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Principles of Nutrition (CHS 261)2 nd semester 1438-1439 H (2017-18)

By

Dr Iftikhar AlamCourse Syllabus

Program in which the course is offered: Health education program and dental hygiene program2 theoretical hours

total contact hours per semester 30 hours

Level at which this course is offered: 3rd level; Health education program and 4hygiene program)

Course prerequisites: None

Sunday: 8:00- 9:50 AM

Classes NO 21, College of Applied Medical Sciences

College member responsible for the course Dr. Iftikhar Alam, Associate Professor

Contact information:Office Number: 2318; 2nd Floor, College of Applied Medical Sciences

Phone : Office:

Email: ialam@ksu.edu.sa

Website: http:// http://fac.ksu.edu.sa/ialam

Office hours: Sunday: 10.0 AM to 12 At Noon

Tuesday: 1.0 P.M to 3.0 Afternoon

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Topics to be Covered

List of Topics No of Weeks

Introduction to Nutrition 2

Food digestion and absorption of nutrients 2

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Water and electrolytes 2

Carbohydrates 1

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2

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Schedule of Assessment Tasks for Students During the SemesterAssessment task (e.g. essay, test, group

project, examination, speech, oral presentation, etc.)

Week Due Proportion of Total Assessment

Mid term I 5- 7 th week

Mid term II 10-12 th week

Class discussion and assignment Continuous

Presentation 4nd to 14th week

End Semester Exam 16th -18 th wk

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CHAPTER 1 - INTRODUCTION

What is Nutrition?1. Nutrition is a nourishing organic process by

which an organism assimilates food and uses it for growth and maintenance. Good nutrition can help prevent disease and promote health.

Why Nutrition is Important?2. Consumption of important fruits and

vegetables ensures lower level of mortality and reduces various degenerative diseases, for instance, cancer, cardiovascular disease, and immune dysfunction in several human cohorts. In addition to the vitamins and minerals found in fruits and vegetables, may contribute to these beneficially protective effects.

Food Provides us with Energy3. The foods we consume provide our bodies

with energy. Just like fuel in cars or cell phone battery, human body requires to be fed every day. Carbohydrate is the main form of energy necessary for human’s body.

Nutrition as a Science4. The aim of nutrition science is to give

definitions to metabolic and physiological feedbacks from the body regarding sufficient diet. Additionally, nutritional science is developing into the study of metabolism, which seeks to separate diet and health through the prism of biochemical process.

5. Nowadays, a plenty of nutrition information is accessible to everyone; from diet books to newspaper articles, everyone seems to have his/her opinions about what we should eat.

Food as a Source of Nutrients6. Food is significant factor to the maintenance,

development, functioning and reproduction of life. During lifetime an individual consumes 30 tons of food on average in seemingly endless dietary varieties. Digestion splits all

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the foods found in all this variety of diets into the same basic nutrients.

7. Food, therefore, is chemistry, and the mixture of chemicals that are represented and divided into four basic categories: (1) nutrients; (2) non-nutritive naturally occurring components (including antinutritives2 and natural toxins); (3) man-made contaminants; and (4) additives. At that, the nutrients account for more than 99.9% of the food contents. The main classes of nutrients are: carbohydrates, proteins, fats, and vitamins, and minerals. The constituents of food are called macronutrients and micronutrients. Macronutrients are the major sources of energy and building materials for humans, while micronutrients are only required in relatively small amounts. Micronutrients can be found in vitamins, minerals and trace elements, and are still required in sufficient amounts to ensure proper functioning of all body cells. In addition, micronutrients, like water, do not provide energy. The majority of macronutrients are essential nutrients for life processes, produced by human body itself. Therefore, these essential nutrients can be received only from the food we eat. Most importantly, macronutrients are constituent and indispensable ingredients of our diets, found in: carbohydrates, fat, protein, water.

Body mass, body fat, and body water

8. The human body consists of materials similar to those found in foods; however, the relative proportions differ, according to genetic dictates as well as to the unique life experience of the individual. The body of a healthy lean man is composed of roughly 62 percent water, 16 percent fat, 16 percent protein, 6 percent minerals, and less than 1 percent carbohydrate, along with very small amounts of vitamins and other miscellaneous substances. Females usually carry more fat

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(about 22 percent in a healthy lean woman) and slightly less of the other components than do males of comparable weight.

9. The body’s different compartments—lean body mass, body fat, and body water—are constantly adjusting to changes in the internal and external environment so that a state of dynamic equilibrium (homeostasis) is maintained. Tissues in the body are continuously being broken down (catabolism) and built up (anabolism) at varying rates. For example, the epithelial cells lining the digestive tract are replaced at a dizzying speed of every three or four days, while the life span of red blood cells is 120 days, and connective tissue is renewed over the course of several years.

10. The composition of the body tends to change in somewhat predictable ways over the course of a lifetime—during the growing years, in pregnancy and lactation, and as one ages—with corresponding changes in nutrient needs during different phases of the life cycle (see the section Nutrition throughout the life cycle). Regular physical exercise can help attenuate the age-related loss of lean tissue and increase in body fat.

Nutritional Status Assessment11. Good nutrition is essential for the

attainment and maintenance of good health. Determining whether a person is at risk requires completion of a nutrition assessment, which should, in fact, become part of a routine exam done by a registered dietitian or other health care professional specifically trained in the diagnosis of at-risk individuals.

Tools/Types of Nutritional Status Assessment (ABCD)

12. A proper nutrition assessment includes anthropometric measurements, clinical examination, biochemical tests, and dietary-social history.

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Anthropometrics13. Anthropometric measurements include

height and weight and measurements of the head (for children), upper arm, and skinfold (Figure-1).

14. The skinfold measurements are done with a caliper. They are used to determine the percentage of adipose and muscle tissue in the body. Measurements out of line with expectations may reveal failure to thrive in children, wasting (cataboism), edema, or obesity, all of which reflect nutrient deficiencies or excesses. During the clinical examination, signs of nutrient deficiencies are noted. Some nutrient deficiency diseases, such as scurvy, rickets, iron deficiency, and kwashiorkor, are obvious; other forms of nutrient deficiency can be far more subtle. Table 2 lists some clinical signs and probable causes of nutrient deficiencies. Biochemical tests include various blood, urine, and stool tests.

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Laboratory Assessment

15. A deficiency or toxicity can be determined by laboratory analysis of the samples. The tests allow detection of malnutrition before signs appear. The following are some of the most commonly used tests for nutritional evaluation. Serum albumin level measures the main protein in the blood and is used to determine protein status. Serum transferrin level indicates iron-carrying protein in the blood. The level will be above normal if iron stores are low and below normal if the body lacks protein. Blood urea nitrogen (BUN) may indicate renal failure, insufficient renal blood supply, or blockage of the urinary tract. Creatinine excretion indicates the amount of creatinine excreted in the urine over a 24-hour period and can be used in estimating body muscle mass. If the muscle mass has been depleted, as in malnutrition, the level will be low. Serum creatinine indicates the amount of creatinine in the blood and is used for evaluating renal function.

16. Examples of other blood tests are hemoglobin (Hgb), hematocrit (Hct), red

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blood cells (RBCs), and white blood cells (WBCs). A low Hgb and Hct can indicate anemia. Not a routine test, but ordered on many clients with heart conditions, isthe lipid profile, which includes total serum cholesterol, high-density lipoprotein(HDL), low-density lipoprotein (LDL), and serum triglycerides. Urinalysis also can detect protein and sugar in the urine, which can indicate kidney disease and diabetes.

Dietary Assessment

17. The dietary-social history involves evaluation of food habits and is very important in the nutritional assessment of any client. It can be difficult to obtain an accurate dietary assessment. The most common method is the 24-hour recall. In this method, the client is usually interviewed by the dietitian and is asked to give the types of, amounts of, and preparation used for all food eaten in the 24 hours prior to admission (PTA). Another method is the food diary. The client is asked to list all food eaten in a 3–4-day period. Neither method is totally accurate because clients forget or are not always totally truthful. They are sometimes inclined to say they have eaten certain foods because they know they should have done so.

Essential nutrients

18. The six classes of nutrients found in foods are carbohydrates, lipids (mostly fats and oils), proteins, vitamins, minerals, and water. Carbohydrates, lipids, and proteins constitute the bulk of the diet, amounting together to about 500 grams (just over one pound) per day in actual weight. These macronutrients provide raw materials for tissue building and maintenance as well as fuel to run the myriad of physiological and metabolic activities that sustain life. In contrast are the micronutrients, which are

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not themselves energy sources but facilitate metabolic processes throughout the body: vitamins, of which humans need about 300 milligrams per day in the diet, and minerals, of which about 20 grams per day are needed. The last nutrient category is water, which provides the medium in which all the body’s metabolic processes occur.

19. A nutrient is considered “essential” if it must be taken in from outside the body—in most cases, from food (Table-3) These nutrients are discussed in this section. Although they are separated into categories for purposes of discussion, one should keep in mind that nutrients work in collaboration with each other in the body, not as isolated entities.

TABLE 3: Dietary Reference Intakes for selected nutrients for adultsBased on a recommended intake of 0.8 g of protein per kg of body weight for a healthy 70-kg (154-pound) man and a healthy nonpregnant, nonlactating 57-kg (126-pound) woman (United States and Canada). 2As retinol activity equivalents (RAE); 1 RAE = 1 μg all-trans-retinol = 12 μg dietary all-trans-betacarotene. One IU of vitamin A activity = 0.3 μg all-trans-retinol = 3.6 μg all-trans-betacarotene. 31 μg cholecalciferol = 40 IU vitamin D. 4As niacin equivalents (NE); 1 mg niacin = 60 mg tryptophan. 5As dietary folate equivalents (DFE); 1 DFE = 1 μg food folate = 0.6 μg folic acid from fortified food or as a supplement consumed with food = 0.5 μg folic acid as a supplement taken without food. Source: National Academy of Sciences, Dietary Reference Intakes (1997, 1998, 2000, 2001, and 2002).

Recommended Daily Intake(Recommended Dietary Allowance or Adequate Intake)women men

Macronutrientscarbohydrates 130 g 130 g

25 g 38 glinoleic acid (omega-6) 12 g 17 galpha-linolenic acid (omega-3) 1.1 g 1.6 g

46 g 56 g

700 μg [2,333 IU] 900 μg [3,000 IU]75 mg 90 mg

(as cholecalciferol)5–15 μg [200–600 IU] 5–15 μg [200–600 IU]

(as alpha-tocopherol) 15 mg 15 mg90 μg 120 μg1.1 mg 1.2 mg1.1 mg 1.3 mg14 mg 16 mg1.3 mg 1.3 mg400 μg 400 μg2.4 μg 2.4 μg

pantothenic acid 5 mg 5 mg30 μg 30 μg

1,000–1,200 mg 1,000–1,200 mg

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TABLE 3: Dietary Reference Intakes for selected nutrients for adults25 μg 35 μg900 μg 900 μg3 mg 4 mg150 μg 150 μg8–18 mg 8 mg310–320 mg 400–420 mg1.8 mg 2.3 mg45 μg 45 μg700 mg 700 mg55 μg 55 μg8 mg 11 mg

CHAPTER 2 – DIGESTION AND ABSOPTION

Food has to be broken down

20. Food is one of the basic requirements of all living organisms. The major components of our food are carbohydrates, proteins and fats. Vitamins and minerals are also required in small quantities. Food provides energy and organic materials for growth and repair of tissues. The water we take in, plays an important role in metabolic processes and also prevents dehydration of the body.

21. Bio-macromolecules in food cannot be utilized by our body in their original form. They have to be broken down and converted into simple substances in the digestive system. This process of conversion of complex food substances to simple absorbable forms is called digestion and is carried out by our digestive system by mechanical and biochemical methods. General organization of the human digestive system is shown in the Figure.

DIGESTIVE SYSTEM

22. The human digestive system consists of the alimentary canal and the associated glands.

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Alimentary Canal

23. The alimentary canal begins with an anterior opening – the mouth, and it opens out posteriorly through the anus. The mouth leads to the buccal cavity or oral cavity. The oral cavity has a number of teeth and a muscular tongue.

Mouth and Teeth – the First Digestive Organs

24. The hard chewing surface of the teeth, made up of enamel, helps in the mastication of food. The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum. The upper

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surface of the tongue has small projections called papillae, some of which bear taste buds.

oesophagus

25. The oral cavity leads into a short pharynx which serves as a common passage for food and air. The oesophagus and the trachea (wind pipe) open into the pharynx. A cartilaginous flap called epiglottis prevents the entry of food into the glottis – opening of the wind pipe – during swallowing. The oesophagus is a thin, long tube which extends posteriorly passing through the neck, thorax and diaphragm and leads to a ‘J’ shaped bag like structure called stomach.

Stomach

26. A muscular sphincter (gastro-oesophageal) regulates the opening of oesophagus into the stomach. The stomach, located in the upper left portion of the abdominal cavity, has three major parts – a cardiac portion into which the oesophagus opens, a fundic region and a pyloric portion which opens into the first part of small intestine.

Small Intestine

27. Small intestine is distinguishable into three regions, a ‘C’ shaped duodenum, a long coiled middle portion jejunum and a highly coiled ileum. The opening of the stomach into the duodenum is guarded by the pyloric sphincter. Ileum opens into the large intestine. It consists of caecum, colon and rectum. Caecum is a small blind sac which hosts some symbiotic micro-organisms. A narrow finger-like tubular projection, the vermiform appendix which is a vestigial

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organ, arises from the caecum. The caecum opens into the colon. The colon is divided into three parts – an ascending, a transverse and a descending part. The descending part opens into the rectum which opens out through the anus.

Villi28. The innermost layer lining the lumen of

the alimentary canal is the mucosa. This layer forms irregular folds (rugae) in the stomach and small finger-like foldings called villi in the small intestine. The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance. These modifications increase the surface area enormously. Villi are supplied with a network of capillaries and a large lymph vessel called the lacteal. Mucosal epithelium has goblet cells which secrete mucus that help in lubrication. Mucosa also forms glands in the stomach (gastric glands) and crypts in between the bases of villi in the intestine (crypts of Lieberkuhn). All the four layers show modifications in different parts of the alimentary canal.

Digestive Glands

29. The digestive glands associated with the alimentary canal include the salivary glands, the liver and the pancreas. Saliva is mainly produced by three pairs of salivary glands, the parotids (cheek), the sub-maxillary/sub-mandibular (lower jaw) and the sublinguals (below the tongue). These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity.

Liver30. Liver is the largest gland of the body

weighing about 1.2 to 1.5 kg in an adult human. It is situated in the abdominal cavity, just below the diaphragm and has two lobes. The hepatic lobules are the structural and

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functional units of liver containing hepatic cells arranged in the form of cords. The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder. The duct of gall bladder (cystic duct) along with the hepatic duct from the liver forms the common bile duct. The bile duct and the pancreatic duct open together into the duodenum.

DIGESTION OF FOOD

31. The process of digestion is accomplished by mechanical and chemical processes. The buccal cavity performs two major functions, mastication of food and facilitation of swallowing. The teeth and the tongue with the help of saliva masticate and mix up the food thoroughly. Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus. The bolus is then conveyed into the pharynx and then into the oesophagus by swallowing or deglutition. The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis. The gastro-oesophageal sphincter controls the passage of food into the stomach. The saliva secreted into the oral cavity contains electrolytes and enzymes, salivary amylase and lysozyme. The chemical process of digestion is initiated in the oral cavity by the hydrolytic action of the carbohydrate splitting enzyme, the salivary amylase. About 30 per cent of starch is hydrolysed here by this enzyme (optimum pH 6.8) into a disaccharide – maltose. Lysozyme present in saliva acts as an antibacterial agent that prevents infections.

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Gastric Glands

32. The mucosa of stomach has gastric glands. Gastric glands have three major types of cells namely (i) mucus neck cells which secrete mucus; (ii) peptic or chief cells which secrete the pro-enzyme pepsinogen; and (iii) parietal or oxyntic cells which secrete HCl and intrinsic factor (factor essential for absorption of vitamin B12). The stomach stores the food for 4-5 hours. The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the chyme.

33. Pepsin enzyme in stomach digests protein.

34. Small amounts of lipases are also secreted by gastric glands.

35. Various types of movements are generated by the muscularis layer of the small intestine. These movements help in a thorough mixing up of the food with various secretions in the intestine and thereby facilitate digestion.

36. The bile, pancreatic juice and the intestinal juice are the secretions released into the small intestine. The pancreatic juice contains enzymes that digest fats in diet.

37. The bile released into the duodenum contains bile pigments (bilirubin and bili-verdin), bile salts, cholesterol and phospholipids but no enzymes. Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates lipases.

38. The breakdown of bio-macromolecules mentioned above occurs in the duodenum region of the small intestine. The simple substances thus formed are absorbed in the

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jejunum and ileum regions of the small intestine. The undigested and unabsorbed substances are passed on to the large intestine. No significant digestive activity occurs in the large intestine.

Functions of Large Intestine

39. The functions of large intestine are: (i) absorption of some water, minerals and certain drugs; (ii) secretion of mucus which helps in adhering the waste (undigested) particles together and lubricating it for an easy passage.

40. The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine through ileo-caecal valve, which prevents the back flow of the faecal matter.

ABSORPTION OF DIGESTED PRODUCTS

What is Absorption

41. Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph. It is carried out by passive, active or facilitated transport mechanisms. Small amounts of monosaccharides like glucose, amino acids and some electrolytes like chloride ions are generally absorbed by simple diffusion.

42. The passage of these substances into the blood depends upon the concentration gradients. However, some substances like glucose and amino acids are absorbed with the help of carrier proteins. This mechanism is called the facilitated transport. Transport of water depends upon the osmotic gradient.

43. Active transport occurs against the concentration gradient and hence requires energy. Various nutrients like amino acids, monosaccharides like glucose, electrolytes like Na+ are absorbed into the blood by this mechanism.

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44. Fatty acids and glycerol being insoluble, cannot be absorbed into the blood. They are first incorporated into small droplets called micelles which move into the intestinal mucosa. They are re-formed into very small protein coated fat globules called the chylomicrons which are transported into the lymph vessels (lacteals) in the villi. These lymph vessels ultimately release the absorbed substances into the blood stream. Absorption of substances takes place in different parts of the alimentary canal, like mouth, stomach, small intestine and large intestine. However, maximum absorption occurs in the small intestine.

45. The absorbed substances finally reach the tissues which utilise them for their activities. This process is called assimilation. The digestive wastes, solidified into coherent faeces in the rectum initiate a neural reflex causing an urge or desire for its removal. The egestion of faeces to the outside through the anal opening (defaecation) is a voluntary process and is carried out by a mass peristaltic movement.

DISORDERS OF DIGESTIVE SYSTEM

46. The inflammation of the intestinal tract is the most common ailment due to bacterial or viral infections. The infections are also caused by the parasites of the intestine like tapeworm, roundworm, threadworm, hookworm, pin worm, etc.

47. Jaundice: The liver is affected, skin and eyes turn yellow due to the deposit of bile pigments.

48. Vomiting: It is the ejection of stomach contents through the mouth. This reflex action is controlled by the vomit centre in the medulla. A feeling of nausea precedes vomiting.

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49. Diarrhoea: The abnormal frequency of bowel movement and increased liquidity of the faecal discharge is known as diarrhoea. It reduces the absorption of food.

50. Constipation: In constipation, the faeces are retained within the rectum as the bowel movements occur irregularly.

51. Indigestion: In this condition, the food is not properly digested leading to a feeling of fullness. The causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning, over eating, and spicy food.

CHAPTER 3 – ENERGY

UTILIZATION OF FOOD BY THE BODY

1. The human body can be thought of as an engine that releases the energy present in the foods that it digests. This energy is utilized partly for the mechanical work performed by the muscles and in the secretory processes and partly for the work necessary to maintain the body’s structure and functions. The performance of work is associated with the production of heat; heat loss is controlled so as to keep body temperature within a narrow range. Unlike other engines, however, the human body is

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continually breaking down (catabolizing) and building up (anabolizing) its component parts. Foods supply nutrients essential to the manufacture of the new material and provide energy needed for the chemical reactions involved.

2. Carbohydrate, fat, and protein are, to a large extent, interchangeable as sources of energy. Typically, the energy provided by food is measured in kilocalories, or Calories. One kilocalorie is equal to 1,000 gram-calories (or small calories), a measure of heat energy. However, in common parlance, kilocalories are referred to as “calories.” In other words, a 2,000-calorie diet actually has 2,000 kilocalories of potential energy. One kilocalorie is the amount of heat energy required to raise one kilogram of water from 14.5 to 15.5 °C at one atmosphere of pressure. Another unit of energy widely used is the joule, which measures energy in terms of mechanical work. One joule is the energy expended when one kilogram is moved a distance of one metre by a force of one newton. The relatively higher levels of energy in human nutrition are more likely to be measured in kilojoules (1 kilojoule = 103

joules) or megajoules (1 megajoule = 106

joules). One kilocalorie is equivalent to 4.184 kilojoules.

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Measurement of Energy

3. The energy present in food can be determined directly by measuring the output of heat when the food is burned (oxidized) in a bomb calorimeter. However, the human body is not as efficient as a calorimeter, and some potential energy is lost during digestion and metabolism. Corrected physiological values for the heats of combustion of the three energy-yielding nutrients, rounded to whole numbers, are as follows: carbohydrate, 4 kilocalories (17 kilojoules) per gram; protein, 4 kilocalories (17 kilojoules) per gram; and fat, 9 kilocalories (38 kilojoules) per gram. Beverage alcohol (ethyl alcohol) also yields energy—7 kilocalories (29 kilojoules) per gram—although it is not essential in the diet. Vitamins, minerals, water, and other food constituents have no energy value, although many of them participate in energy-releasing processes in the body.

4. The energy provided by a well-digested food can be estimated if the gram amounts of energy-yielding substances (non-fibre carbohydrate, fat, protein, and alcohol) in that food are known. For example, a slice of white bread containing 12 grams of carbohydrate, 2 grams of protein, and 1 gram of fat supplies 67 kilocalories (280 kilojoules) of energy. Food composition tables (see table) and food labels provide useful data for evaluating energy and nutrient intake of an individual diet. Most foods provide a mixture of energy-supplying nutrients, along with vitamins, minerals, water, and other substances. Two notable exceptions are table sugar and vegetable oil, which are virtually pure carbohydrate (sucrose) and fat, respectively.

5. Throughout most of the world, protein supplies between 8 and 16 percent of the

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energy in the diet, although there are wide variations in the proportions of fat and carbohydrate in different populations. In more prosperous communities about 12 to 15 percent of energy is typically derived from protein, 30 to 40 percent from fat, and 50 to 60 percent from carbohydrate. On the other hand, in many poorer agricultural societies, where cereals comprise the bulk of the diet, carbohydrate provides an even larger percentage of energy, with protein and fat providing less. The human body is remarkably adaptable and can survive, and even thrive, on widely divergent diets. However, different dietary patterns are associated with particular health consequences.

TABLE 1: Energy value and nutrient content of some common foods

Foods Name energy(kcal)

carbohy-drate

(g)protein

(g)fat(g)

water

(g)

whole wheat bread (1 slice, 28 g) 69 12.9 2.7 1.2 10.6

white bread (1 slice, 25 g) 67 12.4 2.0 0.9 9.2

242 53.4 4.4 0.4 127.5

lowfat milk (2%) (8 fl oz, 244 g) 121 11.7 8.1 4.7 17.7

36 0 0 4.1 0.8

cheddar cheese (1 oz, 28 g) 114 0.4 7.1 9.4 10.4

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TABLE 1: Energy value and nutrient content of some common foods

272 0 24.7 18.5 55.7

168 0 24.8 7.0 50.9

Potato (1 medium, 135 g) 117 27.2 2.5 0.1 103.9

Green Peas (1/2 cup, 80 g) 62 11.4 4.1 0.2 63.6

Cabbage (1/2 cup shredded, 35 g) 9 2.1 0.5 0.1 32.0

Orange (1 fruit, 131 g) 60 15.2 1.3 0.1 113.7

Apple (1 medium, 138 g) 81 21.0 0.3 0.5 115.8

15 4.0 0 0 0

Energy Disorders

6. Under the contemporary conditions of healthcare, diets remain rather important, nevertheless, certain number of people lose the right sense of nutrition in their strive to keep fit. Additionally, we have lost the comprehension that diet is closely related to mental health. In due respect, women are more prone to eating disorders known as Anorexia nervosa, Binge eating, Bulimia, compared to men. Usually, this tendency starts in the teenage period, and is often accompanied by depression and anxiety disorders. Consequently, these cause heart and kidney problems, and even lead to mortality risk.

Obesity

7. The main causes of obesity are: the decreased level of nutrients intake, and sedentary lifestyle. In spite of all available information about nutrition in schools, hospitals, Internet, it is apparent that overeating is a problem. For example, the intake of fast food meals tripled between 1977 and 1995, and calorie level magnified four times during the same period. Nevertheless, it is insufficient explanation of phenomenal rise in the obesity levels in the well-developed countries. Overall, obesity is a

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significant health and social problem, which has reached pandemic levels. In accordance with numerous reports, energy intakes from food in England have decreased over the last 30 years, while the prevalence of obesity has tripled over 20 years, and continue to increase at an alarming rate.

8. A well-balanced diet provides energy and nourishment necessary to lead normal life, and therefore to keep fit. Hence, it is important to provide our body with all the necessary resources and fuels to stay in good health. An unhealthy diet and physical inactivity increase our chances of getting heart disease, cancer, stroke, type 2 diabetes, high blood pressure, breathing problems, arthritis, gallbladder disease, and osteoarthritis.

9. Metabolism, the sum of the chemical reactions that take place within each cell of a living organism and that provide energy for vital processes and for synthesizing new organic material.

10. Living organisms are unique in that they can extract energy from their environments and use it to carry out activities such as movement, growth and development, and reproduction. But how do

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living organisms—or, their cells—extract energy from their environments, and how do cells use this energy to synthesize and assemble the components from which the cells are made?

11. The answers to these questions lie in the enzyme-mediated chemical reactions that take place in living matter (metabolism). Hundreds of coordinated, multistep reactions, fueled by energy obtained from nutrients and/or solar energy, ultimately convert readily available materials into the molecules required for growth and maintenance.

12. At the cellular level of organization, the main chemical processes of all living matter are similar, if not identical. This is true for animals, plants, fungi, or bacteria; where variations occur (such as, for example, in the secretion of antibodies by some molds), the variant processes are but variations on common themes. Thus, all living matter is made up of large molecules called proteins, which provide support and coordinated movement, as well as storage and transport of small molecules, and, as catalysts, enable chemical reactions to take place rapidly and specifically under mild temperature, relatively low concentration, and neutral conditions (i.e., neither acidic nor basic). Proteins are assembled from some 20 amino acids, and, just as the 26 letters of the alphabet can be assembled in specific ways to form words of various lengths and meanings, so may tens or even hundreds of the 20 amino-acid “letters” be joined to form specific proteins. Moreover, those portions of protein molecules involved in performing similar functions in different organisms often comprise the same sequences of amino acids.

13. Energy is needed not only when a person is physically active but even when the body is lying motionless. Depending on an

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individual’s level of physical activity, between 50 and 80 percent of the energy expended each day is devoted to basic metabolic processes (basal metabolism), which enable the body to stay warm, breathe, pump blood, and conduct numerous physiological and biosynthetic activities, including synthesis of new tissue in growing children and in pregnant and lactating women. Digestion and subsequent processing of food by the body also uses energy and produces heat.

14. This phenomenon, known as the thermic effect of food (or diet-induced thermogenesis), accounts for about 10 percent of daily energy expenditure, varying somewhat with the composition of the diet and prior dietary practices. Adaptive thermogenesis, another small but important component of energy expenditure, reflects alterations in metabolism due to changes in ambient temperature, hormone production, emotional stress, or other factors. Finally, the most variable component in energy expenditure is physical activity, which includes exercise and other voluntary activities as well as involuntary activities such as fidgeting, shivering, and maintaining posture. Physical activity accounts for 20 to 40 percent of the total energy expenditure, even less in a very sedentary person and more in someone who is extremely active.

15. Basal or resting energy expenditure is correlated primarily with lean body mass (fat-free mass and essential fat, excluding storage fat), which is the metabolically active tissue in the body. At rest, organs such as the liver, brain, heart, and kidney have the highest metabolic activity and, therefore, the highest need for energy, while muscle and bone require less energy, and body fat even less. Besides body composition, other factors affecting basal metabolism include age, sex, body temperature, and thyroid hormone levels.

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16. The basal metabolic rate (BMR), a precisely defined measure of the energy expenditure necessary to support life, is determined under controlled and standardized conditions—shortly after awakening in the morning, at least 12 hours after the last meal, and with a comfortable room temperature. Because of practical considerations, the BMR is rarely measured; the resting energy expenditure (REE) is determined under less stringent conditions, with the individual resting comfortably about 2 to 4 hours after a meal. In practice, the BMR and REE differ by no more than 10 percent—the REE is usually slightly higher—and the terms are used interchangeably.

17. Energy expenditure can be assessed by direct calorimetry, or measurement of heat dissipated from the body, which employs apparatuses such as water-cooled garments or insulated chambers large enough to accommodate a person. However, energy expenditure is usually measured by the less cumbersome techniques of indirect calorimetry, in which heat produced by the body is calculated from measurements of oxygen inhaled, carbon dioxide exhaled, and urinary nitrogen excreted. The BMR (in kilocalories per day) can be roughly estimated using the following formula: BMR = 70 × (body weight in kilograms)3/4.

18. The energy costs of various activities have been measured (see table). While resting may require as little as 1 kilocalorie per minute, strenuous work may demand 10 times that much. Mental activity, though it may seem taxing, has no appreciable effect on energy requirement. A 70-kg (154-pound) man, whose REE over the course of a day might be 1,750 kilocalories, could expend a total of 2,400 kilocalories on a very sedentary day and up to 4,000 kilocalories on a very active day. A 55-kg (121-pound) woman,

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whose daily resting energy expenditure might be 1,350 kilocalories, could use from 1,850 to more than 3,000 total kilocalories, depending on level of activity.

TABLE 2: Approximate energy expenditure for activity levels

ctivity Categoryenergy as multiple of

Resting Energy Expenditure (REE)

Kilocalories/minute

Resting (sleeping, reclining) REE × 1.0 1-1.2very light (driving, typing, cooking) REE × 1.5 up to 2.5Light (walking on a level surface at 4 to 5 km/hr[2.5 to 3 mph], golf, table tennis)

REE × 2.5 2.5-4.9

Moderate (walking 5.5 to 6.5 km/hr [3.5 to 4 mph], carryinga load, cycling, tennis, skiing, dancing)

REE × 5.0 5.0-7.4

Heavy (walking uphill with a load, basketball, climbing, football, soccer) REE × 7.0 7.5-12.0

19. The law of conservation of energy applies: If one takes in more energy than is expended, over time one will gain weight; insufficient energy intake results in weight loss, as the body taps its energy stores to provide for immediate needs. Excess food energy is stored in small amounts as glycogen, a short-term storage form of carbohydrate in muscle and liver, and as fat, the body’s main energy reserve found in adipose tissue. Adipose tissue is mostly fat (about 87 percent), but it also contains some protein and water. In order to lose 454 grams (one pound) of adipose tissue, an energy deficit of about 3,500 kilocalories (14.6 megajoules) is required.

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CHAPTER 4 - WATER

1. Although often overlooked as a nutrient, water (H2O) is actually the most critical nutrient of all. Humans can survive weeks without food but only a matter of days without water.

2. Water provides the medium in which nutrients and waste products are transported throughout the body and the myriad biochemical reactions of metabolism occur.

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Water allows for temperature regulation, the maintenance of blood pressure and blood volume, the structure of large molecules, and the rigidity of body tissues. It also acts as a solvent, a lubricant (as in joints), and a protective cushion (as inside the eyes and in spinal fluid and amniotic fluid). The flow of water in and out of cells is precisely controlled by shifting electrolyte concentrations on either side of the cell membrane. Potassium, magnesium, phosphate, and sulfate are primarily intracellular electrolytes; sodium and chloride are major extracellular ones.

3. Water makes up about 50 to 70 percent of body weight, approximately 60 percent in healthy adults and an even higher percentage in children. Because lean tissue is about three-quarters water, and fatty tissue is only about one-fifth water, body composition—the amount of fat in particular—determines the percentage of body water. In general, men have more lean tissue than women, and therefore a higher percentage of their body weight is water.

4. Water is consumed not only as water itself and as a constituent of other beverages but also as a major component of many foods, particularly fruits and vegetables, which may contain from 85 to 95 percent water. Water also is manufactured in the body as an end product of metabolism. About 2.5 litres (about 2.6 quarts) of water are turned over daily, with water excretion (primarily in urine, water vapour from lungs, sweat loss from skin, and feces) balancing intake from all sources. Because water requirements vary with climate, level of activity, dietary composition, and other factors, there is no one recommendation for daily water intake. However, adults typically need at least 2 litres (8 cups) of water a day, from all sources. Thirst is not reliable as a register for dehydration, which typically occurs before

30

the body is prompted to replace fluid. Therefore, water intake is advised throughout the day, especially with increased sweat loss in hot climates or during vigorous physical activity, during illness, or in a dehydrating situation such as an airplane flight.

5. The body contains a large variety of ions, or electrolytes, which perform a variety of functions. Some ions assist in the transmission of electrical impulses along cell membranes in neurons and muscles. Other ions help to stabilize protein structures in enzymes. Still others aid in releasing hormones from endocrine glands. All of the ions in plasma contribute to the osmotic balance that controls the movement of water between cells and their environment.

6. Electrolytes in living systems include sodium, potassium, chloride, bicarbonate, calcium, phosphate, magnesium, copper, zinc, iron, manganese, molybdenum, copper, and chromium. In terms of body functioning, six electrolytes are most important: sodium, potassium, chloride, bicarbonate, calcium, and phosphate.

Roles of Electrolytes

7. These six ions aid in nerve excitability, endocrine secretion, membrane permeability, buffering body fluids, and controlling the movement of fluids between compartments. These ions enter the body through the digestive tract. More than 90 percent of the calcium and phosphate that enters the body is incorporated into bones and teeth, with bone serving as a mineral reserve for these ions. In the event that calcium and phosphate are needed for other functions, bone tissue can be broken down to supply the blood and other tissues with these minerals. Phosphate is a normal constituent of nucleic acids; hence, blood levels of phosphate will increase whenever nucleic acids are broken down.

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8. Excretion of ions occurs mainly through the kidneys, with lesser amounts lost in sweat and in feces. Excessive sweating may cause a significant loss, especially of sodium and chloride. Severe vomiting or diarrhea will cause a loss of chloride and bicarbonate ions. Adjustments in respiratory and renal functions allow the body to regulate the levels of these ions in the ECF.

Electrolyte and Ion Reference Values

NameChemical symbol Plasma CSF Urine

Sodium Na+ 136.00–146.00 (mM)

138.00–150.00 (mM) 40.00–220.00 (mM)

Potassiu K+ 3.50–5.00 (mM) 0.35–3.5 (mM) 25.00–125.00 (mM)

Chloride Cl– 98.00–107.00 (mM)

118.00–132.00 (mM)

110.00–250.00 (mM)

Bicarbonate HCO3

– 22.00–29.00 (mM) —— ——

Calcium Ca++ 2.15–2.55 (mmol/day) —— Up to 7.49

(mmol/day)

Phosphat HPO42−HPO42−

0.81–1.45 (mmol/day) —— 12.90–42.00

(mmol/day)

Sodium

9. Sodium is the major cation of the extracellular fluid. It is responsible for one-half of the osmotic pressure gradient that exists between the interior of cells and their surrounding environment. People eating a typical Western diet, which is very high in NaCl, routinely take in 130 to 160 mmol/day of sodium, but humans require only 1 to 2 mmol/day. This excess sodium appears to be a major factor in hypertension (high blood

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pressure) in some people. Excretion of sodium is accomplished primarily by the kidneys. Sodium is freely filtered through the glomerular capillaries of the kidneys, and although much of the filtered sodium is reabsorbed in the proximal convoluted tubule, some remains in the filtrate and urine, and is normally excreted.

Potassium

10. Potassium is the major intracellular cation. It helps establish the resting membrane potential in neurons and muscle fibers after membrane depolarization and action potentials. In contrast to sodium, potassium has very little effect on osmotic pressure. The low levels of potassium in blood and CSF are due to the sodium-potassium pumps in cell membranes, which maintain the normal potassium concentration gradients between the ICF and ECF. The recommendation for daily intake/consumption of potassium is 4700 mg. Potassium is excreted, both actively and passively, through the renal tubules, especially the distal convoluted tubule and collecting ducts. Potassium participates in the exchange with sodium in the renal tubules under the influence of aldosterone, which also relies on basolateral sodium-potassium pumps.

Chloride

11. Chloride is the predominant extracellular anion. Chloride is a major contributor to the osmotic pressure gradient between the ICF and ECF, and plays an important role in maintaining proper hydration. Chloride functions to balance cations in the ECF, maintaining the electrical neutrality of this fluid. The paths of secretion and reabsorption of chloride ions in the renal system follow the paths of sodium ions.

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Bicarbonate

12. Bicarbonate is the second most abundant anion in the blood. Its principal function is to maintain your body’s acid-base balance by being part of buffer systems. This role will be discussed in a different section.

13. Bicarbonate ions result from a chemical reaction that starts with carbon dioxide (CO2) and water, two molecules that are produced at the end of aerobic metabolism. Only a small amount of CO2 can be dissolved in body fluids. Thus, over 90 percent of the CO2 is converted into bicarbonate ions, HCO3–, through the following reactions:

CO2 + H2O ↔ H2CO3 ↔ H2CO3– + H+

14. The bidirectional arrows indicate that the reactions can go in either direction, depending on the concentrations of the reactants and products. Carbon dioxide is produced in large amounts in tissues that have a high metabolic rate. Carbon dioxide is converted into bicarbonate in the cytoplasm of red blood cells through the action of an enzyme called carbonic anhydrase. Bicarbonate is transported in the blood. Once in the lungs, the reactions reverse direction, and CO2 is regenerated from bicarbonate to be exhaled as metabolic waste.

Calcium

15. About two pounds of calcium in body in bone, which provides hardness to the bone and serves as a mineral reserve for calcium and its salts for the rest of the tissues. Teeth also have a high concentration of calcium within them. A little more than one-half of blood calcium is bound to proteins, leaving the rest in its ionized form. Calcium ions, Ca2+, are necessary for muscle contraction, enzyme activity, and blood coagulation. In addition, calcium helps to stabilize cell membranes and is essential for the release of

34

neurotransmitters from neurons and of hormones from endocrine glands.

16. Calcium is absorbed through the intestines under the influence of activated vitamin D. A deficiency of vitamin D leads to a decrease in absorbed calcium and, eventually, a depletion of calcium stores from the skeletal system, potentially leading to rickets in children and osteomalacia in adults, contributing to osteoporosis.

Phosphate

17. Phosphate is present in the body in three ionic forms: H2PO4−, HPO42, and PO43−. The most common form is HPO42−HPO42−. Bone and teeth bind up 85 percent of the body’s phosphate as part of calcium-phosphate salts. Phosphate is found in phospholipids, such as those that make up the cell membrane, and in ATP, nucleotides, and buffers.

CHAPTER 4 - CARBOHYDRATES

18. Carbohydrates, which are composed of carbon, hydrogen, and oxygen, are the major supplier of energy to the body, providing 4 kilocalories per gram. In most carbohydrates, the elements hydrogen and oxygen are present in the same 2:1 ratio as in water, thus “carbo” (for carbon) and “hydrate” (for water).

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Glucose

19. The simple carbohydrate glucose is the principal fuel used by the brain and nervous system and by red blood cells. Muscle and other body cells can also use glucose for energy, although fat is often used for this purpose. Because a steady supply of glucose is so critical to cells, blood glucose is maintained within a relatively narrow range through the action of various hormones, mainly insulin, which directs the flow of glucose into cells, and glucagon and epinephrine, which retrieve glucose from storage.

20. The body stores a small amount of glucose as glycogen, a complex branched form of carbohydrate, in liver and muscle tissue, and this can be broken down to glucose and used as an energy source during short periods (a few hours) of fasting or during times of intense physical activity or stress. If blood glucose falls below normal (hypoglycemia), weakness and dizziness may result. Elevated blood glucose (hyperglycemia), as can occur in diabetes, is also dangerous and cannot be left untreated.

21. Glucose can be made in the body from most types of carbohydrate and from protein, although protein is usually an expensive source of energy. Some minimal amount of carbohydrate is required in the diet—at least 50 to 100 grams a day. This not only spares protein but also ensures that fats are completely metabolized and prevents a condition known as ketosis, the accumulation of products of fat breakdown, called ketones, in the body. Although there are great variations in the quantity and type of carbohydrates eaten throughout the world, most diets contain more than enough.

Other sugars and starch

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22. The simplest carbohydrates are sugars, which give many foods their sweet taste but at the same time provide food for bacteria in the mouth, thus contributing to dental decay. Sugars in the diet are monosaccharides, which contain one sugar or saccharide unit, and disaccharides, which contain two saccharide units linked together. Monosaccharides of nutritional importance are glucose, fructose, and galactose; disaccharides include sucrose (table sugar), lactose (milk sugar), and maltose. A slightly more complex type of carbohydrate is the oligosaccharide (e.g., raffinose and stachyose), which contains three to 10 saccharide units; these compounds, which are found in beans and other legumes and cannot be digested well by humans, account for the gas-producing effects of these foods. Larger and more complex storage forms of carbohydrate are the polysaccharides, which consist of long chains of glucose units. Starch, the most important polysaccharide in the human diet—found in grains, legumes, potatoes, and other vegetables—is made up of mainly straight glucose chains (amylose) or mainly branching chains (amylopectin). Finally, nondigestible polysaccharides known as dietary fibre are found in plant foods such as grains, fruits, vegetables, legumes, seeds, and nuts.

23. In order to be utilized by the body, all complex carbohydrates must be broken down into simple sugars, which, in turn, must be broken down into monosaccharides—a feat, accomplished by enzymes, that starts in the mouth and ends in the small intestine, where most absorption takes place. Each dissacharide is split into single units by a specific enzyme; for example, the enzyme lactase breaks down lactose into its constituent monosaccharides, glucose and galactose. In much of the world’s population, lactase activity declines during childhood and

37

adolescence, which leads to an inability to digest lactose adequately. This inherited trait, called lactose intolerance, results in gastrointestinal discomfort and diarrhea if too much lactose is consumed. Those who have retained the ability to digest dairy products efficiently in adulthood are primarily of northern European ancestry.

Dietary fibre

24. Dietary fibre, the structural parts of plants, cannot be digested by the human intestine because the necessary enzymes are lacking. Even though these nondigestible compounds pass through the gut unchanged (except for a small percentage that is fermented by bacteria in the large intestine), they nevertheless contribute to good health. Insoluble fibre does not dissolve in water and provides bulk, or roughage, that helps with bowel function (regularity) and accelerates the exit from the body of potentially carcinogenic or otherwise harmful substances in food. Types of insoluble fibre are cellulose, most hemicelluloses, and lignin (a phenolic polymer, not a carbohydrate).

25. Major food sources of insoluble fibre are whole grain breads and cereals, wheat bran, and vegetables. Soluble fibre, which dissolves or swells in water, slows down the transit time of food through the gut (an undesirable effect) but also helps lower blood cholesterol levels (a desirable effect).

26. Types of soluble fibre are gums, pectins, some hemicelluloses, and mucilages; fruits (especially citrus fruits and apples), oats, barley, and legumes are major food sources. Both soluble and insoluble fibre help delay glucose absorption, thus ensuring a slower and more even supply of blood glucose. Dietary fibre is thought to provide important protection against some gastrointestinal diseases and to reduce the risk of other chronic diseases as well. (See nutritional disease.)

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CHAPTER 4 - LIPIDS

27. Lipids also contain carbon, hydrogen, and oxygen but in a different configuration, having considerably fewer oxygen atoms than are found in carbohydrates. Lipids are soluble in organic solvents (such as acetone or ether) and insoluble in water, a property that is readily seen when an oil-and-vinegar salad dressing separates quickly upon standing.

28. The lipids of nutritional importance are triglycerides (fats and oils), phospholipids (e.g., lecithin), and sterols (e.g., cholesterol). Lipids in the diet transport the four fat-soluble vitamins (vitamins A, D, E, and K) and assist in their absorption in the small intestine. They also carry with them substances that impart sensory appeal and palatability to food and provide satiety value, the feeling of being full and satisfied after eating a meal.

29. Fats in the diet are a more concentrated form of energy than carbohydrates and have an energy yield of 9 kilocalories per gram. Adipose (fatty) tissue in the fat depots of the body serves as an energy reserve as well as helping to insulate the body and cushion the internal organs.

Triglycerides

30. The major lipids in food and stored in the body as fat are the triglycerides, which consist of three fatty acids attached to a backbone of glycerol (an alcohol). Fatty acids are essentially hydrocarbon chains with a carboxylic acid group (COOH) at one end, the

39

alpha (α) end, and a methyl group (CH3) at the other, omega (ω), end. They are classified as saturated or unsaturated according to their chemical structure. A point of unsaturation indicates a double bond between two carbon atoms, rather than the full complement of hydrogen atoms that is present in saturated fatty acids. A monounsaturated fatty acid has one point of unsaturation, while a polyunsaturated fatty acid has two or more.

31. The common fatty acids in foods are listed in the table. Fatty acids found in the human diet and in body tissues range from a chain length of 4 carbons to 22 or more, each chain having an even number of carbon atoms. Of particular importance for humans are the 18-carbon polyunsaturated fatty acids alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid); these are known as essential fatty acids because they are required in small amounts in the diet.

32. The omega designations (also referred to as n-3 and n-6) indicate the location of the first double bond from the methyl end of the fatty acid. Other fatty acids can be synthesized in the body and are therefore not essential in the diet. About a tablespoon daily of an ordinary vegetable oil such as safflower or corn oil or a varied diet that includes grains, nuts, seeds, and vegetables can fulfill the essential fatty acid requirement.

33. Essential fatty acids are needed for the formation of cell membranes and the synthesis of hormone-like compounds called eicosanoids (e.g., prostaglandins, thromboxanes, and leukotrienes), which are important regulators of blood pressure, blood clotting, and the immune response. The consumption of fish once or twice a week provides an additional source of omega-3 fatty acids that appears to be healthful.

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TABLE 4: Common Fatty Acids In Foods*The number to the left of the colon indicates the number of carbon atoms; the number to the right of the colon indicates the number of double bonds.

shorthand* typical source

4:0 butterfat6:0 butterfat8:0 coconut oil10:0 coconut oil12:0 coconut oil, palm kernel oil14:0 butterfat, coconut oil16:0 palm oil, animal fat18:0 cocoa butter, animal fat20:0 peanut oil22:0 peanut oil

10:1 butterfat12:1 butterfat14:1 butterfat16:1 some fish oils, beef fat18:1 olive oil, canola oil20:1 some fish oils22:1 canola oil

18:2 most vegetable oils, especially safflower, corn, soybean, cottonseed

18:3 soybean oil, canola oil, walnuts, wheat germ oil, flaxseed oil

20:4 lard, meatseicosapentaenoic (EPA; omega- 20:5 some fish oils, shellfish

22:6 some fish oils, shellfish

34. A fat consisting largely of saturated fatty acids, especially long-chain fatty acids, tends to be solid at room temperature; if unsaturated fatty acids predominate, the fat is liquid at room temperature.

35. Fats and oils usually contain mixtures of fatty acids, although the type of fatty acid in greatest concentration typically gives the food its characteristics. Butter and other animal fats are primarily saturated; olive and canola oils, monounsaturated; and fish, corn, safflower, soybean, and sunflower oils, polyunsaturated.

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36. Although plant oils tend to be largely unsaturated, there are notable exceptions, such as coconut fat, which is highly saturated but nevertheless semiliquid at room temperature because its fatty acids are of medium chain length (8 to 14 carbons long).

37. Saturated fats tend to be more stable than unsaturated ones. The food industry takes advantage of this property during hydrogenation, in which hydrogen molecules are added to a point of unsaturation, thereby making the fatty acid more stable and resistant to rancidity (oxidation) as well as more solid and spreadable (as in margarine). However, a result of the hydrogenation process is a change in the shape of some unsaturated fatty acids from a configuration known as cis to that known as trans. Trans-fatty acids, which behave more like saturated fatty acids, may also have undesirable health consequences.

Phospholipids

38. A phospholipid is similar to a triglyceride except that it contains a phosphate group and a nitrogen-containing compound such as choline instead of one of the fatty acids. In food, phospholipids are natural emulsifiers, allowing fat and water to mix, and they are used as food additives for this purpose. In the body, phospholipids allow fats to be suspended in fluids such as blood, and they enable lipids to move across cell membranes from one watery compartment to another. The phospholipid lecithin is plentiful in foods such as egg yolks, liver, wheat germ, and peanuts. However, the liver is able to synthesize all the lecithin the body needs if sufficient choline is present in the diet.

Sterols

39. Sterols are unique among lipids in that they have a multiple-ring structure. The well-

42

known sterol cholesterol is found only in foods of animal origin—meat, egg yolk, fish, poultry, and dairy products. Organ meats (e.g., liver, kidney) and egg yolks have the most cholesterol, while muscle meats and cheeses have less. There are a number of sterols in shellfish but not as much cholesterol as was once thought.

40. Cholesterol is essential to the structure of cell membranes and is also used to make other important sterols in the body, among them the sex hormones, adrenal hormones, bile acids, and vitamin D. However, cholesterol can be synthesized in the liver, so there is no need to consume it in the diet.

41. Cholesterol-containing deposits may build up in the walls of arteries, leading to a condition known as atherosclerosis, which contributes to myocardial infarction (heart attack) and stroke. Furthermore, because elevated levels of blood cholesterol, especially the form known as low-density lipoprotein (LDL) cholesterol, have been associated with an increased risk of cardiovascular disease, a limited intake of saturated fat—particularly medium-chain saturated fatty acids, which act to raise LDL cholesterol levels—is advised.

42. Trans-fatty acids also raise LDL cholesterol, while monounsaturated and polyunsaturated (cis) fats tend to lower LDL cholesterol levels. Because of the body’s feedback mechanisms, dietary cholesterol has only a minor influence on blood cholesterol in most people; however, since some individuals respond strongly to cholesterol in the diet, a restricted intake is often advised, especially for those at risk of heart disease. The complex relationships between various dietary lipids and blood cholesterol levels, as well as the possible health consequences of different dietary lipid patterns, are discussed in the article

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nutritional disease.

CHAPTER 5 - PROTEINS

44

43. Proteins, like carbohydrates and fats, contain carbon, hydrogen, and oxygen, but they also contain nitrogen, a component of the amino chemical group (NH2), and in some cases sulfur. Proteins serve as the basic structural material of the body as well as being biochemical catalysts and regulators of genes. Aside from water, protein constitutes the major part of muscles, bones, internal organs, and the skin, nails, and hair. Protein is also an important part of cell membranes and blood (e.g., hemoglobin). Enzymes, which catalyze chemical reactions in the body, are also protein, as are antibodies, collagen in connective tissue, and many hormones, such as insulin.

44. Tissues throughout the body require ongoing repair and replacement, and thus the body’s protein is turning over constantly, being broken down and then resynthesized as needed.

45. Tissue proteins are in a dynamic equilibrium with proteins in the blood, with input from proteins in the diet and losses through urine, feces, and skin. In a healthy adult, adjustments are made so that the amount of protein lost is in balance with the amount of protein ingested. However, during periods of rapid growth, pregnancy and lactation, or recuperation after illness or depletion, the body is in positive nitrogen balance, as more protein is being retained than excreted. The opposite is true during illness or wasting, when there is negative nitrogen balance as more tissue is being broken down than synthesized.

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General scheme of protein and amino acid metabolism.

46. The proteins in food—such as albumin in egg white, casein in dairy products, and gluten in wheat—are broken down during digestion into constituent amino acids, which, when absorbed, contribute to the body’s metabolic pool. Amino acids are then joined via peptide linkages to assemble specific proteins, as directed by the genetic material and in response to the body’s needs at the time.

47. Each gene makes one or more proteins, each with a unique sequence of amino acids and precise three-dimensional configuration. Amino acids are also required for the synthesis of other important nonprotein compounds, such as peptide hormones, some neurotransmitters, and creatine.

48. Food contains approximately 20 common amino acids, 9 of which are considered essential, or indispensable, for humans; i.e., they cannot be synthesized by the body or cannot be synthesized in sufficient quantities and therefore must be taken in the diet. The essential amino acids for humans are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine,

46

tryptophan, and valine. Conditionally indispensable amino acids include arginine, cysteine, and tyrosine, which may need to be provided under special circumstances, such as in premature infants or in people with liver disease, because of impaired conversion from precursors.

49. The relative proportions of different amino acids vary from food to food (Table 5). Foods of animal origin—meat, fish, eggs, and dairy products—are sources of good quality, or complete, protein; i.e., their essential amino acid patterns are similar to human needs for protein. (Gelatin, which lacks the amino acid tryptophan, is an exception.) Individual foods of plant origin, with the exception of soybeans, are lower quality, or incomplete, protein sources. Lysine, methionine, and tryptophan are the primary limiting amino acids; i.e., they are in smallest supply and therefore limit the amount of protein that can be synthesized. However, a varied vegetarian diet can readily fulfill human protein requirements if the protein-containing foods are balanced such that their essential amino acids complement each other. For example, legumes such as beans are high in lysine and low in methionine, while grains have complementary strengths and weaknesses. Thus, if beans and rice are eaten over the course of a day, their joint amino acid patterns will supplement each other and provide a higher quality protein than would either food alone. Traditional food patterns in native cultures have made good use of protein complementarity. However, careful balancing of plant proteins is necessary only for those whose protein intake is marginal or inadequate. In affluent populations, where protein intake is greatly in excess of needs, obtaining sufficient good quality protein is usually only a concern for young children who are not provided with animal proteins.

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TABLE 5: Essential amino acids in some common foods**In milligrams. **Underscored number indicates limiting amino acid

Egg(50 g)

Beef(3.5 oz)

Milk(8 fl

oz)

soybeans(1/2 cup)

Cornmeal(1

cup)

whole

wheat,(1

cup)

tryptophan 77 277 115 494 70** 254

threonine 302 1,080 366 1,478 372 474

isoleucine 343 1,111 490 1,651 355 610

leucine 538 1,954 795 2,772 1,215 1,11

1lysine 452 2,057 64

4 2,265 278 454

methionine 196 633 205 459 207 254

phenylalanine 334 965 393 1,777 487 775

valine 384 1,202 544 1,699 501 742

histidine 149 846 220 918 303 380

Protein intake

50. The World Health Organization recommends a daily intake of 0.75 gram of good quality protein per kilogram of body weight for adults of both sexes. Thus, a 70-kg (154-pound) man would need 52.5 grams of protein, and a 55-kg (121-pound) woman would need about 41 grams of protein. This recommendation, based on nitrogen balance studies, assumes an adequate energy intake. Infants, children, and pregnant and lactating women have additional protein needs to support synthesis of new tissue or milk production.

51. Protein requirements of endurance athletes and bodybuilders may be slightly

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higher than those of sedentary individuals, but this has no practical significance because athletes typically consume much more protein than they need.

52. Protein consumed in excess of the body’s needs is degraded; the nitrogen is excreted as urea, and the remaining keto acids are used for energy, providing 4 kilocalories per gram, or are converted to carbohydrate or fat. During conditions of fasting, starvation, or insufficient dietary intake of protein, lean tissue is broken down to supply amino acids for vital body functions. Persistent protein inadequacy results in suboptimal metabolic function with increased risk of infection and disease.

CHAPTER 6 - VITAMINS

53. Vitamins are organic compounds found in very small amounts in food and required for normal functioning—indeed, for survival. Humans are able to synthesize certain vitamins to some extent. For example, vitamin D is produced when the skin is exposed to sunlight; niacin can be synthesized from the amino acid tryptophan; and vitamin K and biotin are synthesized by bacteria living in the gut. However, in general, humans depend on their diet to supply vitamins. When a vitamin is in short supply or is not able to be utilized properly, a specific deficiency syndrome results. When the deficient vitamin is resupplied before irreversible damage occurs, the signs and symptoms are reversed. The amounts of

49

vitamins in foods and the amounts required on a daily basis are measured in milligrams and micrograms.

54. Unlike the macronutrients, vitamins do not serve as an energy source for the body or provide raw materials for tissue building. Rather, they assist in energy-yielding reactions and facilitate metabolic and physiologic processes throughout the body. Vitamin A, for example, is required for embryonic development, growth, reproduction, proper immune function, and the integrity of epithelial cells, in addition to its role in vision.

55. The B vitamins function as coenzymes that assist in energy metabolism; folic acid (folate), one of the B vitamins, helps protect against birth defects in the early stages of pregnancy. Vitamin C plays a role in building connective tissue as well as being an antioxidant that helps protect against damage by reactive molecules (free radicals). Now considered to be a hormone, vitamin D is involved in calcium and phosphorus homeostasis and bone metabolism.

56. Vitamin E, another antioxidant, protects against free radical damage in lipid systems, and vitamin K plays a key role in blood clotting. Although vitamins are often discussed individually, many of their functions are interrelated, and a deficiency of one can influence the function of another.

57. Vitamin nomenclature is somewhat complex, with chemical names gradually replacing the original letter designations created in the era of vitamin discovery during the first half of the 20th century. Nomenclature is further complicated by the recognition that vitamins are parts of families with, in some cases, multiple active forms. Some vitamins are found in foods in precursor forms that must be activated in the

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body before they can properly fulfill their function. For example, beta(β)-carotene, found in plants, is converted to vitamin A in the body.

58. The 13 vitamins known to be required by human beings are categorized into two groups according to their solubility. The four fat-soluble vitamins (soluble in nonpolar solvents) are vitamins A, D, E, and K. Although now known to behave as a hormone, the activated form of vitamin D, vitamin D hormone (calcitriol), is still grouped with the vitamins as well. The nine water-soluble vitamins (soluble in polar solvents) are vitamin C and the eight B-complex vitamins: thiamin, riboflavin, niacin, vitamin B6, folic acid, vitamin B12, pantothenic acid, and biotin. Choline is a vitamin-like dietary component that is clearly required for normal metabolism but that can be synthesized by the body. Although choline may be necessary in the diet of premature infants and possibly of those with certain medical conditions, it has not been established as essential in the human diet throughout life.

59. Different vitamins are more or less susceptible to destruction by environmental conditions and chemical agents. For example, thiamin is especially vulnerable to prolonged heating, riboflavin to ultraviolet or fluorescent light, and vitamin C to oxidation (as when a piece of fruit is cut open and the vitamin is exposed to air). In general, water-soluble vitamins are more easily destroyed during cooking than are fat-soluble vitamins.

60. The solubility of a vitamin influences the way it is absorbed, transported, stored, and excreted by the body as well as where it is found in foods. With the exception of vitamin B12, which is supplied by only foods of animal origin, the water-soluble vitamins are synthesized by plants and found in both plant and animal foods. Strict vegetarians

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(vegans), who eat no foods of animal origin, are therefore at risk of vitamin B12 deficiency. Fat-soluble vitamins, on the other hand, are found in association with fats and oils in foods and in the body and typically require protein carriers for transport through the water-filled compartments of the body.

61. Water-soluble vitamins are not appreciably stored in the body (except for vitamin B12) and thus must be consumed regularly in the diet. If taken in excess they are readily excreted in the urine, although there is potential toxicity even with water-soluble vitamins; especially noteworthy in this regard is vitamin B6. Because fat-soluble vitamins are stored in the liver and fatty tissue, they do not necessarily have to be taken in daily, so long as average intakes over time—weeks, months, or even years—meet the body’s needs. However, the fact that these vitamins can be stored increases the possibility of toxicity if very large doses are taken. This is particularly of concern with vitamins A and D, which can be toxic if taken in excess. Under certain circumstances, pharmacological (“megadose”) levels of some vitamins—many times higher than the amount typically found in food—have accepted medical uses. Niacin, for example, is used to lower blood cholesterol levels; vitamin D is used to treat psoriasis; and pharmacological derivatives of vitamin A are used to treat acne and other skin conditions as well as to diminish skin wrinkling. However, consumption of vitamins or other dietary supplements in amounts significantly in excess of recommended levels is not advised without medical supervision.

62. Vitamins synthesized in the laboratory are the same molecules as those extracted from food, and they cannot be distinguished by the body. However, various forms of a vitamin are not necessarily equivalent. In the particular case of vitamin E, supplements

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labeled d-α-tocopherol (or “natural”) generally contain more vitamin E activity than those labeled dl-α-tocopherol. Vitamins in food have a distinct advantage over vitamins in supplement form because they come associated with other substances that may be beneficial, and there is also less potential for toxicity. Nutritional supplements cannot substitute for a healthful diet.

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CHAPTER 7 - MINERALS

63. Unlike the complex organic compounds (carbohydrates, lipids, proteins, vitamins) discussed in previous sections, minerals are simple inorganic elements—often in the form of salts in the body—that are not themselves metabolized, nor are they a source of energy.

64. Minerals constitute about 4 to 6 percent of body weight—about one-half as calcium and one-quarter as phosphorus (phosphates), the remainder being made up of the other essential minerals that must be derived from the diet. Minerals not only impart hardness to bones and teeth but also function broadly in metabolism—e.g., as electrolytes controlling the movement of water in and out of cells, as components of enzyme systems, and as constituents of many organic molecules.

65. As nutrients, minerals are traditionally divided into two groups according to the amounts present in and needed by the body. The major minerals (macrominerals)—those required in amounts of 100 milligrams or more per day—are calcium, phosphorus (phosphates), magnesium, sulfur, sodium, chloride, and potassium. The trace elements (microminerals or trace minerals), required in much smaller amounts of about 15 milligrams per day or less, include iron, zinc, copper, manganese, iodine (iodide), selenium, fluoride, molybdenum, chromium, and cobalt (as part of the vitamin B12 molecule). Fluoride is considered a beneficial nutrient because of its role in protecting against dental caries, although an essential function in the strict sense has not been established in human nutrition.

66. The term ultratrace elements is

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sometimes used to describe minerals that are found in the diet in extremely small quantities (micrograms each day) and are present in human tissue as well; these include arsenic, boron, nickel, silicon, and vanadium. Despite demonstrated roles in experimental animals, the exact function of these and other ultratrace elements (e.g., tin, lithium, aluminum) in human tissues and indeed their importance for human health are uncertain.

67. Minerals have diverse functions, including muscle contraction, nerve transmission, blood clotting, immunity, the maintenance of blood pressure, and growth and development. The major minerals, with the exception of sulfur, typically occur in the body in ionic (charged) form: sodium, potassium, magnesium, and calcium as positive ions (cations) and chloride and phosphates as negative ions (anions). Mineral salts dissolved in body fluids help regulate fluid balance, osmotic pressure, and acid-base balance.

68. Sulfur, too, has important functions in ionic forms (such as sulfate), but much of the body’s sulfur is nonionic, serving as an integral part of certain organic molecules, such as the B vitamins thiamin, biotin, and pantothenic acid and the amino acids methionine, cysteine, and cystine. Other mineral elements that are constituents of organic compounds include iron, which is part of hemoglobin (the oxygen-carrying protein in red blood cells), and iodine, a component of thyroid hormones, which help regulate body metabolism. Additionally, phosphate groups are found in many organic molecules, such as phospholipids in cell membranes, genetic material (DNA and RNA), and the high-energy molecule adenosine triphosphate (ATP).

69. The levels of different minerals in foods

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are influenced by growing conditions (e.g., soil and water composition) as well as by how the food is processed. Minerals are not destroyed during food preparation; in fact, a food can be burned completely and the minerals (ash) will remain unchanged. However, minerals can be lost by leaching into cooking water that is subsequently discarded.

70. Many factors influence mineral absorption and thus availability to the body. In general, minerals are better absorbed from animal foods than from plant foods. The latter contain fibre and other substances that interfere with absorption. Phytic acid, found principally in cereal grains and legumes, can form complexes with some minerals and make them insoluble and thereby indigestible.

71. Only a small percentage of the calcium in spinach is absorbed because spinach also contains large amounts of oxalic acid, which binds calcium. Some minerals, particularly those of a similar size and charge, compete with each other for absorption. For example, iron supplementation may reduce zinc absorption, while excessive intakes of zinc can interfere with copper absorption. On the other hand, the absorption of iron from plants (nonheme iron) is enhanced when vitamin C is simultaneously present in the diet, and calcium absorption is improved by adequate amounts of vitamin D. Another key factor that influences mineral absorption is the physiological need for the mineral at the time.

72. Unlike many vitamins, which have a broader safety range, minerals can be toxic if taken in doses not far above recommended levels. This is particularly true for the trace elements, such as iron and copper. Accidental ingestion of iron supplements has been a major cause of fatal poisoning in

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young children.