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3 Macronutrients

Lipids

Contents

Key terms 46

Introduction 47

Common properties and specific types 47

• Fatty acids 47

• Triglycerides 50

• Phospholipids 50

• Sterols 50

Transporting lipids in the body 51

• Transport from the small intestine 52

• Transport from the liver to the body cells 52

Food sources of lipids 52

Major roles of lipids in the body 55

• Energy source and reserve 56

• Insulation and protection 56

• Components of cell membrane 56

Health implications of lipids 57

• Omega- 3 fatty acids 57

• Trans fat 58

• Obesity: excessive adiposity 58

• Cancer 59

Summary 59

Case study 60

Review questions 61

Suggested reading 61

Glossary 61

Key terms

• Atherosclerosis • Chylomicron

• Fatty acids • High- density lipoprotein

• Lipids • Lipogenesis

Macronutrients: lipids 47

• Lipolysis • Lipoprotein

• Low- density lipoprotein • Monounsaturated fatty acids

• Phospholipids • Polyunsaturated fatty acids

• Saturated fatty acids • Sterols

• Trans- fatty acids • Triglycerides

• Unsaturated fatty acids • Very low- density lipoprotein Introduction

Lipids are required for every physiological system in the body and are thus essential nutri-ents. For many people the thought of fatty foods invokes images of unhealthy living. We often shop for “fat- free” foods and try to avoid fats altogether. Food manufacturers have even developed “fat substitutes” to replace the fats normally found in food. However, although diets high in fat can lead to health complications such as obesity and heart disease, getting enough of the right types of fat is just as essential for optimal health.

What are the right types of fat? Should we put butter or margarine on our toast?

Should we use canola or corn oil in cooking? There are hundreds of oils, butters, and margarines from which to choose. Some are solid, some are liquid, some come from plants, and some come from animals. Some are said to increase your risk of heart disease while others claim to do the opposite. Recommendations for a healthy diet suggest that we consume a diet moderate in fat and low in saturated fat, trans fat, and cholesterol. In order to follow these guidelines, we must know how much and what types of fats are in the foods we choose.

Common properties and specific types

Lipid is the chemical term for what is commonly known as fats and oils. Lipids are a diverse group of chemical compounds. They share one main characteristic: they do not readily dissolve in water. For example, think of an oil- and-vinegar salad dressing. The oil is not soluble in the water- based vinegar; the two separate into distinct layers, with oil on top and vinegar on the bottom. Lipids in the diet and in our bodies provide a concen-trated source of energy. Recall in Chapter 1 that each gram of fat provides 9 kcal compared with only 4 kcal per gram from carbohydrate and protein. The major lipid classes include fatty acids, triglycerides, phospholipids, and sterols. The triglycerides predominate both in foods and in the body.

Fatty acids

In the body and in foods, fatty acids are found in the main form of lipids, triglycerides. A fatty acid is basically a long chain of carbons bonded together and flanked by hydrogen (Figure 3.1). At one end of the molecule is an acid group (COOH). At the other end, which is often referred to as the omega end, is a methyl group (CH3). Most naturally occurring fatty acids contain even numbers of carbon in their chains, usually 12 to 22, although some may be as short as 4 or as long as 26 carbons. Fatty acids with fewer than 8 carbons are called short- chain fatty acids; those with 8 to 12 carbons are medium- chain fatty acids; and those with more than 12 carbons are long- chain fatty acids. The long- chain (12 to 24 carbons) fatty acids are most common in the diet and are found prim-arily in meat, fish, and vegetable oils, while short- or medium- chain (6 to 10 carbons) fatty acids occur mainly in dairy products. The chain length of a fatty acid affects its chemical properties and physiological functions. In general, fatty acids with a shorter chain length tend to be liquid at room temperature, less stable, and more water soluble.

Another way in which fatty acids differ is by the types of chemical bonds between the carbon atoms (Figure 3.1). These carbon–carbon bonds may either be single bonds or double bonds. If a fatty acid contains all single carbon–carbon bonds, it is saturated fatty acid. The most common saturated fatty acids are palmitic acid, which has 16 carbons, and stearic acid, which has 18 carbons. These are found most often in animal foods such as meat and dairy products. Vegetable sources of saturated fatty acids include palm oil, palm kernel oil, and coconut oil. These are often called tropical oils because they are found in plants common in tropical climates. Most fats with long- chain saturated fatty acids are solid at room temperature.

Fatty acids containing one or more double bonds are unsaturated fatty acids (Figure 3.1). In other words, an unsaturated fatty acid contains some carbons that are not satur- ated with hydrogen. More specifically, fatty acids with one double bond are monounsatu-rated fatty acids; those with two or more double bonds are polyunsatuated with hydrogen. More specifically, fatty acids with one double bond are monounsatu-rated fatty acids.

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Figure 3.1 Chemical structure of saturated, monounsaturated, and polyunsaturated fatty acids. Each contains 18 carbons, but they differ from each other in the number and location of double bonds

Macronutrients: lipids 49 In our diet, the most common monounsaturated fatty acid is oleic acid, which is pre-valent in olive and canola oils. The most common polyunsaturated fatty acid is linoleic acid, found in corn, safflower, and soybean oils. Unsaturated fatty acids melt at cooler temperatures than saturated fatty acids of the same chain length. Therefore, the more unsaturated bonds a fatty acid contains, the more likely it is to be liquid at room temperature. There are different categories of unsaturated fatty acids, depending on the loca-tion of the first double bond in the chain. As shown in Figure 3.1, if the first double bond occurs between the third and fourth carbons, counting from the omega end of the chain, the fat is said to be an omega- 3 (ω-3) fatty acid. Alpha- linolenic acid, found in vegetable oils, and eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), found in fish oil, are omega- 3 fatty acids. If the first double bond occurs between the sixth and seventh carbons from the omega end, the fatty acid is called an omega- 6 (ω-6) fatty acid. Linoleic acid, found in corn and safflower oils, is the major omega- 6 fatty acid in the North American diet. Our bodies cannot synthesize double bonds in the omega- 3 and omerga- 6 positions. Therefore, both alpha- linolenic acid (ω-3) and linoleic acid (ω-6) are also referred to as essential fatty acids and they must be obtained from the diet. Omega- 3 fatty acids are important for the structure and function of cell mem-branes, particularly in the retina of the eye and the central nervous system. Omega- 6 fatty acids are important for growth, skin integrity, fertility, and maintaining red blood cell structure.

The position of the hydrogen atoms around a double bond is another way of classify-ing unsaturated fatty acids. Unsaturated fatty acids can exist in two different structural forms: the cis and trans forms (Figure 3.2). Most naturally occurring fatty acids are usually in the cis form in which the hydrogens are on the same side of the carbon–carbon double bond. During certain types of food processing, some hydrogens are transferred

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Figure 3.2 Cis- versus trans-fatty acids

to opposite sides of the carbon–carbon double bond, creating the trans form, or a trans- fatty acid. The cis bond causes the fatty acid backbone to bend. However, the trans bond allows the fatty acid backbone to remain straight, which makes it similar to the shape of saturated fatty acid. For this reason, trans fatty acids are also more likely to be solid at room temperature. Trans fatty acids are found in small amounts in nature and are formed during food processing involving high heat and high pressure.

Triglycerides

Most fatty acids do not exist in their free or unbound form in foods or in the body.

Instead, they are part of larger, more complex molecules called triglycerides or in smaller molecules called diglycerides and monoglycerides. When three fatty acids are attached to a backbone of the three- carbon molecule glycerol, the molecule is called a triglyceride (Figure 3.3a). When one fatty acid is attached, the molecule is called a monoglyceride, and when two fatty acids are attached, it is a diglyceride. Before most dietary fats are absorbed in the small intestine, the two outer fatty acids are typically removed from triglycerides. This produces a mixture of fatty acids and monoglycerides that can be absorbed into intestine cells. After absorption, the fatty acids and monoglycerides are mostly rejoined to form triglycerides. Triglycerides may contain any combina-tion of fatty acids: long, medium, short, saturated, or unsaturated. Triglycerides make up most of the lipids in foods and in the body, and are usually what is referred to when the term “fat” is used.

Phospholipids

Phospholipids are another class of lipids. They are important constitutes of cell mem-branes. Like triglycerides, they are built on a backbone of glycerol. However, at least one fatty acid is replaced with a compound containing phosphorus and often other elements such as nitrogen and choline (Figure 3.3b). Lecithin is a common example of phospho-lipids that is attached with a molecule of choline. The fatty acid end of phospholipids is soluble in fat or hydrophobic, whereas the phosphate end is water soluble or hydrophilic. Phospholipids are amphipathic, meaning they contain both polar (hydrophilic) and nonpolar (hydrophobic) potions. The structure allows phospholipids to be major components of cell membranes because they are able to mix with both water and fat. Having such polarized configuration makes phospholipids important in carrying out the digestion, absorption, and transport of lipids. Phospholipids are also found in food sources such as eggs, liver, soybeans, wheat germ, and peanuts.

Sterols

In addition to triglycerides and phospholipids, the lipids include the sterols, compounds with a multiple- ring structure (Figure 3.3c). A sterol can be attached to a fatty acid via an ester bond, forming a sterol ester. The most famous sterol is cholesterol. Cholesterol is a weakly polar compound. Although some free or unbound cholesterol is found in the body, most is bonded to a fatty acid. This cholesterol fatty acid is called cholesteryl ester.

Cholesteryl esters are more hydrophobic than free cholesterol. Cholesterol can be manufactured by almost every tissue in the body, especially the liver. Therefore, choles-terol is regarded as a nonessential nutrient. More than 90 percent of cholesterol in the body is found in cell membranes. It is also part of myelin, the coating on many nerve cells. Cholesterol is found only in foods from animal sources. Plant foods do not contain cholesterol unless animal products are combined with them in cooking or processing.

Macronutrients: lipids 51 Transporting lipids in the body

Because of the inherent difficulties related to the hydrophobic nature of lipids, the process of lipid circulation in the body is more complex than it is for other macronutri-ents. Generally, water- insoluble lipids are transported through the blood coated in a water- soluble envelope created when the lipids combine with phospholipids and pro- teins to form transport particles called lipoproteins. Fat- soluble vitamins are also trans-ported in lipoproteins. Lipoproteins help transport both dietary lipids from the small intestine and stored or newly synthesized lipids from the liver.

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Figure 3.3 Chemical forms of common lipids: (a) triglyceride, (b) phospholipids (e.g., lecithin), and (c) sterol (e.g., cholesterol)

Transport from the small intestine

After absorption into the intestinal mucosal cells, lipids that are somewhat water soluble, such as short- and medium- chain fatty acids and phospholipids, can enter the blood.

Lipids that are not soluble in water, such as long- chain fatty acids and cholesterol, cannot enter the bloodstream directly. These fatty acids are first assembled into triglyc- erides by the mucosal cell. These triglycerides are then combined with cholesterol, phos-pholipids, and a small amount of protein to form lipoproteins called chylomicrons.

Chylomicrons are absorbed into the lymphatic system and then enter the bloodstream without first passing through the liver. As chylomicrons circulate in the blood, the enzyme lipoprotein lipase, present on the surface of the cell lining the blood vessels, breaks the triglycerides down into fatty acids and glycerol, which enter the surrounding cells. The fatty acids can be used either as fuel or resynthesized into triglycerides for storage. What remains of the chylomicrons composed mostly of cholesterol and protein goes to the liver to be disassembled.

Transport from the liver to the body cells

The liver is the major lipid- producing organ where excess protein, carbohydrate, or alcohol can be broken down and used to make triglycerides or cholesterol. Triglycerides made in the liver are incorporated into lipoprotein particles called very low- density lipoproteins (VLDLs). VLDLs are rich in triglycerides and thus are very low in density. The VLDL trans-ports lipids out of the liver and delivers triglycerides to body cells. Once in the blood-stream, as with chylomicrons, the enzyme lipoprotein lipase breaks down the triglycerides in the VLDL so that the fatty acids can be taken up by the surrounding cells.

As its triglycerides are released, the VLDL becomes proportionately denser. Much of what eventually remains of the VLDL fraction is then called low- density lipoproteins (LDLs); these are composed primarily of the remaining cholesterol. The primary func-tion of the LDL is to transport cholesterol to tissues. For LDLs to be taken up by the cells, a protein on the surface of the LDL particle must bind to a receptor on the cell membrane. This allows LDLs to be removed from circulation and to enter cells where their cholesterol and other components can be used. If LDLs are not readily cleared from the bloodstream, endothelial cells of the arteries will take them up, leading to atherosclerosis, a condition in which an artery wall thickens as the result of a build- up of fatty materials such as cholesterol. High levels of LDL in the blood have been associated with an increased risk for heart disease.

Since most body cells cannot effectively break down cholesterol, it must be returned to the liver to be eliminated from the body. This reverse cholesterol transport is accomp-lished by the densest of the lipoprotein particles called high- density lipoproteins (HDLs). The liver and intestine produce most of the HDLs in the blood. The HDLs pick up cholesterol from dying cells and other lipoproteins, and function as a temporary storage site for lipids. Some of the cholesterol in HDLs is taken directly to the liver for disposal, and some is transferred to organs that have a high requirement for cholesterol, such as those involved in steroid hormone synthesis. High levels of HDL in the blood are associated with a reduction in heart disease risk.

Food sources of lipids

The fat content in foods can vary from 100 percent, as found in most cooking oils and spreads such as butter, margarine, and mayonnaise, to minor trace amounts, less than 5 percent, as found in most fruits and vegetables. Some foods obviously have a high fat

Macronutrients: lipids 53 content. For example, foods high in fat include nuts, bologna, avocados, and bacon, which have about 80 percent of calories as fat; these are followed by peanut butter, cheddar cheese, steak, hamburgers, ice cream, doughnuts, and whole milk (Table 3.1).

However, in other foods, the fat content may be high but not as obvious. This is known as hidden fat. For example, some baked goods such as cakes, muffins, croissants, cookies, crackers, and chips contain considerable amounts of fat, but people often remain unaware of this. A 5-oz baked potato contains 145 kcal with about 3 percent fat, but people often ignore the fact that the same size serving of potato chips contains 795 kcal, over 60 percent of them from fat.

The type of fat in food is important to consider along with the total amount of fat.

Animal fats are the chief contributors of saturated fatty acids. About 40 to 60 percent of the total fat in dairy and meat products is in the form of saturated fatty acids (Figure 3.4). In contrast, plant oils contain mostly unsaturated fatty acids, ranging from 70 to 95 percent of total fat. Some of the plant oils are good sources of monounsaturated fatty acids such as canola, olive, and peanut oils. Corn, sunflower, soybean, and safflower oils contain mostly polyunsaturated fatty acids. These plant oils supply the majority of the alpha- linoleic (omega- 3) and linoleic (omega- 6) in the North American food supply. These fatty acids are considered as essential fatty acids, meaning that they must be obtained through the diet because human cells lack the enzymes needed to produce these fatty acids. Both omega- 3 and omega- 6 fatty acids perform important roles in immune function and vision, help form cell membrane, and produce hormone- like compounds. Table 3.2 exhibits amounts of omega- 3 fatty acid of commonly chosen fish and seafood products.

As mentioned earlier, wheat germ, peanuts, egg yolks, soybeans, and organ meats are rich sources of phospholipids. Phospholipids such as lecithin, a component of egg yolks, are often added to salad dressings. Lecithin is used as an emulsifier because of its ability to prevent mixtures of lipids and water from separating. Emulsifiers are added to salad dress-ings to keep the vegetable oil suspended in water. The fact that eggs are added to cake batters is another example of phospholipids being used to emulsify the fat with water.

Table 3.1 Fat content of commonly selected foods

Foods Serving size Fat (g) Calories from fat (%)

Canola oil 1 tablespoon 14 100

Margarine 1 tablespoon 12 100

Butter 1 tablespoon 12 100

Avocado 1/2 cup 11 86

Mixed nuts 1 ounce 16 78

Peanut butter 1 tablespoon 8 76

Cheddar cheese 1 ounce 10 74

T-bone steak 3 ounces 17 66

Flax seeds 1 tablespoon 3 62

Whole milk 1 cup 8 49

Snack crackers 1 ounce 7 45

Doughnut 1 5 45

Hamburger 1 12 39

Chocolate candies 1 ounces 6 39

Chicken breast with skin 3 ounces 7 36

2% milk 1 cup 5 36

Chicken breast without skin 3 ounces 6 32

Baked beans 1/2 cup 7 31

Yogurt 8 ounces 7 28

Low-fat yogurt 8 ounces 4 18

Table 3.2 Omega-3 fatty acid content of fish and seafood

Food Omega-3 fatty acid (g)

Salmon 1.15

Swordfish 1.15

Trout 1.15

Shark 0.83

Flounder 0.48

Sole 0.44

Cod 0.44

Squid 0.40

Crab 0.35

Oyster 0.30

Shrimp 0.27

Scallop 0.27

Mussel 0.26

Clam 0.26

Tuna 0.23

Lobster 0.07

Source: adapted from USDA Nutrient Data Laboratory.

NoteAll values represent estimated amounts in a 3-ounce cooked portion and these values may vary markedly with species, season, diet, packaging, and cooking methods.

Coconut oil Whole milk Butter Cream cheese Palm oil Beef Lard Chicken Salmon Olive oil Stick margarine Tub margarine Peanut oil Corn oil Soybean oil Sunflower oil Flaxseed oil Safflower oil Canola oil

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Saturated fatty acids

Monounsaturated fatty acids

Polyunsaturated fatty acids

Fatty acid content (%)

Figure 3.4 Saturated, monounsaturated, and polyunsaturated fatty acid content of various sources of dietary lipid

Macronutrients: lipids 55 Cholesterol, a common example of sterol and widespread in plasma membrane of all cells, is obtained either through the diet or through cellular synthesis. Cholesterol obtained from the diet is referred to as exogenous cholesterol, while cholesterol pro-duced within the body is referred to as endogenous cholesterol. Even if an individual maintains a “cholesterol- free” diet, endogenous cholesterol synthesis varies between 500 and 2000 mg per day. More endogenous cholesterol forms with a diet high in saturated fatty acids. Exogenous cholesterol is found only in animal foods (Table 3.3). Eggs are our main source of cholesterol, along with meat and whole milk. One egg yolk contains about 200 mg of cholesterol. Organ meats contain about 300 mg per 3-oz serving. Lean red meat and chicken contains 100 mg, whereas fish contains 50 mg in 3 oz. The produc-tion of endogenous cholesterol is usually sufficient to meet the body’s needs; hence severely reducing cholesterol intake may cause little harm except in pregnant women and infants.

Major roles of lipids in the body

The blood carries lipids to various sites around the body. Once they arrive at their destinations, the lipids can get to work providing energy, insulating against temperature extremes, protecting against shock, and maintaining cellular integrity. The following sections describe each of these roles in more detail.

Table 3.3 Cholesterol content of commonly selected foods

Foods Serving size Cholesterol content (mg)

Skim milk 1 cup 4

Mayonnaise 1 tablespoon 10

Butter 1 pat 11

Lard 1 tablespoon 12

Cottage cheese 1/2 cup 15

Low-fat milk (2%) 1 cup 22

Half-and-half 1/4 cup 23

Hot dog 1 29

Ice cream 1/2 cup 30

Cheddar cheese 1 ounce 30

Whole milk 1 cup 34

Oyster 3 ounces 40

Salmon 3 ounces 40

Clam 3 ounces 55

Tuna 3 ounces 55

Chicken 3 ounces 70

Turkey 3 ounces 70

Beef 3 ounces 75

Pork 3 ounces 75

Lamb 3 ounces 85

Crab 3 ounces 85

Shrimp 3 ounces 110

Lobster 3 ounces 110

Heart 3 ounces 165

Egg yolk 1 210

Beef liver 3 ounces 410

Kidney 3 ounces 540

Source: USDA National Nutrient Database for Standard Reference, Release 22, 2009.

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