens arefound in tobacco smoke (see Chapter 20). Not surprisingly,smoking—especially cigarette smoking—is associated withseveral cancers: lung, upper respiratory tract, esophageal,bladder, and pancreatic. Tobacco smoke is also implicated instomach, liver, kidney, and pancreas cancers and in leukemia.

Diet is second only to smoking as a major cause of can-cer in the United States. Red meat has been identified asan associative cause of colon, rectal, and prostate cancers.Obesity in adults is linked to cancers of the uterus, breast,colon, kidney, and gallbladder. Abusive drinking of alcoholic

beverages enhances the risk of cancers of the upper respi-ratory tract, digestive tract, and liver. Eating heavily saltedfoods and drinking extremely hot beverages have beenlinked to cancers of the stomach and esophagus, respec-tively, in countries outside the United States where thesedietary habits are customary.

Ironically, the latest research suggests that what is miss-ing in our diets may be far more important in causing can-cer than what we actually eat or drink. Components calledphytochemicals, which occur naturally in vegetables, fruits,grains, and seeds, have been investigated for their protec-tive action in the prevention of cancer and other diseases.

Cruciferous vegetables (broccoli, cauliflower, cabbage, andso on) are excellent sources of one class of chemopreventivechemicals: the dithiolthiones. In laboratory animals treatedwith a synthetic version of one of the dithiolthiones, tumors ofthe lung, colon, mammary glands, and bladder were inhibited.One of the most potent dithiolthiones is sulforaphane, foundin broccoli, which has been shown to inhibit breast cancer inrats. Both of these phytochemicals apparently work by acti-vating liver enzymes that destroy carcinogens in the body.

Genistein, a compound found in soy products (derivedfrom the soybean), prevents the formation of breast tumorsin rats in a different manner. It inhibits the formation andgrowth of blood vessels to a growing tumor. Without thesesources of nutrients and oxygen, the tumor cannot grow.

products should be limited to one or two servings daily. Theconsumption of red meat, saturated fats, refined grains, andstarchy fruits and vegetables should be restricted. Trans fattyacids should be avoided completely. A daily multiple vitaminand mineral supplement should be taken. Alcohol consump-tion of beer, wine, or distilled spirits in moderation is accept-able because of evidence of its benefit to the cardiovascularsystem (see Chapter 24).

Recently, Willett conducted a study on the health of menand women who followed his dietary suggestions. He foundthat they had reduced the risk of cardiovascular disease by 30%

for women and 40% for men. These findings and others haveresulted in major revisions within the USDA Dietary Guidelines,2010 and illustrated in the release of MyPlate.

Meatless AlternativesWith the awareness about the dangers of saturated fat andcholesterol inherent in animal products, many Americansare incorporating a greater percentage of vegetables, fruits,grains, and legumes into their diets. Some even choose atotally vegetarian lifestyle. There are many different forms of

vegetarianism: lacto vegetarians, lacto-ovo vegetarians, andvegans. Some vegetarians, such as the lacto and the lacto-ovo,consume dairy products or dairy products and eggs but do notconsume animal flesh. Vegans are pure vegetarians, consum-ing no animal products at all. Some other vegetarians stretchthe concept by consuming fish and poultry, avoiding only redmeat.

There are several health benefits to increasing the con-sumption of plant products while decreasing the consump-tion of animal products. Vegetarians are less likely to sufferfrom the chronic diseases that afflict many Americans whosediets are high in animal products. Blood cholesterol andtriglyceride levels usually reflect the amount of animal fat inthe diet and are lowest in vegans, who, consequently, have alower incidence of heart disease. Those cancers linked to redmeat and dairy consumption (colon and prostate) are lesscommon in vegetarians. High fiber in a vegetarian diet alsoplays a role in reducing risks of type 2 diabetes and loweringcholesterol levels. (See A Closer Look 10.2: Eat Broccolifor Cancer Prevention.) A further benefit of high-fiber dietsis in weight control; the filling effect of fiber suppressesovereating.

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Studying populations that drink quantities of green tea(made from unfermented tea leaves, Chapter 16) has revealeda lower incidence of many cancers, especially those of thebreast and prostate. Green tea contains a high percentageof chemical agents (flavonoids) known as catechins, and inparticular, epigallocatechin gallate. For a cancer to metas-tasize, certain enzymes are needed. One of these crucialenzymes is urokinase, and it appears that catechins inhibitit and thus prevent the invasion and spread of cancer cellsto distant sites. In the processing of black tea, the type oftea most commonly drunk by Americans, epigallocatechingallate is destroyed and thus does not afford the same pro-tective benefits associated with the drinking of green tea.In 2010, it was shown that epigallocatechin gallate (EGCG)can interfere with certain anticancer drugs, such as borte-zomib. Bortezomib is given to patients who are sufferingfrom multiple myeloma, an incurable but treatable form ofcancer of blood cells in the bone marrow. Bortezomib actsby deactivating proteasomes (see Chapter 2); however, in thepresence of EGCG, its anticancer activity is inhibited. It wasobserved, however, that only proteasome deactivators thatcontain boron, as does bortezomib, are inhibited by EGCG.Green tea does not interfere with the proteasome inhibitionof other anticancer drugs that lack boron. Patients undergo-ing treatment with bortezomib should refrain from drinkinggreen tea or taking supplements of green tea extracts.

Phytochemicals appear to be most effective if eaten infoods rich in these cancer-preventive agents rather thanadministered as supplements. In fact, the National CancerInstitute initiated the 5-A-Day Program in 1991 to encouragethe public to eat five or more servings of vegetables andfruits every day. In 2007, the CDC and the Produce for BetterHealth Foundation (PBH) released Fruits and Veggies—More

Matters™ which calculates the servings of fruits and vege-tables on the basis of age, gender and activity level. Severalstudies appear to support this belief. Beta-carotene, theyellow-orange pigment associated most commonly with car-rots, was given to people at risk of developing lung canceras a supplement in a chemoprevention trial sponsored bythe National Cancer Institute. Surprisingly, the group givenbeta-carotene had a slightly higher rate of lung cancerthan the group given a placebo. A 2007 review of nearly70 studies also found that adults who take supplements ofbeta- carotene, vitamin A, and/or vitamin E had higher mor-tality rates than those who did not or who were given a pla-cebo. Adults who took supplements of vitamin C or selenium

showed no difference in death rates than the control group.Recent large, long-term studies looking at a diet high in

fruits and vegetables to reduce the risk of breast cancerhave failed to substantiate the promise of earlier laboratorywork. Those who consumed the greatest amount of fruitsand vegetables did, however, show a 25% reduction in therisk of cardiovascular disease. Critics point out flaws inquestionnaires and the difficulties that participants have inaccurately recording and quantifying their diets. Also, somesuggest that the protective effect may be conferred duringa critical period in childhood, but no study has examined theconnection between a girl’s diet and breast cancer. Anotherstudy found that women who had the highest blood con-centration of carotenoids, such as beta-carotene, showedthe lowest incidence of breast cancer. But the same studyshowed no difference in the incidence of breast cancer withthe amount of vegetables and fruits consumed, suggestingthat individual differences in metabolism to harvest pro-tective phytochemicals may be more important. The workcontinues.

Vegetarians, especially vegans, must be knowledgeableabout all the nutritional requirements and use care in selectingplant sources that meet those requirements. Special attentionmust be given to ensure that iron, the B vitamins (especiallyB12), calcium, vitamin D, and the essential amino acids aresupplied. Complementing the essential amino acids is of primeimportance because all plant proteins are incomplete. This canbe easily accomplished by eating legumes and grains that, incombination, provide an excellent source of protein for the diet.In fact, one study indicates that proteins from plant foods may bepreferable to animal protein because consumption of excessiveanimal protein itself, not just animal fat, may be linked to heartdisease and cancer. The suggestion is that meat should no longerbe the centerpiece of a meal but instead relegated to a side dishfor a vegetarian entrée. Perhaps vegetarians have the right idea.

CHAPTER SUMMARY1. The nutritional needs of the human diet can be categorized

into the macronutrients (carbohydrates, proteins, lipids)and micronutrients (vitamins and minerals).

2. Carbohydrates include simple sugars, such as the mono-saccharides fructose and glucose and the disaccharidessucrose and lactose. Complex carbohydrates, or polysac-charides, include starch and glycogen and act as the body’sfuel. Dietary fiber includes both soluble and insoluble formsthat provide beneficial health effects, acting as roughage,promoting regularity, and lowering blood cholesterol levels.

3. All plant proteins are incomplete, deficient in one or moreessential amino acids. But complementing incompleteplant proteins, such as combining legumes and cereals in asingle meal, can overcome this deficiency. Gluten in wheatcan trigger celiac disease in individuals who have an inher-ited sensitivity to this protein.

4. Lipids include fats and oils, and most of the dietary lipidsare classified as triglycerides. A diet high in saturated fatsraises the risk of cardiovascular disease. Unsaturated fats,whether monounsaturated or polyunsaturated, lower therisk of cardiovascular disease by lowering LDL cholesterollevels. Foods of animal origin generally are high in satu-rated fats and cholesterol, but foods of plant origin lackcholesterol and contain mainly unsaturated fats.

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5. Deficiency diseases result if a diet lacks any of the essentialvitamins and minerals. Fat-soluble vitamins (A, D, E, andK) can be stored by the body but can build up to toxic levelsif excessive amounts are taken. Water-soluble vitamins (Cand B complex) cannot be stored in appreciable quantitiesby the body. Most vitamins and minerals (the exceptionis vitamin B12) can be found in foods of plant origin. The category of minerals (major or trace) is defined by thequantity needed in the body.

6. Americans should modify their diets for a healthier life-style. Some fats, such as monounsaturated and polyunsat-urated fats, are heart-healthy and should become a regularpart of the diet. Saturated fats, however, should be limitedand trans fats avoided completely. Limit the consumptionof white bread, white rice, white potato, and other highglycemic index carbohydrates to reduce the risk of type 2diabetes. Limit dairy products that are high in kilocaloriesand saturated fat. Eat plenty of nonstarchy fruits and veg-etables, and take a daily vitamin and mineral supplement.The most important factor for a healthier life is to keepweight under control by limiting total caloric consumptionand exercising regularly.

REVIEW QUESTIONS1. If fiber is largely indigestible, why is it required for a

healthy diet?

2. What are the essential amino acids? Why is it importantthat they be consumed?

3. What is the role of gluten in celiac disease?

4. What is the role of lipids in the body? What is the dietarysignificance of saturated and unsaturated fats?

5. Describe the following deficiency diseases: scurvy, rickets,marasmus, beriberi, osteoporosis.

6. How closely do you follow the recommended dietary guidelines? Evaluate your diet in terms of a healthy lifestyle.

7. What are the dietary causes of anemia?

8. What differentiates simple carbohydrates from complexcarbohydrates?

9. What dietary factors influence the development of cardio-vascular diseases or cancer?

10. Estimate the amount of protein you consume in a typicalday. What portion is from animal food sources, and whatportion is from plant foods?

11. Your friend insists that a vegetarian diet is unhealthybecause proteins found in plants are never complete. Howwould you respond?

12. Concerned about your risk of cardiovascular disease, youundergo a blood test for cholesterol at a health fair at thelocal supermarket. The test results indicate your total cholesterol count is 190. Is further testing necessary?

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