Nutrition 330 Introductory Nutrition

Study Guide: Unit 5

Lipids: Fats, Oils, Phospholipids, and Sterols

Dietary lipids have gained much attention over the years because of evidence of their association with chronic lifestyle diseases such as heart disease, cancer, and obesity. However, dietary lipids do have an essential role in nutrition. They provide the most concentrated form of energy (nine kcalories per gram of fat), and are the source of essential fatty acids. As with other nutrients, their overconsumption will result in adverse health effects.

In this unit, we examine the chemical and physical characteristics of various types of lipids and discuss their sources. We study the processes of digestion, absorption, and transport of lipids. We review the functions of lipids and the effects of diet on blood cholesterol. Finally, we look at the present trends in fat consumption by Canadians, the current recommendations for fat intake, and the effects of food processing on dietary lipids.

This unit consists of four sections:

5.1—Chemistry and Classification
5.2—Digestion, Absorption, and Transport
5.3—Functions and Health Effects
5.4—Lipids in the Diet

Objectives

After completing this unit, you should be able to

1. identify the chemical characteristics of the three main categories of lipids—triglycerides, phospholipids, and sterols—and give examples of their food sources.
2. name the classifications of fatty acids based on the chemical characteristics of chain length and degree of unsaturation; relate these chemical characteristics to the physical properties of fats and oils; and give examples of saturated, monounsaturated, and polyunsaturated fats.
3. describe the physical effects and nutritional implications of hydrogenation and oxidation of dietary fats.
4. identify the two families of essential fatty acids, identify their “parent” fatty acids, and give examples of dietary sources.
5. outline the steps in the digestion of lipids, identifying the sites, the substrates, the enzymes, and the products of digestion; describe the mechanism of lipid absorption for various types of lipids.
6. describe the transport of lipids in the body, and discuss the significance of the different lipoproteins in cardiovascular disease.
7. describe the roles of fat in the body.
8. discuss the effects of diet on blood cholesterol levels, and the implication of high blood cholesterol for cardiovascular disease.
9. describe a heart‑healthy diet and give examples of suitable food choices in such a diet.
10. list and describe the nutritional roles of essential fatty acids and identify the causes and health effects of essential fatty acid deficiency.
11. describe the pattern of fat consumption in Canada, and discuss the implication of lipid consumption on health.
12. state the current recommendations for fat consumption and describe the food pattern outlined in Canada’s Food Guide, which achieves these recommendations.
13. identify dietary sources of saturated fatty acids (SFA), monounsaturated fatty acids (MUFA), polyunsaturated fatty acids (PUFA), and omega‑3 fatty acids.

5.1 Chemistry and Classification

Introduction

Lipids are organic compounds composed primarily of carbon, hydrogen, and small amounts of oxygen. In general terms, they can be defined as organic substances that are insoluble in water but soluble in organic solvents, such as chloroform or ether, and that can be utilized by living organisms. Even though the terms lipids and fats are often used interchangeably (as in this course), in strict chemical terms, fats belong to a subgroup under the general nutrient class of lipids. This subgroup, called the triglycerides, includes fats that are usually solid and oils that are usually liquid at room temperature. Triglycerides make up 95% of dietary lipids. The other 5% is contributed by phospholipids and sterols plus fat‑soluble vitamins, waxes, and other minor complex lipid compounds. In the body, triglycerides make up 99% of the stored fats.

Objectives

After completing this section, you should be able to

• identify the chemical characteristics of the three main categories of lipids—triglycerides, phospholipids, and sterols—and give examples of their food sources.
• name the classifications of fatty acids based on the chemical characteristics of chain length and degree of unsaturation; relate these chemical characteristics to the physical properties of fats and oils; and give examples of saturated, monounsaturated, and polyunsaturated fats.
• describe the physical effects and nutritional implications of hydrogenation and oxidation of dietary fats.
• identify the two families of essential fatty acids, identify their “parent” fatty acids, and give examples of dietary sources.

Key Terms

After completing section 5.1, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 5: Introduction and “The Chemist’s View of Fatty Acids and Triglycerides,” pages 135–138 (to “Degree of Unsaturation Revisited”)
• Chapter 5: “The Chemist’s View of Phospholipids and Sterols,” pages 142–144

Triglycerides

A triglyceride is composed of a glycerol molecule, to which three fatty acids are attached (organic acids that consist of hydrocarbon chains). Each chain has a carboxylic acid at one end and a methyl group at the other. One fatty acid attached to a glycerol molecule forms a monoglyceride; two fatty acids attached to a glycerol molecule forms a diglyceride. Monoglycerides and diglycerides are products of the food industry. They are used as emulsifiers to give foods a smooth texture and to keep fats evenly dispersed. Foods such as ice cream, baked goods, margarine, lard, and shortening may contain monoglycerides and diglycerides. Monoglycerides can also be found in the human gastrointestinal tract after a fatty meal, as a product of fat digestion.

Dietary lipids are typically composed of mixed triglycerides, meaning that three different fatty acids are attached to each glycerol (see Figure 5‑5, p. 139). These fatty acids can vary in length (even‑numbered, from four to thirty carbon atoms), and in degree of unsaturation (number of double carbon bonds along the chains). The nutritional significance of triglycerides is governed by the composition of these fatty acids, which we discuss later. Triglycerides dominate in dietary fats; some are visible in such foods as butter, margarine, lard, vegetable oil, salad dressing, the fatty portion of meat, and chicken skin. Examples of foods that contribute invisible fats include cream, milk, cheese, egg yolks, meat, fish, poultry, nuts, seeds, olives, avocados, some whole‑grain cereals, bakery items, and fried foods.

A phospholipid is a lipid that contains phosphorus. Phospholipids are found in almost all cells as a major constituent of cell membranes. Unlike triglycerides and sterols, they have the ability to attract water‑soluble as well as fat‑soluble substances; they can therefore facilitate the passage of fats in and out of cells through the cell membranes. Phospholipids in the blood and body fluids also act as emulsifiers, keeping fats in solution.

An example of a phospholipid is lecithin, which has the water‑soluble base choline plus a phosphate molecule, along with two water‑insoluble fatty acids attached to a glycerol molecule (see Figure 5‑9, p. 142). Lecithin and other phospholipids occur naturally in liver, egg yolks, and soybeans. As a food additive, lecithin serves as an emulsifier in such foods as cheese, margarine, and chocolate bars. Phospholipids make up a very small proportion of dietary lipids. When ingested, they are hydrolyzed by intestinal enzymes and can be used to produce energy. They are not an essential nutrient, as the body can synthesize them as needed.

A sterol is a lipid with a characteristic hydrocarbon ring structure; all sterols are derived from cholesterol, and resemble chicken wire (see Figure 5‑11, p. 143). Like triglycerides, sterols are hydrophobic (water‑insoluble). They do not contribute energy to the body.

The best-known example of a sterol is cholesterol. About 80% of the body’s cholesterol is synthesized in the body (mainly by the liver and intestine); it is called endogenous cholesterol. Cholesterol is also found in foods of animal origin; plants do not make cholesterol. Food manufacturers that advertise vegetable oils, peanut butters, or margarines as “cholesterol free” are simply taking advantage of a characteristic that already exists in these foods. Cholesterol from dietary sources, called exogenous cholesterol, contributes the remaining body cholesterol. Major sources of dietary cholesterol are egg yolks, meat, poultry, fish, and dairy products.

In the body, cholesterol is used as an essential component of cell membranes and is the precursor for the synthesis of bile acids, hormones, and vitamin D.

A high blood cholesterol level is a major risk factor for coronary heart disease, as we will see later in the unit.

Fatty Acids

Fatty acids are the building blocks of lipids. They can be classified according to the chain length:

• Short‑chain fatty acids have fewer than 6 carbons.
• Medium‑chain fatty acids have 6–10 carbons.
• Long‑chain fatty acids have 12–24 carbons.

Another way of classifying fatty acids is by the degree of unsaturation, that is, the number of double carbon bonds formed along the chain:

• Saturated fatty acids (SFA) have no double bonds.
• Monounsaturated fatty acids (MUFA) have one double bond.
• Polyunsaturated fatty acids (PUFA) have two or more double bonds.

The length of the carbon chains and the degree of unsaturation of fatty acids determine the hardness of a fat or oil at room temperature: the shorter and more unsaturated the fatty acids, the more liquid the fat or oil. Dietary lipids that contain saturated short‑ and medium‑chain fatty acids are butterfat and coconut oil. Most fats and oils found in nature are made up of long‑chain fatty acids. Palmitic acid is the most abundant saturated fatty acid in fats, and linoleic acid is the most abundant polyunsaturated fatty acid in oils. The following table lists some of the common dietary fatty acids, their chain length, their number of double bonds, and their consistency.

Table 5.1 Common Dietary Fatty Acids

Natural fats and oils contain a great diversity of fatty acids and triglycerides. In general, fats of animal origin tend to contain large amounts of palmitic and stearic acids, which are saturated and therefore solid at room temperature. Oils found in plants and seeds, however, usually have large amounts of oleic and linoleic acids, which are unsaturated and therefore liquid at room temperature (hence they are called oils). Exceptions are coconut and palm oils (tropical oils) that are of plant origin, but highly saturated (92% and 51% saturated, respectively). Among the animal fats, the degree of unsaturation also varies, depending on the diet of the animal. For instance, beef tallow and mutton fat are harder and more saturated than pork fat, and pork fat is harder and more saturated than poultry fat. Consequently, the beef industry has been experimenting with different feeds to reduce the amount of saturated fats in beef in order to meet the needs of health‑conscious consumers. Fish fat is softer than fats from land animals because it contains large amounts of unsaturated fatty acids (18‑, 20‑, and 22‑carbon members of the omega‑3 series, which have three, five, and six double bonds, respectively). Further details of the health effects of this family of fatty acids will be discussed later. Figure 5‑6 (p. 140) compares dietary fats; note the major sources of the different types of fats.

Reading Assignment

• Chapter 5: “Degree of Unsaturation Revisited,” pages 138–142

Oxidation and Hydrogenation

Oxidation occurs when oxygen combines with an unsaturated fatty acid at the double bond. This gives the oil the unpleasant odour and flavour characteristic of rancid fats. Oils that contain a high percentage of polyunsaturated fatty acids are the most susceptible to oxidation. Conditions that accelerate oxidation include the presence of oxygen, light, and heat, and exposure to metals, such as iron and copper.

Oils high in polyunsaturated fatty acids can be protected from oxidation in a number of ways. They should be stored in air‑tight, dark containers at cool—not cold—temperatures. Antioxidants, such as BHA and BHT, may be added to the oil or, for some foods, the packaging material. Antioxidants act by combining with oxygen, serving as oxygen scavengers. Vitamin E is a biological antioxidant, but is not often used as a food additive because of its cost. A final option to prevent oxidation is to partially hydrogenate the oil to reduce the number of double bonds.

Hydrogenation is a chemical process by which hydrogen is added to unsaturated oils to reduce the number of double bonds, thus making them more saturated and more resistant to oxidation. This common food processing procedure is used to improve shelf life, and to attain desired consistency by changing a vegetable oil to a more solid fat, such as margarine or vegetable shortening. The degree of hydrogenation varies in products and determines the hardness of a fat. The textbook explains that total hydrogenation of an oil to produce a solid fat rarely occurs in food manufacturing. However, partial hydrogenation is a common procedure in the production of soft (tub) margarines, and some types of shortening (e.g., Crisco). In this process, hydrogen is added to some of the double bonds in the original oil, turning double bonds into single bonds. This makes the oil semi‑solid, or spreadable. Some of the remaining double bonds, however, have their hydrogens rearranged, changing cis forms to trans forms. Figure 5.8 (p. 141) shows how the trans form of a fatty acid has the hydrogen atoms on opposite sides of its double bond, which effectively changes the physical shape of the fat molecule.

Although hydrogenation reduces oxidation of fats and prolongs shelf life, it has some unwelcome effects. It changes polyunsaturated fat to saturated fat. It changes the chemical structures of essential fatty acids (EFA) by reducing the number of double bonds, thereby eliminating their ability to function as essential fatty acids. Also, during hydrogenation, double bonds transform from a cis to a trans form. Most natural fats are in the cis form, and there is now a consensus that the body handles large amounts of trans fat poorly. In particular, a strong body of evidence has accumulated indicating that these fats are far worse than saturated fats in increasing the risk of heart disease.

In short, hydrogenated fats contain less polyunsaturated fat than was present in the original oil, and of the polyunsaturated fat that remains, much is now in the unnatural trans form—a double disadvantage, nutritionally. Because of the concern about the detrimental effects of trans fat, nearly all soft or “tub” margarines, once a major source of trans fat in the diet, are now non‑hydrogenated and free of trans fat. The label will indicate whether the margarine contains partially hydrogenated oil. Hard or “block” margarines and vegetable shortenings are more highly hydrogenated: they are high in SFA and trans fat. The small amounts of trans fat occurring naturally in beef and milk are not considered to be harmful; the fats in beef and milk, however, are saturated.

Reading Assignment

• Chapter 5: “Essential Fatty Acids,” pages 151–152

Essential Fatty Acids

When essential appears with a nutrient, it means that the body is unable to produce that nutrient in amounts sufficient for normal functioning; the nutrient must be supplied by the diet. However, as the textbook points out, defining the essentialness of individual fatty acids is not simple. Our knowledge of the body’s ability to convert one fatty acid to another and of whether sufficient amounts are synthesized for normal body functioning remains incomplete.

In general, two families of polyunsaturated fatty acids are considered essential: the omega‑6 (w‑6) and the omega‑3 (w‑3) fatty acid families. The chemical difference between the two groups is the position of the farthest double bond from the methyl end of the fatty acid molecule. In an omega‑6 fatty acid this double bond is six carbons from the methyl end; in an omega‑3 fatty acid, this double bond is three carbons from the methyl end. These fatty acid series are not interconvertible.

The nutritionally significant omega‑6 fatty acids are linoleic acid (18 carbons, two double bonds) and arachidonic acid (20 carbons, four double bonds). Arachidonic acid can be synthesized in the body from its precursor, linoleic acid, and is also provided in the diet by meats. Seed oils (safflower, sunflower, corn, and soybean oils) are excellent sources of linoleic acid.

Linolenic acid (18 carbons, three double bonds) is the “parent” fatty acid of the omega‑3 series. Linolenic acid has its own distinct nutritional functions, and serves as the precursor for two other fatty acids in this series: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The richest sources of linolenic acid are flax, canola, and soybean oils, fatty fish, and walnuts. Fatty fish such as mackerel, sardines, lake trout, herring, salmon, tuna, whitefish, and anchovies are sources of preformed EPA and DHA. We will discuss the functions of the essential fatty acids later in the unit.

Study Questions

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5.2 Digestion, Absorption, and Transport

Introduction

Dietary lipids, which are primarily triglycerides, are water‑insoluble. Lipid molecules are non‑polar (neutral) with no net charge. Consequently, they tend to separate from watery fluids (water is polar, with both positive and negative charges). This tendency presents the body with two challenges. The first is to mix lipids with the watery digestive juices so that digestive enzymes can gain access to the lipids. Mixing is made possible by means of the emulsifying action of bile. Bile can attract both hydrophilic and hydrophobic substances, thus dispersing and suspending fat droplets in the digestive juices. The second challenge is to transport lipids around the body via the watery body fluids. After digestion and absorption, lipids (triglycerides, phospholipids, and cholesterol) cluster together with protein, forming lipoprotein complexes called chylomicrons. These complexes can be transported in the aqueous media of lymph and blood without separating.

Objectives

After completing this section, you should be able to

• outline the steps in the digestion of lipids, identifying the sites, the substrates, the enzymes, and the products of digestion; describe the mechanism of lipid absorption for various types of lipids.
• describe the transport of lipids in the body, and discuss the significance of the different lipoproteins in cardiovascular disease.

Key Terms

After completing section 5.2, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 5: “Digestion, Absorption, and Transport of Lipids,” pages 144–150

Digestion, Absorption, and Transport of Lipids

Figure 5‑12 on page 145 outlines the steps of lipid digestion in the GI tract. After lipids are hydrolyzed into monoglycerides, glycerol, and free fatty acids in the small intestine, absorption takes place in one of two ways, depending on the size of the fat molecule. Glycerol and short‑ and medium‑chain fatty acids are absorbed directly into the bloodstream by the capillaries lining the small intestine. Long‑chain fatty acids are absorbed by the lymphatic system as chylomicrons. The text explains both systems briefly (p. 146).

Bile is also essential for fat absorption, and the text explains how bile can be reabsorbed and recycled (p. 146–147 and Figure 5‑16). Soluble dietary fibre from fruits, oats, and legumes can bind or trap bile in the intestine, effectively reducing the volume of bile that is recycled and increasing bile excretion through the feces. This signals the body to synthesize more bile from its precursor, cholesterol, which can result in the lowering of blood cholesterol level. This is one of the important health benefits of soluble dietary fibre that you learned in Unit 4.

Lipid transport through the bloodstream is made possible by a variety of protein‑lipid complexes known as lipoproteins. All lipoproteins contain triglycerides, phospholipids, cholesterol, and proteins in varying proportions. You are not required to remember the exact percentage composition for the four types of lipoproteins, but you should be aware of the most significant component of each lipoprotein, and should be able to relate lipoprotein levels to cardiovascular disease risk.

Chylomicrons, the lipoproteins formed in the mucosal cells after absorption of dietary lipids, are composed mainly of triglycerides, making them very low in density. After a person eats a meal, chylomicrons are first released into the lymphatic system, then into the bloodstream at a point near the heart; from there, they circulate to different parts of the body. Cells all over the body remove the lipids from the chylomicrons as they pass by. Liver and adipose cells are particularly active in taking up lipids from the chylomicrons.

Very‑low‑density lipoproteins (VLDL) are the lipoproteins formed in the liver. The liver picks up various compounds from the blood as well as excess carbohydrates, proteins, and alcohol, and converts them into triglycerides, cholesterol, fatty acids, and other compounds. Like chylomicrons, VLDL are composed of a high proportion of triglycerides. VLDL are transported throughout the body, where cells remove the triglycerides and use them for energy, storage, or synthesis of other body compounds. Removal of the triglycerides results in the formation of low‑density lipoproteins.

Low‑density lipoproteins (LDL) contain little triglyceride but much cholesterol. As LDL circulate throughout the body, peripheral tissues (outside the liver), such as muscles, arterial walls, adipose tissues, and mammary glands pick up cholesterol and phospholipids for synthesis of membranes, hormones, etc. Special LDL receptors in the liver normally remove LDL from circulation, thereby controlling the cholesterol concentration in the blood. Lack of LDL receptors results in high blood cholesterol levels, which greatly increase the risk of cardiovascular disease. Hypercholesterolemia, the condition of inadequate LDL receptors, tends to run in families.

High‑density lipoproteins (HDL) contain much protein. HDL differ from LDL in their destination in the body. Instead of transporting cholesterol from the liver to peripheral tissues, HDL carry cholesterol and phospholipids from peripheral tissues back to the liver for recycling or disposal. Consequently, people with high levels of HDL seem to have a lower risk of developing cardiovascular disease. Cholesterol can be excreted by the body via bile, and the liver synthesizes bile acids from cholesterol.

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5.3 Functions and Health Effects

Introduction

The study of lipids and their roles in health and disease has been the focus of research for many years, particularly since the relationship between the blood cholesterol level and heart disease was demonstrated. A phenomenal number of research studies have been reported, many of which are controversial. Nevertheless, strong scientific evidence confirms some of the health effects of consuming dietary fats. Most experts agree that sufficient data exist to support the general dietary recommendation to reduce total intake of fats—particularly of saturated fats—to prevent heart disease.

Objectives

After completing this section, you should be able to

• describe the roles of fat in the body.
• discuss the effects of diet on blood cholesterol levels, and the implication of high blood cholesterol for cardiovascular disease.
• describe a heart‑healthy diet and give examples of suitable food choices in such a diet.
• list and describe the nutritional roles of essential fatty acids—EPA and DHA—and identify the causes and health effects of essential fatty acid deficiency.

Key Terms

After completing section 5.3, you should be able to define and use the following terms in context:

Reading Assignment

• Chapter 5: “Lipids in the Body,” pages 150–153 (note: you read “Essential Fatty Acids” in section 5.1)
• Chapter 5: “Health Effects and Recommended Intakes of Saturated Fats, Trans Fats, and Cholesterol,” (up to “Recommended Intakes of Monounsaturated and Polyunsaturated Fats”), pages 153–157
• Chapter 5: “Highlight 5: High‑Fat Foods: Friend or Foe?” pages 168–174

How Lipids Affect Health

The majority of fat in the body is associated with energy storage and release. Fat also insulates and protect the body and its organs. Several specific functions, summarized on page 153 of the textbook (section headed “REVIEW IT”), are associated with the essential fatty acids and their related compounds. “A Preview of Lipid Metabolism” (pp. 152–153) describes how the body stores fat and uses it for energy. Unit 7 covers the metabolism of the energy-containing nutrients —carbohydrates, lipids, and proteins—in greater detail.

Dietary fat has been shown to play a significant role in the development of cardiovascular disease. Cardiovascular disease is multi‑factorial; it involves a complex interplay of genes, diet, and lifestyle; it is the major cause of death in North America. An elevated level of blood cholesterol, especially LDL cholesterol, is one of the major risk factors for heart disease. Volumes of research have shown that diet affects total blood cholesterol and affects the distribution of cholesterol in different types of lipoproteins.

In the body, endogenous cholesterol can supply as much as 70–80% of the total cholesterol. The rest comes from dietary cholesterol. The major dietary sources of cholesterol include eggs, milk products, meat, poultry, and shellfish. The body is normally able to regulate synthesis and excretion of cholesterol so that when dietary intakes are high, blood levels remain constant. Therefore, dietary cholesterol has a significant effect only on blood cholesterol in some people.

The type of dietary fat ingested has a greater influence on blood cholesterol levels than does dietary cholesterol. Saturated fats (such as coconut oil, palm oil, and animal fats) increase total blood cholesterol and LDL cholesterol. Therefore, reducing saturated fats is the most effective dietary way of decreasing blood cholesterol. Since animal fats are the principal source of saturated fat as well as the only dietary source of cholesterol, a diet low in animal fat is the best way to lower the intake of both saturated fat and cholesterol.

In contrast, polyunsaturated fats—particularly those rich in the omega‑6 fatty acids found in corn oil, sunflower oil, cottonseed oil, and safflower oil—lower blood cholesterol and LDL cholesterol. Sources of omega‑6 fatty acids are given on page 140.

The textbook emphatically states that saturated fats play a major causative role in heart disease (Figure H5‑1, page 171). This has been a widespread view for many years. In fact, the evidence supporting this is quite sparse. The strongest evidence of just how little role saturated fat plays in heart disease has come from cohort studies. In these studies, thousands of healthy people are tracked carefully over several years. Whereas these studies have consistently found a clear relationship between an array of dietary components (such as whole grains, fish, and alcohol) and risk of heart disease, they have failed to detect any such association between intake of saturated fat and risk of heart disease. Likewise, there is no evidence that people who consume eggs, a major source of cholesterol, are at increased risk of heart disease. The current consensus is that people should aim to reduce their intake of saturated fat and select foods rich in polyunsaturated fats (Mozaffarian, 2016). In other words, the current advice is not to simply “cut down on saturated fat” but rather to “replace some of the saturated fat with sources of polyunsaturated fats.”

The omega‑3 fats, most notably EPA and DHA, benefit the cardiovascular system, and regular intake appears to reduce risk of stroke and heart attack. The ways in which EPA and DHA protect against heart disease are listed on page 170.

Monounsaturated fatty acids—found in high quantities in olive oil and canola oil—are neutral in their effect on blood cholesterol levels; that is, they neither raise nor lower cholesterol levels. Diets rich in MUFA, like the Mediterranean diet, are associated with a lower risk of cardiovascular disease. As with polyunsaturated fats, consuming excessive amounts of monounsaturated fats may contribute to obesity.

It is now generally agreed that hydrogenated fats (and with them trans fat) should be reduced as much as possible because they increase the blood cholesterol level and are associated with increased risk of heart disease. Major sources are partially hydrogenated soft margarines, hard margarine, vegetable shortening, snack foods, and baked goods (e.g., cakes, cookies, and doughnuts). The quantity of these fats present in foods has been much reduced in recent years. Nutrition Facts labels on prepared foods now clearly identify trans-fat content.

Another dietary factor affecting the blood cholesterol level is soluble dietary fibre (pectin and gums). This class of fibre is thought to lower blood cholesterol by binding with bile acids, thus hindering their reabsorption. The body is then forced to synthesize additional bile acids from cholesterol.

Excess energy intake leading to obesity has been reported to increase total blood cholesterol and LDL cholesterol. Conversely, a decrease in body weight has been shown to decrease total blood cholesterol and increase HDL cholesterol.

In summary, a heart‑healthy diet has the following attributes:

• It is low in trans fat.It is low in saturated fat and rich in unsaturated fat, especially omega‑6 fatty acids.
• It contains a dietary source of DHA and EPA, provided by consuming fatty fish at least twice per week.
• It is rich in a variety of dietary fibre.
• It supplies an energy intake that supports a healthy BMI.

Eating Well with Canada’s Food Guide is an ideal guide to plan and shop for a heart‑healthy diet (textbook pp. 168–174).

These themes are explored in greater depth in Athabasca University’s Nutrition 405: Nutrition in Health and Disease. Your Nutrition 330 textbook discusses heart disease in more detail on pages 636–643, but this section is not required reading.

Dietary fats have been linked to other aspects of health in addition to heart disease. The textbook summarizes what we know about the possible role of dietary fats in cancer (pp. 156–157). The textbook also refers to the possible role of dietary fat in contributing to obesity (p. 157). The evidence is quite weak that a high‑fat diet plays an important role in obesity.

Essential Fatty Acids

Functions of essential fatty acids:

• They are constituents of lipoproteins that are essential for fat transport.
• They are required to maintain the function and integrity of cellular membranes.
• The 20‑carbon fatty acids of both series (arachidonic acid and EPA) are precursors of the hormone‑like compounds called eicosanoids. They function by regulating many of the body systems: blood pressure, nerve impulses, inflammation responses, hormone responses, blood clotting, the immune response, and the inflammation response to injury and infection.

In recent years, much attention has been given to the nutritional importance of the omega‑3 fatty acids, particularly EPA and DHA, which can be produced in the body in limited amounts from linolenic acid (an 18‑carbon PUFA). Because synthesis of EPA and DHA is very limited, it is recommended that people consume dietary sources of these fatty acids.

DHA (22‑carbon fatty acid) has been found to concentrate in the photoreceptor membranes of the retina and in neural membranes of the brain, suggesting its role in retinal and brain development during rapid growth—especially in the third trimester of pregnancy and in infancy. Human breast milk contains DHA as well as EPA to meet the needs of the newborn infant.

Deficiency of essential fatty acids is relatively rare in populations that consume a mixed diet. However, cases of deficiency have occurred in

• infants fed with a low‑fat formula or skim milk. Note: The requirement for essential fatty acids is high during periods of rapid growth, especially during the third trimester of pregnancy and in infancy.
• patients suffering from serious burns, with loss of body fluids.
• people with bowel resections, who may suffer from severe fat malabsorption.
• children or adults on long‑term, low‑fat, total parenteral nutrition (TPN), or gastric tube feedings.

Classic signs of essential fatty acid deficiency in humans are

• growth retardation.
• skin lesions with characteristic dermatitis (eczema) and dry, scaly skin.
• increased susceptibility to infections.
• possible peripheral neuropathy and blurred vision.

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5.4 Lipids in the Diet

Introduction

Consumers are increasingly aware of the adverse effects of a high fat intake. However, it is all too easy to overconsume fat. This is because fat is widespread in our food supply, especially in fast foods and processed foods, which are very much a part of the modern lifestyle.

Objectives

After completing this section, you should be able to

• describe the pattern of fat consumption in Canada.
• state the current recommendations for fat consumption and describe the food pattern outlined in Canada’s Food Guide that achieves these recommendations.
• identify dietary sources of SFA, MUFA, PUFA, EPA, and DHA.

Fat in the Canadian Diet

Surveys of the typical Canadian diet indicate a substantial decrease in fat intake since 1970. Whereas fat supplied 40% of energy in 1970, by 1997 the figure had fallen to 30% (Grey‑Donald et al., 2000). Ten percent of the energy in the Canadian diet comes from saturated fat. The average cholesterol intake is about 200–400 mg per day.

Reading Assignment

• Chapter 5: “Recommended Intakes of Fat” (to “Fat Replacers”), pages 157–161

Recommendations for Fat Intake

The chart on page 159 of the textbook emphasizes a reduced intake of fats from dairy foods and meat. But, as noted earlier, recent findings dispute the value of that advice, at least with regard to risk of heart disease. There is growing evidence that many people should reduce their intake of meat, especially processed meat such as bacon, ham, and salami.

In recent years, consumption of animal fats has decreased as people have switched to leaner meats and low‑fat milk. At the same time, intake of hidden fat in such foods as cakes, cookies, French fries, and salad dressing has probably been rising. As a result, the latter foods may now be the main dietary source of saturated fat.

There is no RDA for fat as a general nutrient class. However, there are suggested minimum and maximum intakes for total fat and specific recommendations for the essential fatty acids. Recommendations for intake of dietary fat are continually changing as new information becomes available; it is important to stay informed. The current recommendations are summarized below:

• DRI (or AMDR) for total fat is 20–35% of total energy. For a 2000‑kcal diet, recommended fat intake is 45–75 grams per day.
• There is no DRI for SFA, but many health organizations, including the Canadian Diabetes Association, recommend that SFA intake be less than 10% of total energy.
• Recommendations for PUFA are broken down into two groups:
  1. Linoleic acid (omega‑6): DRI or AMDR is 5–10% of total energy. The text also lists an AI of 17 grams/day for men and 12 grams/day for women.
  2. Linolenic acid (omega‑3): DRI or AMDR is 0.6–1.2% of total energy. The text lists AI of 1.6 and 1.1 grams per day for men and women, respectively.

The omega‑6 to omega‑3 ratio should range from 4:1 to no more than 10:1. These two fatty acids must be consumed in proper balance because they have different abilities to stimulate or slow down processes in the body. An important example is inflammatory processes: omega‑6 metabolites are significantly more inflammatory than omega‑3 metabolites.

• MUFA should supply the remaining energy from fat, or about 10–15% of total energy.
• There is no DRI for EPA and DHA (both are in the omega‑3 family). The Canadian Heart and Stroke Foundation suggests that dietary sources of EPA and DHA (i.e., fatty fish two to three times per week) are safe and effective; the Heart and Stroke Foundation has not endorsed a specific amount of fish oil. Consuming fatty fish two times per week, as recommended in Canada’s Food Guide, will provide a sufficient amount of EPA and DHA for normal adults.
• Some trans fat occurs naturally in beef and dairy products. It is recommended that trans fat from processed food be limited as much as possible.
• There is no specific recommendation for cholesterol intake.

Sources of Specific Dietary Fats

Sources of the various dietary fats are found throughout Chapter 5 in the textbook. The dietary sources of SFA, trans fat, and cholesterol are listed in the margins on pages 154 and 155. Figure 5‑6 (p. 140) compares the linoleic (omega‑6) and linolenic (omega‑3) acid levels in common dietary fats. The average Canadian diet contains a plentiful supply of omega‑6 fatty acids, as most vegetable oils contain ample amounts. Omega‑3 fatty acids are less plentiful; they are restricted to flaxseed, canola, soybean, and walnut oils. The omega‑3 fatty acid in these vegetable oils (linolenic acid) is sometimes listed as ALA (alpha‑linolenic acid) in Nutrition Facts labels. DHA and EPA are also omega‑3 fatty acids, but they are found only in fatty fish or their oils; fish obtain them from the algae in their diet. Table 5‑2, on page 156, lists the sources of omega‑6 and omega‑3 fatty acids.

Food manufacturers are capitalizing on the consumer demand for healthy omega‑3 fats by adding flaxseed oil and/or fish oil to foods, often including a diet‑related health claim like “DHA, an omega‑3 fatty acid, supports the normal development of the brain, eyes, and nerves.” Some examples of these “functional foods” include Becel Omega3Plus margarine, Naturegg Omega 3 eggs, Yoplait Source Omega 3 yogourt, and Neilson Dairy Oh! milk. Keep in mind that a food labelled as containing omega‑3 may not necessarily contain EPA and DHA. You always need to look closely at both the Nutrition Facts and the ingredient list.

This section on recommendations for fat intake may seem overwhelming. For this course, you should remember the AMDR for total fat and SFA and the general recommendations for and sources of trans fat, and the omega‑6 and omega‑3 fatty acids. You should also be able to apply Canada’s Food Guide to make healthy choices with regard to fat. Reading Nutrition Facts labels and ingredient lists is becoming an essential part of choosing foods wisely when it comes to fat.

Study Questions

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References