Lipids
Unfolding Case
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Dylan Masters
Dylan is a 7-year-old boy who has up to 20 seizures a day due to epilepsy. Because antiseizure medications have failed to control his seizures, he is a candidate for a ketogenic diet. Although there are several different levels of the diet, it is characterized as a high-fat, adequate-protein, very low carbohydrate diet. Dylan’s classic ketogenic diet will provide 1200 calories, 120 g of fat, 18 g of protein, and 12 g of carbohydrate. The family has undergone extensive counseling and has agreed to try the diet for at least 12 weeks; they understand the risks and that all foods must be carefully prepared and weighed on a gram scale. Dylan will be under close medical and nutritional supervision and will start the diet in the hospital where he will be closely monitored by a neurologist and registered dietitian. Lab work will be used to identify metabolic abnormalities and evaluate serum nutrient levels.
Check Your Knowledge
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Learning Objectives
Upon completion of this chapter, you will be able to
1 Compare saturated, monounsaturated, and polyunsaturated fatty acids.
2 Propose ways to limit saturated fat present in a sample eating pattern.
3 Explain the functions of fat in the body.
4 Discuss the digestion and absorption of fat.
5 Give examples of foods that provide omega-3 fatty acids.
6 Discuss strategies for decreasing saturated fat intake and increasing oil intake within the context of a healthy eating pattern.
7 Explain why there is no Recommended Dietary Allowance for total fat, trans fat, and saturated fat.
“Avoid too much fat” appeared in the first edition of the Dietary Guidelines for Americans published in 1980 (http://health.gov/dietaryguidelines/1980thin.pdf). Since then, decades of study have shown that the relationship between fat and health is far more complex than that statement implies. The evolving understanding of individual fatty acids and fatty acid groups is emerging as a key factor in health (Vannice & Rasmussen, 2014). Some fatty acids have a positive impact on health (e.g., omega-3 fatty acids) and should be eaten in moderation; other fatty acids have a negative impact on health (e.g., trans fats) and should be limited. Eating less of some fatty acid groups and more of others is a recommendation among many American and international health and government agencies. There are three classes of lipids, which are referred to as fat throughout the rest of this chapter and book: triglycerides (fats and oils), which account for 98% of the fat in food; phospholipids (e.g., lecithin); and sterols (e.g., cholesterol). This chapter describes the classes of fats, their dietary sources, and how they are handled in the body. The functions of fat are presented, as are recommendations regarding intake.
Lipids a group of water-insoluble, energy-yielding organic compounds composed of carbon, hydrogen, and oxygen atoms.
TRIGLYCERIDES
Chemically, triglycerides are made of the same elements as carbohydrates, namely, carbon, hydrogen, and oxygen. Because there are proportionately more carbon and hydrogen atoms to oxygen atoms, triglycerides yield more calories per gram than carbohydrates. Structurally, triglycerides are composed of a three-carbon atom glycerol backbone with three fatty acids attached (Fig. 4.1). An individual triglyceride molecule may contain one, two, or three different types of fatty acids.
Triglycerides a class of lipids composed of a glycerol molecule as its backbone with three fatty acids attached.
Fatty Acids
Fatty acids are basically chains of carbon atoms with hydrogen atoms attached (Fig. 4.2). At one end of the chain is a methyl group (CH3), and at the other end is an acid group (COOH). Fatty acids are commonly abbreviated by a C followed by the number of carbon atoms, a colon, and the number of double bonds. For example, stearic acid, an 18-carbon length fatty acid with no double bonds, is abbreviated as C18:0. The most common fatty acids and their sources are listed in Table 4.1.
Fatty Acids organic compounds composed of a chain of carbon atoms to which hydrogen atoms are attached. An acid group (COOH) is attached at one end, and a methyl group (CH3) at the other end.
Glycerol a three-carbon atom chain that serves as the backbone of triglycerides.
Fatty acids attach to glycerol molecules in various ratios and combinations to form a variety of triglycerides within a single food fat. The types and proportions of fatty acids present influence the sensory and functional properties of the food fat. For instance, butter tastes and acts differently from corn oil, which tastes and acts differently from lard. Fatty acids vary in the length of their carbon chain and in the degree of unsaturation. These variables account for the differences in physiologic function and impact on disease risk seen between individual fatty acids (Vannice & Rasmussen, 2014).
 Figure 4.1 ▲ Generic triglyceride molecule. |
Carbon Chain Length
Almost all naturally occurring fatty acids have an even number of carbon atoms in their chain, generally between 4 and 24. Long-chain fatty acids (containing more than 12 carbon atoms) predominate in meats, fish, and vegetable oils and are the most common length fatty acid in the diet. Smaller amounts of medium-chain (8-12 carbon atoms) and short-chain (up to 6 carbon atoms) fatty acids are found primarily in dairy products. The body absorbs short- and medium-chain fatty acids differently than long-chain fatty acids.
 Figure 4.2 ▶ Fatty acid configurations. |
Degree of Saturation
As dictated by nature, each carbon atom in a fatty acid chain must have four bonds connecting it to other atoms. When all the carbon atoms in a fatty acid have four single bonds each, the fatty acid is saturated with hydrogen atoms. The majority of naturally occurring saturated fatty acids are straight-line molecules that can pack tightly together; thus, they are solid at room temperature.
Saturated Fatty Acids fatty acids in which all the carbon atoms are bonded to as many hydrogen atoms as they can hold so no double bonds exist between carbon atoms.
Unsaturated Fatty Acids fatty acids that are not completely saturated with hydrogen atoms, so one or more double bonds form between the carbon atoms.
Monounsaturated Fatty Acids fatty acids that have only one double bond between two carbon atoms.
Polyunsaturated Fatty Acids fatty acids that have two or more double bonds between carbon atoms.
An “unsaturated” fatty acid does not have all the hydrogen atoms it can potentially hold; therefore, one (monounsaturated) or more (polyunsaturated) double bonds form between carbon atoms in the chain. Because of the double bond, unsaturated fatty acids are physically kinked and unable to pack together tightly; they are liquid at room temperature and are referred to as “oils.”
All food fats contain a mixture of saturated, monounsaturated, and polyunsaturated fatty acids. When applied to sources of fat in food, “unsaturated” and “saturated” are not absolute terms used to describe the only types of fatty acids present; rather, they are relative descriptions that indicate which kinds of fatty acids are present in the largest proportion. For instance, butter is classified as a saturated fat; however, 34% of
its fatty acids are unsaturated. Similarly, olive oil is known as a monounsaturated fat because 77% of its fatty acids are monounsaturated, not because all of them are.
its fatty acids are unsaturated. Similarly, olive oil is known as a monounsaturated fat because 77% of its fatty acids are monounsaturated, not because all of them are.
Table 4.1 The Most Common Fatty Acids in Food | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Saturated Fats
Fats with a high percentage of saturated fatty acids are referred to as solid fats because they are solid at room temperature. Saturated fatty acids occur to the greatest extent in animal fats—the fat in meats, egg yolks, and whole-milk dairy products (the fat in milk does not appear as a solid due to the process of homogenization). The only vegetable oils that are saturated are palm oil, palm kernel oil, and coconut oil.
As a category, saturated fat is commonly known as a “bad” fat based on early studies that showed a correlation between dietary saturated fat and coronary heart disease (Kato, Tillotson, Nichaman, Rhoads, & Hamilton, 1973). More recent studies have shown a positive, inverse, or no association between dietary saturated fat and cardiovascular disease (CVD) morbidity and/or mortality (Siri-Tarino, Sun, Hu, & Krauss, 2010). In reality, individual saturated fatty acids do not all have the same impact on low-density lipoprotein (LDL) cholesterol. The saturated fatty acids lauric, myristic, and palmitic acids increase LDL cholesterol (Nicolosi, 1997), whereas the saturated fatty acid stearic acid has a neutral impact on LDL cholesterol (Grande, Anderson, & Keys, 1970). However, stearic acid is found in foods that also provide the saturated fatty acids that raise LDL cholesterol. Thus, it is recommended that the intake of foods high in saturated fat be limited even though not all saturated fatty acids may be considered “bad.”
Low-Density Lipoprotein (LDL) Cholesterol the major class of atherogenic lipoproteins that carry cholesterol from the liver to the tissues.
Unsaturated Fatty Acids
Categories of unsaturated fatty acids are polyunsaturated (PUFA) and monounsaturated (MUFA), commonly known as “good fats.” Strong consistent evidence shows that replacing saturated fat with unsaturated fat, especially PUFA, is associated with a decrease in LDL cholesterol and lowers the risk of cardiovascular events and death (U.S. Department of Health and Human Services [US-DHHS] & U.S. Department of Agriculture [USDA], 2015). There is some evidence that MUFA also lower CVD risk, but it is not as strong. MUFAs are the predominate fat in olives, olive oil, canola oil, avocado, peanut oil, and most other nuts. Meat fat contains moderate amounts of monounsaturated fats, providing approximately 50% of MUFAs in a typical American eating pattern (National Research Council, 2005). PUFAs are less ubiquitous than monounsaturated fats. They are the predominate fat in corn, soybean, safflower, and cottonseed oils and also in fish.
Unsaturated fatty acids can be classified according to the location of their double bonds along the carbon chain. The most common method of identifying the bond is to count the number of carbon atoms from the methyl (CH3) end, as denoted by the term “n” or “omega.” A PUFA with its first double-bond three carbons from the methyl end is an omega-3 or n-3 fatty acid. Likewise, an omega-6 or n-6 PUFA has its first double-bond 6 carbons from the methyl end. Omega-9 or n-9 fatty acids are monounsaturated fats.
Omega-3 (n-3) Fatty Acid an unsaturated fatty acid whose endmost double bond occurs three carbon atoms from the methyl end of its carbon chain.
Omega-6 (n-6) Fatty Acid an unsaturated fatty acid whose endmost double bond occurs six carbon atoms from the methyl end of its carbon chain.
Essential Fatty Acids fatty acids that cannot be synthesized in the body and thus must be consumed through food.
The location of the first double bond is significant because it determines the essentiality of a fatty acid. The body is unable to synthesize fatty acids with double bonds closer than n-9, so one n-6 fatty acid (linoleic acid) and one n-3 fatty acid (alpha-linolenic acid) are essential fatty acids and must be consumed through food. MUFAs are n-9 fatty acids and are not essential because they can be synthesized in the body.
Linoleic Acid. Linoleic acid, the essential n-6 PUFA, is the most highly consumed PUFA in Western diets. The richest sources are soybean, corn, and safflower oils; poultry, nuts, and seeds are also sources. The body can make other n-6 fatty acids, such as arachidonic acid, from linoleic acid. However, if a deficiency of linoleic acid develops, arachidonic acid becomes “conditionally essential” because the body is unable to synthesize it without a supply of linoleic acid.
Alpha-Linolenic Acid. Alpha-linolenic acid, the essential n-3 fatty acid, is the most prominent n-3 fatty acid in most Western diets. It is found in walnuts, flaxseed, chia and hemp seeds, and canola and soybean oils.
To a very limited extent, humans can convert alpha-linolenic acid to the n-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These two n-3 fatty acids are commonly referred to as “fish oils” because they are primarily found in fatty fish, especially salmon, anchovy, sardines, tuna, herring, and mackerel. Food products fortified with EPA and/or DHA are available, such as soy milks, cooking oils, margarine-like spreads, breakfast cereals, baked goods, infant formulas, and baby food and juices.
Fish Oils a common term for the long-chain, polyunsaturated omega-3 fatty acids EPA and DHA found in the fat of fish, primarily in cold-water fish.
Omega-3 fatty acids are probably best known for their heart health benefits, which are attributed to their anti-inflammatory, anticlotting, and anti-arrhythmic effects. Other potential benefits include improvements in symptoms related to hypertension, depression, joint pain, and other rheumatoid issues (International Food Information Council Foundation [IFICF], 2014).
Stability of Fats
Although all fats can become oxidized when exposed to light and oxygen over time, the greater the number of double bonds, the greater is the susceptibility to rancidity. Therefore, polyunsaturated fats are most susceptible to rancidity, saturated fats are least susceptible, and monounsaturated fats are somewhere in between.
Rancidity the chemical change that occurs when fats are oxidized, which causes an offensive taste and smell and the loss of fat-soluble vitamins A and E.
Hydrogenation a process of adding hydrogen atoms to unsaturated vegetable oils (usually corn, soybean, cottonseed, safflower, or canola oil), which reduces the number of double bonds; the number of saturated and monounsaturated bonds increases as the number of polyunsaturated bonds decreases.
Cis Fats unsaturated fatty acids whose hydrogen atoms occur on the same side of the double bond.
Trans Fats unsaturated fatty acids that have at least one double bond whose hydrogen atoms are on the opposite sides of the double bond; “trans” means across in Latin.
To help extend the shelf life of foods, manufacturers may add antioxidants, such as butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), to polyunsaturated fat-rich foods and oils. Another commercial method to make oils more stable is hydrogenation.
Hydrogenation
Hydrogenation is a process that adds hydrogen atoms to polyunsaturated oils to saturate some of the double bonds so that the resulting product is less susceptible to rancidity and has improved function. Hydrogenation varies in degrees from “light” to “partial” according to the desired outcome. Lightly hydrogenated oils are more stable than polyunsaturated oils because they have fewer double bonds but are still in liquid form. Partial hydrogenation results in a more solid (more saturated) product, such as stick margarine and shortening, yet still maintains some unsaturated (double) bonds. Fully hydrogenated products are virtually completely saturated.
Initially, hydrogenated fats appeared to be superior to both saturated and unsaturated fats, albeit for different reasons. Compared to the saturated fats in butter and lard, hydrogenated fats seemed more heart healthy because they still provide some unsaturated fatty acids. And compared to liquid oils, products made with partially hydrogenated fats have a longer shelf life and improved functionality. Hydrogenated fats make pie crusts flakier, French fries crispier, and frosting creamier. Seemingly a have-your-cake-and-eat-it-too type of product, partially hydrogenated fats permeated the food supply and quickly became a dietary staple in the 1970s.
However, the process of hydrogenation changes the placement of the hydrogen atoms around the remaining double bonds from the natural cis position to the rare trans position (Fig. 4.3). Lightly or partially hydrogenated margarines, shortenings, and oils as well as processed foods made with those products, such as fried foods (doughnuts, French fries, potato chips), baked goods (cakes, cookies, pie crusts), and frozen pizza became the major source of artificial trans fats in the food supply. Because fully hydrogenated fats are completely saturated, they are trans fat free. Only small amounts of trans fats occur naturally in some animal foods, such as beef, lamb, and dairy products.
Trans Fats
Over time, mounting evidence revealed that trans fats increase LDL cholesterol and reduce high-density lipoprotein (HDL) cholesterol, the lipoprotein that is protective against CVD. Even at low intake levels, trans fat intake is associated with abnormal lipid levels, inflammation, and increased cardiovascular mortality; growing evidence also suggests an increased risk of diabetes, cancer, and stroke (Kiage et al., 2013). Changes in food manufacturing have reduced the prevalence of trans fats in foods. By limiting processed foods and fast foods that are made with light or partially hydrogenated oils, consumers should be able to ideally lower their trans fat intake to <1% of total calories (Vannice & Rasmussen, 2014). Currently, manufactured and natural trans fats account for 3.7% of total calories consumed.
 Figure 4.3 ▶ Cis and trans fatty acid configuration. |
Trans fatty acid content is listed on the “Nutrition Facts” label. However, because amounts less than 0.5 g per serving can be rounded down to zero, the label can claim to have “zero trans fats” even if partially hydrogenated oils or shortenings appear on the ingredient list. And although 0.5 g trans fat per serving may sound insignificant, it can add up. For instance, most people will eat 3 cups of microwave popcorn at a time, which is actually three servings according to the label. At slightly under 0.5 g trans fat per serving, the total trans fat intake comes to almost 1.5 g. Only when the label says “no trans fats” is it actually free of trans fat.
Based on extensive research and public input, the U.S. Food and Drug Administration (FDA) released a final determination in June 2015 that states partially hydrogenated oils are no longer Generally Recognized as Safe (GRAS) (FDA, 2016). A 3-year compliance period gives food manufacturers time to reformulate products without partially hydrogenated oils and/or petition the FDA to permit specific uses of partially hydrogenated oils. In the meantime, consumers are urged to read ingredient labels and choose products that do not contain partially hydrogenated oils. Because trans fatty acids occur naturally in meat and dairy products, they will not be eliminated from the food supply.
Generally Recognized as Safe (GRAS) compounds exempt from the definition of “food additive” because they are generally recognized as safe based on “a reasonable certainty of no harm from a product under the intended conditions of use.”
FUNCTIONS OF FAT IN THE BODY
The primary function of fat is to fuel the body. At rest, fat provides about 60% of the body’s calorie needs. All fat, whether saturated or unsaturated, cis or trans, provides 9 cal/g, more than double the amount of calories as an equivalent amount of either carbohydrate or protein. Although fat is an important energy source, it cannot meet all of the body’s energy needs because certain cells, such as brain cells and cells of the central nervous system, normally rely solely on glucose for energy.
Fat has other important functions in the body. Fat deposits insulate and cushion internal organs to protect them from mechanical injury. Fat under the skin helps to regulate body temperature by serving as a layer of insulation against the cold. And dietary fat facilitates the absorption of the fat-soluble vitamins A, D, E, and K when consumed at the same meal.
Specific types of fatty acids have particular functions in the body. For instance,
Saturated fatty acids provide structure to cell membranes and facilitate normal function of proteins.
MUFAs are components of lipid membranes, especially nervous tissue myelin.
Both essential fatty acids play a role in maintaining healthy skin and promoting normal growth in children.
Omega-6 PUFAs are involved in the synthesis of fatty acids, are components of cell membranes, and play a role in cell signaling pathways.
Arachidonic acid and EPA are precursors of eicosanoids (e.g., prostaglandins, thromboxanes, and leukotrienes), a group of hormone-like substances that help regulate blood pressure, blood clotting, and other body functions. Observational and randomized controlled trial evidence suggest that fish or fish oil intake may reduce inflammation, improve endothelial function, and normalize variations in heart rate and, at high doses, inhibit platelet aggregation (Mozaffarian, Appel, & Van Horn, 2011).
EPA and DHA may play a role in preventing and treating heart disease through their anti-inflammatory, antiarrhythmic, and anticlotting effects (IFICF, 2014). They are essential for normal growth and development. DHA is a structural component of red blood cell membranes and is abundant in retinal tissue, neuron cells, the liver, and testes.
Phospholipids
Like triglycerides, phospholipids have a glycerol backbone with fatty acids attached. What makes them different from triglycerides is that a phosphate group replaces one of the fatty acids. Although phospholipids occur naturally in almost all foods, they make up a very small percentage of total fat intake.
Phospholipids a group of compound lipids that is similar to triglycerides in that they contain a glycerol molecule and two fatty acids. In place of the third fatty acid, phospholipids have a phosphate group and a molecule of choline or another nitrogen-containing compound.
Emulsifier a stabilizing compound that helps to keep both parts of an emulsion (oil and water mixture) from separating.
Phospholipids are both fat soluble (because of the fatty acids) and water soluble (because of the phosphate group), a unique feature that enables them to act as emulsifiers. This role is played out in the body as they emulsify fats to keep them suspended in blood and other body fluids. As a component of all cell membranes, phospholipids not only provide structure but also help to transport fat-soluble substances across cell membranes. Phospholipids are also precursors of prostaglandins.
Lecithin is the best-known phospholipid. Claims that it lowers blood cholesterol; improves memory; controls weight; and cures arthritis, hypertension, and gallbladder problems are unfounded. Studies show no benefit from taking supplements because lecithin is digested in the gastrointestinal tract into its component parts and is not absorbed intact to perform super functions. Lecithin is not even an essential nutrient because it is synthesized in the body. Many people who take lecithin supplements do not realize that they provide 9 cal/g, just like all other fats.
Cholesterol
Cholesterol is a sterol, a waxy substance whose carbon, hydrogen, and oxygen molecules are arranged in a ring. Cholesterol occurs in the tissues of all animals. It is found in all cell membranes and in myelin. Brain and nerve cells are especially rich in cholesterol. The body synthesizes bile acids, steroid hormones, and vitamin D from cholesterol. Although cholesterol is made from acetyl-coenzyme A (acetyl-CoA), the body cannot break down cholesterol into CoA molecules to yield energy, so cholesterol does not provide calories.
Sterols one of three main classes of lipids that include cholesterol, bile acids, sex hormones, the adrenocortical hormones, and vitamin D.
Cholesterol is found exclusively in animals, with organ meats and egg yolks the richest sources. The cholesterol in food is just cholesterol; descriptions of “good” and “bad” cholesterol refer to the lipoprotein packages that move cholesterol through the blood (see Chapter 20). You cannot eat more “good” cholesterol, but you can make lifestyle changes, such as quitting smoking, exercising, and losing weight if overweight, that increase the amount of “good” cholesterol in the blood.
Because all body cells are capable of making enough cholesterol to meet their needs, cholesterol is not an essential nutrient. In fact, daily endogenous cholesterol synthesis is approximately two to three times more than average cholesterol intake. When dietary cholesterol decreases, endogenous cholesterol production increases to maintain an adequate supply. The body makes cholesterol from acetyl-CoA, which can originate from carbohydrates, protein, fat, or alcohol. Thus, eating an excess of calories, regardless of the source, can increase cholesterol synthesis.
HOW THE BODY HANDLES FAT
Digestion
A minimal amount of chemical digestion of fat occurs in the mouth and stomach through the action of lingual lipase and gastric lipases, respectively (Fig. 4.4).
 Figure 4.4 ▶ Fat digestion. |
Fat entering the duodenum stimulates the release of the hormone cholecystokinin, which in turn stimulates the gallbladder to release bile. Bile, an emulsifier produced in the liver from bile salts, cholesterol, phospholipids, bilirubin, and electrolytes, prepares fat for digestion by suspending the hydrophobic molecules in the watery intestinal fluid. Emulsified fat particles have enlarged surface areas on which digestive enzymes can work.
Most fat digestion occurs in the small intestine. Pancreatic lipase, the most important and powerful lipase, splits off one fatty acid at a time from the triglyceride molecule, working from the outside in until two free fatty acids and a monoglyceride remain. Usually, the process stops at this point, but sometimes, digestion continues and the monoglyceride splits into a free fatty acid and a glyceride molecule. The end products of digestion—mostly monoglycerides with free fatty acids and little glycerol—are absorbed into intestinal cells. It is normal for a small amount of fat (4-5 g) to escape digestion and be excreted in the feces.
Monoglyceride a glyceride molecule with only one fatty acid attached.
The digestion of phospholipids is similar, with the end products being two free fatty acids and a phospholipid fragment. Cholesterol does not undergo digestion; it is absorbed as is.
Absorption
About 95% of consumed fat is absorbed, mostly in the duodenum and jejunum. Small fat particles, such as short- and medium-chain fatty acids and glycerol, are absorbed directly through the mucosal cells into capillaries. They bind with albumin and are transported to the liver via the portal vein.
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