chapter 4 lipids 1instructor: dr. may hamdan. introduction the term lipids refers to fat, oils and...

Instructor: Dr. May Hamdan 1

Chapter 4liPiDs


Introduction

• The term lipids refers to fat, oils and cholesterol

• Usually The term fat usually refers to lipids

• Most of the lipids in our food and in our bodies are triglycerides.


Triglycerides and Fatty Acids

• A triglyceride is made up of three fatty acids attached to a 3-carbon molecule called glycerol.

(Figure 4.1)


When one fatty acid is attached, the molecule is called a monoglyceride;

When two fatty acids are attached, it is a diglyceride.

Triglycerides may contain any combination of fatty acids.

• Their fatty acid composition determines the following characteristics: 1. their taste, 2. texture, 3. physical characteristics,4. and health effects


• Fatty acids consist of chains of carbon atoms linked together.

Some fatty acids contain only a few carbons in the chain

Others may have 20 carbons or more.


• Classification of fatty acids depending on the Length of fatty acid chain:

1. Short-chain fatty acids (SCFA) contain fewer than six carbons. 2. Medium-chain fatty acid (MCFA) contain 6–12 carbons,.3. Long-chain fatty acid (LCFA) contain more than 12 carbons.4. Very long chain fatty acid (VLCFA) contain more than 22 carbons

• Each carbon atom in the fatty acid chain forms four chemical bonds that can link it to as many as four other atoms.


• When each carbon in the fatty acid chain is bound to 2 hydrogen atoms, it is a called a saturated fatty acid

because the carbon chain is saturated with hydrogen atoms.

• If the chain contains carbons that are not bound to 2 hydrogen atoms, a double bond is formed between 2 carbons.

Fatty acids containing one or more double bonds are called unsaturated fatty acids.


Saturated Fats

• Saturated Fats are Triglycerides containing a high proportion of saturated fatty acids

• Saturated fats are found primarily in animal foods such as:1. meat, 2. milk, 3. and cheese.

They are also found in plant sources:

4. in palm oil,

5. and coconut oil.


• These saturated vegetable oils are used by the food industry in:

1. cereals, 2. crackers, 3. salad dressings, 4. and cookies

Because saturated oils are more resistant to spoilage and therefore have a longer shelf life than unsaturated oils.


Unsaturated Fats

• Unsaturated fats are triglycerides that contain a high proportion of unsaturated fatty acids.

• Unsaturated fatty acids contain one or more unsaturated (double) bonds.

• Those that contain one carbon-carbon double bond are called monounsaturated fatty acids (MUFA).

Oils that are high in monounsaturated fatty acids include • olive,• peanut,• and canola oils.


• Fatty acids with more than one double bond in their carbon chains are called polyunsaturated fatty acids (PUFA).

Good sources of polyunsaturated fatty acids include 1. corn,2. soybean, 3. and safflower oils.

Diets high in mono- and polyunsaturated fats are associated with a reduced risk of heart disease.


• A type of unsaturated fatty acid that is not good for the heart is trans fat.

• Trans fatty acids are found in hydrogenated fats.

• Hydrogenation is a process that adds hydrogen atoms to unsaturated fatty acids, converting some carbon-carbon double bonds to saturated bonds.


• Hydrogenation makes the fat:

1. more solid at room temperature

2. And less likely to spoil.

3. it also changes some of the carbon-carbon double bonds that remain….

Food manufacturers use hydrogenation to increase the shelf life of oils.


• Most unsaturated fatty acids have carbon-carbon double bonds with both hydrogen atoms on the same side of the bond. This is known as the cis configuration.


• Hydrogenation results in trans fatty acids, which have carbon-carbon double bonds with the hydrogen atoms on opposite sides of the double bond (Figure 4.2).


• Unsaturated fatty acids are categorized based on the location of the first double bond in the carbon chain.

• If the first double bond occurs after the sixth carbon the fat is said to be an omega-6 fatty acid.

• The major omega-6 fatty acid is linoleic acid, which is plentiful in: vegetable oils.


• Omega-6 fatty acids are important for:

1. growth,

2. skin integrity,

3. fertility,

4. and maintaining red blood cell structure.


• Unsaturated fatty acids with the first double bond after the third carbon, are called omega-3 fatty acids.

found in• vegetable oils• found in fish oils

• Types of omega-3 fatty acids1. Eicosa pentaenoic acid (EPA) 2. and docosa hexaenoic acid (DHA), found in fish oils,


Omega-3 fatty acids play an important role in:1. the structure and function of cell membranes,

particularly those in the retina of the eye

2. and the central nervous system.

3. Diets high in omega-3 fatty acids help reduce the risk of heart disease.


Essential Fatty Acids

• linoleic acid (omega-6) and alpha-linolenic acid (omega-3) are essential fatty acids and must be included in the diet.

Because the human body is unable to synthesize double bonds in the omega-6 and omega-3 positions,


• They can be used to make other omega-6 and omega-3 fatty acids.

For example:

1. if the diet is low in linoleic acid, the body cannot make the omega-6 fatty acid, known as arachidonic acid, and so it becomes a dietary essential.

2. EPA and DHA are omega-3 fatty acids that are synthesized from alpha-linolenic acid.


• People who do not consume essential fatty acids in adequate amounts are likely to suffer from symptoms of essential fatty acid deficiency.

These symptoms include:1. scaly, dry skin; 2. Liver abnormalities; 3. wounds that heal poorly; 4. and impaired vision and hearing. 5. Infants with this deficiency fail to grow properly.


• Luckily, essential fatty acid deficiency is rare:

because the human body’s requirement for essential fatty acids is well below the amounts that people typically consume.


Phospholipids

• Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group and a glycerol compound.

• Phospholipids are important in foods and in the body because they allow water and oil to mix.

They consist of:• a backbone of glycerol• with two fatty acids • and a phosphate group

Instructor: Dr. May Hamdan 25Phospholipid


• The positive chemical charge on the phosphate end of the phospholipid molecule allows this end to dissolve in water.

• In contrast, the fatty acid end of the molecule is soluble in fat.

• In foods, phospholipids act as emulsifiers.

• For example, the phospholipid lecithin is used in salad dressings to keep the oil and water portions from separating.


• In the body, phospholipids form the structure of cell membranes.

• Cell membranes consist of two layers of phospholipid molecules called a lipid bilayer.

• The molecules are oriented so that the water-soluble phosphate end of the phospholipid molecules faces the watery environments inside and outside of the cell and the fat-soluble fatty acids face each other.


Cholesterol• Cholesterol is probably the best-known sterol.

• Sterols are lipids that have chemical rings as the basis of their structure.

• Importance of Cholesterol in the body:

1. cholesterol is part of cell membranes 2. part of the insulating sheath that covers the nerves and is needed for

proper nerve function.3. is also needed to synthesize vitamin D, 4. to synthesize bile acids, 5. And to synthesize a number of hormones, including the sex hormones


• Although it is important in the body, cholesterol is not a dietary essential because it is made in the liver.

• Dietary cholesterol is found only in animal foods such as:

1. meats, 2. eggs, 3. and dairy products.

• A diet high in cholesterol can increase the risk of heart disease.


• Two main kinds of lipoproteins carry cholesterol in the blood:

1. Low density lipoprotein, or LDL,

which also is called the “bad” cholesterol

because it carries cholesterol to tissues, including the arteries.

Most of the cholesterol in the blood is the LDL form.

The higher the level of LDL cholesterol in the blood, the greater the risk for heart disease.


2. High density lipoprotein, or HDL,

which also is called the “good” cholesterol

because it takes cholesterol from tissues to the liver, which removes it from the body.

A low level of HDL cholesterol increases the risk for heart disease.

LDL molecules contain much more cholesterol than HDL molecules


What do Lipids Do?

• Lipids have the following functions:

1. an important structural component of cell membranes, particularly in the brain and nervous system.

2. Lipids also have important regulatory roles.

3. The lipid cholesterol is used to synthesize certain hormones, such as the sex hormones,


4. fatty acids are used to make hormone-like molecules called eicosanoids.

Eicosanoids help regulate:blood clotting, blood pressure,Immune function,


5. Fat is also an important energy source.

• Triglycerides consumed in the diet can be used in two main forms:

1. as an immediate source of energy or

2. Be stored in the adipose tissue for future use.

• triglycerides are a concentrated energy source (9 calories per gram),


6. Adipose tissue also insulates the body from changes in temperature,

7. and provides a cushion to protect against shock


Lipids and Heart Disease

Diets that are high in:• saturated fat, • cholesterol, • and trans fat

May increase the risk of developing heart disease, in particular, atherosclerosis.


• Atherosclerosis is a type of heart disease in which a fatty substance called plaque builds up in arteries.

• Plaque causes an artery to narrow and lose its elasticity


• The buildup of plaque can become so great that it completely blocks the artery, or the plaque can rupture and cause a blood clot to form.

The blood clots can block the artery at that spot or break loose and block a smaller artery elsewhere.


• When blood flow is blocked the cells that the artery supplies with blood are starved for oxygen and die.

• If an artery in the heart is blocked, heart muscle cells die, resulting in a heart attack, or myocardial infarction.

• If the blood flow to the brain becomes blocked, brain cells die and a stroke results.


• There are many factors that affect the risk of developing atherosclerosis.

• Risk factors of heart disease:

1. The risk increases as people get older 2. and increases with high blood pressure, 3. diabetes, 4. obesity, 5. high blood LDL cholesterol,6. and low blood HDL cholesterol


Digesting , Absorbing , andTransporting Lipids

• Most lipid digestion takes place in the small intestine through the action of enzymes that are called lipases.

• Digestion and absorption of lipids are aided by bile.

• Bile is produced in the liver and stored in the gallbladder. It contains bile acids and helps break fat into small globules that can be accessed more easily by digestive enzymes.


The enzymes break triglycerides into:• Fatty acids, • glycerol, • and monoglycerides.

• The fatty acids and monoglycerides mix with bile and other lipids to form tiny droplets called micelles, which facilitate fat absorption.


• After their absorption into the cells lining the small intestine,

shorter fatty acids, which are water soluble, enter the blood and are transported to the liver.

Larger fatty acids and monoglycerides are reassembled (recollected) into triglycerides and then combined with cholesterol, phospholipids, and protein to form particles called chylomicrons.


• Chylomicrons are a type of lipoprotein that transports lipids from the intestines.

• Chylomicrons move from the intestinal cells into the lymph, a fluid that then carries them to the bloodstream.

• Chylomicrons travel in the blood, delivering triglycerides to the body cells.

• To enter body cells, an enzyme must first break the triglycerides into fatty acids and glycerol. What remains of the chylomicron goes to the liver.


• When they reach the liver, the remnants of chylomicrons, as well as the triglycerides and cholesterol that are synthesized in the liver, are incorporated into lipoprotein particles called very-low-density lipoproteins (VLDLs).

• VLDLs transport lipids from the liver and deliver triglycerides to body cells.


• To enter the cells an enzyme breaks the triglycerides in VLDLs into

fatty acids and glycerol so they can be taken up by cells.

• After triglycerides have been removed from the VLDLs, these particles can either be returned to the liver or transformed into lipoproteins called low-density lipoproteins (LDLs).

LDLs deliver cholesterol to body cells.


• For LDLs to be taken up by cells, the LDL particle must bind to a protein on the cell membrane, which is called an LDL receptor.

• High levels of LDLs in the blood are associated with an increased risk for heart disease and, therefore, are thought of as “bad” cholesterol.


• Cholesterol that is not used by cells can be returned to the liver by lipoproteins called high-density lipoproteins (HDLs).

• High levels of HDLs in the blood are associated with a reduction in heart disease risk,

therefore HDLs are thought of as “good” cholesterol.

chapter 4 lipids 1instructor: dr. may hamdan. introduction the term lipids refers to fat, oils and...

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