Unsweetened coconut, mayonnaise, sour cream, blue cheese, salad dressing, almonds, pecans, olives, avocados, and sausages—what do all these foods have in common? More than 50% of the kilocalories in each of these foods come from fat, a vital nutrient in our diet.
Added fats and oils provide more kilocalories in the average American diet than any other food group. Examination of U.S. food supply trends indicates total fat intake, which remains at a high level, increased slightly since 2000. The trend reflects a very slight increase in fats provided by vegetable oils (Fig. 6-1).1 Between 1977 and 2008, daily consumption of total fat declined from 39.7% to 33.4% of total caloric intake.2 Consumers have become more aware of healthy food choices, but changes in eating patterns are difficult. Food manufacturers, producers, and grocers have responded to concerns by (a) trimming fat from meats, (b) providing leaner cuts of beef and pork, (c) replacing tropical oils and trans fats in processed foods, and (d) manufacturing foods containing less fat. In addition, some consumers have increased their consumption of fish and poultry and substituted lower fat milk for whole milk. The fat content of very lean beef and pork cuts currently compares favorably with a skinless chicken breast. Added fats and oils provide most of the kilocalories Americans consume (Fig. 6-2).
Fats in the diet should actually be called lipids. Lipids contain the same three elements as carbohydrates: carbon, hydrogen, and oxygen. Lipids contain less oxygen in proportion to hydrogen and carbon than carbohydrates. The structure and function of lipids are covered in detail in Chapter 2 and on the Evolve website. Because of their structure, they provide more energy per gram than either carbohydrates or proteins.
The two classes of water-insoluble substances are (a) simple lipids, or triglycerides, which occur in foods and in the body, and (b) structural lipids, which are produced by the body for specific functions. The structural component of lipids is fatty acids. Triglycerides with one or more of the fatty acids replaced with carbohydrate, phosphate, or nitrogenous compounds are called compound lipids. Dietary lipids used physiologically include triglycerides, fatty acids, phospholipids, and cholesterol. Lipoproteins are found solely in the body.
A fatty acid is a chain of carbon atoms attached to hydrogen atoms with an acid grouping on one end. Glycerol is the alcohol portion of a triglyceride to which the fatty acids attach. Triglycerides are the most common fat present in animal or protein foods (Fig. 6-3). Monoglycerides and diglycerides are found in the small intestine and result from the breakdown of triglycerides during digestion. Free fatty acids, monoglycerides, and glycerol can cross cell membranes.
Each of the three fatty acids attached to the triglyceride can be different: they can be long, medium, or short, and saturated or unsaturated. Medium-chain and short-chain fatty acids are readily digested and absorbed, but most fats in foods (especially vegetable fats) contain predominantly long-chain fatty acids. Short-chain fatty acids contain less than six carbon atoms, medium-chain fatty acids contain 6 to 10 carbon atoms, and long-chain fatty acids contain 12 or more carbon atoms.
As discussed in Chapter 2, fatty acids are classified according to their degree of saturation. Saturation of a fatty acid depends on the number of hydrogen atoms attached to the carbon chain. Saturated fatty acids (SFAs) contain only single bonds, with each carbon atom having two hydrogen atoms attached to it (see Chapter 2, Fig. 2-15). Palmitic and stearic acids (see Chapter 2, Table 2-4), the two most prevalent SFAs, are structural components of tooth enamel and dentin.
When adjacent carbon atoms are joined by a double bond because two hydrogen atoms are lacking, there is a gap between the hydrogen atoms in the chain; it is called an unsaturated fatty acid. Monounsaturated fatty acids (MUFAs) contain only one double bond (see Chapter 2, Fig. 2-12). The most abundant MUFA is oleic acid. Oleic acid is also a structural component of the tooth.
Hydrogenation is a commercial process in which vegetable oil is converted to a solid margarine or shortening by adding hydrogen to the oil. This process results in naturally unsaturated vegetable oils being changed to a SFA by changing unsaturated bonds to saturated bonds. Hydrogenation can be controlled, so “tub” or “soft” margarine is “partially hydrogenated,” or not completely saturated. The hydrogenation process not only increases the proportion of SFAs, but also changes the shape of the fatty acid. When the hydrogen atoms are rotated so that they are on opposite sides of the bond, in the “trans” position (see Chapter 2, Fig. 2-16), the fatty acid is called a trans fatty acid. Partial hydrogenation results in large numbers of fatty acids having this altered shape. Foods with trans fatty acids have a longer shelf life, and flavors are stable. The most common trans fatty acid is elaidic acid, found in partially hydrogenated vegetable oils, such as tub margarines and cooking oils. A naturally-occurring trans fatty acid, vaccenic acid, with double bonds on adjacent carbons, is present in small amounts in milk and meat of ruminants (cows, sheep, and deer). Limited research suggests that these trans fats may possibly have health-enhancing potential.
When carbons in a fatty acid are connected by two or more double bonds, the fatty acid is polyunsaturated (see Chapter 2, Fig. 2-13). Linoleic acid and arachidonic acid are polyunsaturated fatty acids (PUFAs). These PUFAs are omega-6 fatty acids. Their first double bond is on the sixth carbon from the omega (terminal) end; they are also referred to as n-6 PUFAs.
Omega-3 fatty acids, or α-linolenic acids, make up another class of PUFAs. As shown in Chapter 2, Figure 2-13A, these fatty acids are unique in that the first double bond is located three carbon atoms from the omega end of the molecule; hence they are called omega-3s or n-3s. Omega-3 fatty acids include α-linolenic acid, which has 18 carbon atoms and two double bonds, and eicosapentaenoic acid (EPA), which has 20 carbon atoms and five double bonds.
The carbon chain length and degree of saturation determine various properties of fats, including their flavor and hardness, or melting point (the temperature at which a product becomes a liquid). Most SFAs are solid at room temperature; e.g., animal fats, being solid at room temperature, are predominantly saturated fats. Short-chain fatty acids (6 carbon atoms or less), MUFAs, and PUFAs that are liquid at room temperature are oils. Milk fat contains a large amount of short-chain SFAs.
Fats with a high proportion of unsaturated fatty acids may deteriorate or become rancid, resulting in unpleasant flavors and odors. Fats become rancid when subject to high temperatures and exposure to light, which cause oxidation and decomposition of fats. The decomposition results in peroxides that may be toxic in large amounts. Vitamin E, a fat-soluble vitamin, is an antioxidant and, to some degree, protects the oil to which it is added. However, in doing so, vitamin E is inactivated and cannot then be used by the body.
Phospholipids contain phosphorus and a nitrogenous base in addition to fatty acids and glycerol. Detail on the biochemistry of phospholipids can be found online in Evolve. Fats from plant and animal foods contain phospholipids, but they are not required in the diet because the body produces adequate amounts of phospholipids. These substances cannot be absorbed intact; they are broken down into their chemical components before absorption. As a structural component of cell membranes, tooth enamel, and dentin, they are the second most prevalent form of fat in the body. As such, these substances are not used for energy, even in a state of severe starvation. Although the mechanism is not fully understood, phospholipids are involved in the initiation of calcification and mineralization in teeth and bones, and are present in higher amounts in the enamel matrix of teeth than in dentin.
Phospholipids are important in fat absorption and transport of fats in the blood. Phospholipids can mix with either fat-soluble or water-soluble ingredients and transport these products across membrane barriers.
Phospholipids include lecithin, cephalin, and sphingomyelins. Lecithin, the most widely distributed phospholipid, is present in all cells. Lecithin supplements have been marketed for reducing the risk of atherosclerosis (a complex disease of the arteries in which the interior lining of arteries becomes roughened and clogged with fatty deposits that hinder blood flow), for weight loss, and for other chronic health conditions. However, the value of lecithin in this role is questionable because lecithin is digested before its absorption. Cephalin is present in thromboplastin, which is necessary for blood clotting. Sphingomyelins are important constituents of brain tissue and the myelin sheath around nerve fibers. Phospholipids, especially lecithin, are used as additives in commercial products to prevent fat and water components from separating.
Lipoproteins are produced by the body to transport insoluble fats in the blood. Lipoproteins are compound lipids composed of triglycerides, phospholipids, and cholesterol combined with protein (see Chapter 2, Fig. 2-18). The liver and intestinal mucosa produce lipoproteins. Four different types of lipoproteins are present in the blood: high-density lipoproteins (HDLs), low-density lipoproteins (LDLs), very-low-density lipoproteins (VLDLs), and chylomicrons.
The ratio of lipid to protein in lipoproteins varies widely; these variations affect their density. Density increases as lipids decrease and protein increases. Lipoproteins can be classified according to their density and composition, as shown in Figure 6-4. Phospholipids in lipoproteins are present in approximately the same proportions in all individuals.
HDLs, which have been thought to protect against development of CHD, contain greater amounts of protein and less lipid. LDL cholesterol typically constitutes 60% to 70% of the total blood cholesterol. It is considered the main agent in elevated serum cholesterol levels, or the “bad” cholesterol. Serum HDL, LDL, and VLDL are important predictors of heart disease, as discussed in Health Application 6.
Cholesterol is a fatlike, waxy substance classified as a sterol derivative with a complex ring structure (see Chapter 2, Fig. 2-17). More details on the structure and function of cholesterol are available online in the Evolve website. Because the body frequently produces more cholesterol than it absorbs, cholesterol intake is not essential. Cholesterol has important functions as a constituent of the brain, nervous tissue, and bile salts; a precursor of vitamin D and steroid hormones; and a structural component of cell membranes and teeth. Lipoproteins transport cholesterol in the blood.
Dietary fats are a concentrated source of energy, furnishing 9 kcal/g. Foods high in fats are generally referred to as calorie-dense, a beneficial quality in some cases. Calorie-dense foods are high in fats (or fat and sugar) and low in vitamins, minerals, and other nutrients. A characteristic of calorie-dense foods is that less volume of food is needed to furnish energy requirements. As an energy source, fats are also referred to as protein-sparing because they allow protein to be used for the important functions of building and repairing tissues.
Dietary fats are important for their satiety value. Fats contribute to a feeling of fullness for a longer time than carbohydrates or protein because digestion of high-fat meals is slower than other energy-containing nutrients. This has given rise to such descriptions as “sticks to the ribs” in reference to rich, high-fat meals. The higher the fat content of a meal, the longer the food remains in the stomach. Nevertheless, approximately 95% of ingested fats are absorbed. Soft fats that are liquids at body temperature (e.g., margarine) are digested more quickly than hard fats (e.g., meat fats).
Fats contribute to the palatability and flavor of foods. In cooking, they improve texture. A receptor on the tongue and a potential pathway for detection of a “fatty taste” has been identified, which may affect food preferences.3,4 Preference for high-fat foods develops at an early age and persists through adulthood.
Linoleic acid, an omega-6 fatty acid with 18 carbon atoms and two double bonds (see Chapter 2, Fig. 2-12), cannot be synthesized by the body and must be supplied from dietary sources. If linoleic acid is not furnished in the diet, signs of deficiency, including growth retardation, skin lesions, and reproductive failure, result. For this reason, linoleic acid is an essential fatty acid (EFA).
Arachidonic acid (18-carbon chain with four double bonds) and linolenic acid (18-carbon chain with three double bonds) are also considered EFAs, but healthy individuals can produce them from sufficient quantities of linoleic acid (see Chapter 2, Fig. 2-13). Linolenic acid can be converted rapidly into omega-3 fatty acids in the body. The conversion of linolenic acid to EPA and the conversion of linoleic acid to arachidonic acid are competitive because the processes use the same enzyme. Studies suggest that less than 10% of linolenic acid is converted to EPA. When intake of linoleic acid is substantially higher than intake of linolenic acid, less EPA is available. Linolenic acid may be a protective factor against CHD (Table 6-1).5
|Fatty Acid Classification||Common Food Sources||Physiological Action|
|Saturated Fatty Acid (SFA)|
|Coconut oil, butter fat, most fats and oils, cocoa butter, fully hydrogenated vegetable oils||Raises total, LDL and HDL cholesterol (except stearic acid)|
|Monounsaturated Fatty Acid (MUFA)|
|Cis configuration||Some fish oils, beef fat, most fats and oils, nuts, seeds, avocados||Decreases total and LDL cholesterol when substituted for SFAs and decreases total cholesterol compared with dietary carbohydrate|
|Trans configuration||Partially hydrogenated vegetable oils||Raises total and LDL cholesterol similar to SFAs, decreases HDL more than saturated fatty acids, and raises total-to-HDL ratio more than SFAs|
|Trans configuration||Dairy fat, meat from ruminating animals (beef, lamb)||May have beneficial health effects, especially vaccenic acid; more research needed|
|Polyunsaturated Fatty Acid (PUFA)|
|n-6 Fatty Acids|
|Linoleic acid||Liquid vegetable oils, nuts, seeds||Decreases total and LDL cholesterol|
|Arachidonic acid||Meat, poultry, fish, eggs||Precursor for important biologically active substances; substrate for synthesis of a variety of proinflammatory compounds|
|n-3 Fatty Acids|
|α-Linolenic acid||Flaxseed, canola oil, soybean oil, walnuts||Decreases cardiovascular risk|
|Eicosapentaenoic acid (EPA)
Docosahexaenoic acid (DHA)
|Fish oil, algae||Decreases risk of sudden death from cardiovascular conditions and has beneficial effects on nervous system development and health|
Adapted from Kris-Etherton P, Innis S: Position of the American Dietetic Association and Dietitians of Canada: dietary fatty acids. J Am Diet Assoc 2007; 107(9):1599-1611.
Omega-3 fatty acids are used to produce compounds regulating blood pressure, clotting, immune responses, gastrointestinal secretions, and cardiovascular functions; they also prevent heart arrhythmias and decrease triglyceride levels. Omega-3 fatty acids are essential for the development of brain and retinal tissues in fetal and neonatal development, and ongoing cognitive development in childhood. The presence of omega-3 fatty acids in the diet has been linked to reduction or amelioration of several chronic diseases, including CHD, atherosclerosis and atherosclerotic plaque, and mortality risk from CHD (Fig. 6-5),6 rheumatoid arthritis, psoriasis, inflammatory and immune disorders, and serious eye problems such as macular degeneration (see Table 6-1). However, in numerous studies, supplementation has not been associated with either lower risk of all-cause mortality or major CHD outcomes.7 Studies are exploring the relationship of omega-3 fatty acids with mental aging and Alzheimer disease.
Excess dietary carbohydrates and protein are converted to fat and stored in adipose tissue. Fatty acids can be used as an energy source by all cells except red blood cells and central nervous system cells. People have been known to survive total starvation for 30 to 40 days with only water to drink.
Dietary fats are essential for oral health because they are incorporated into the tooth structure. There is some evidence from epidemiological and laboratory studies that fats may have a cariostatic effect. Individuals, whose diet may contain 80% fat from animal and seafood sources (e.g., Alaskan natives), have a very low incidence of dental caries, but this could possibly be a result of their low carbohydrate intake. Dietary fats probably have local rather than systemic influence because fats added to foods protect the teeth more than foods naturally high in fat. Precisely how fats reduce the caries rate is unknown; however, several hypotheses have been explored, as follows:
Bacterial inflammation and systemic immune response are believed to play a central role in the initiation and propagation of atherosclerosis. Periodontal diseases (including gingivitis and periodontitis) are oral conditions caused by bacteria (and poor oral hygiene) that are risk factors contributing to coronary artery disease. When bacteria are allowed to grow rampantly in the mouth, inflammation may occur throughout the body. Bacteria from dental plaque biofilm can cause blood clots when they escape into the bloodstream and could be involved in inflammation of the lining of blood vessels and atherosclerosis. This inflammation may serve as a base for development of arterial atherosclerotic plaques, but—contrary to some concerns—omega-6 fatty acids do not seem to promote inflammation.8 Research studies show that the inflammatory process can be attenuated by n-3 fatty acids.9 Consumption of greater amounts of docosahexaenoic acid (DHA) and—to a lesser degree—EPA were associated with lower prevalence of periodontitis.10
A certain amount of fat is needed to provide adequate amounts of fat-soluble vitamins and EFAs. The acceptable macronutrient distribution range for fat is estimated to be 20% to 35% of energy intake for adults (Table 6-2). The lower limit for fat intake was established to minimize the increase in blood triglyceride levels and decrease in HDL cholesterol levels that occur with higher intakes of carbohydrates. The upper limit of 35% kcal from fat was based on information indicating higher fat intake is associated with a greater intake of energy and SFA, which may be detrimental to health. Box 6-1 shows a method for calculating the IOM recommendation for dietary fat.
|Classification of Fat||Dietary Guidelines||IOM† Reference Dietary Allowance (RDA)/Adequate Intake‡ (AI)||IOM† Acceptable Macronutrient Distribution Range (AMDR)||American Heart Association§||American Diabetes Association¶||Canada’s Food Guide**|
|Total fat||20% to 35% kcal||—||20% to 35% kcal||25% to 35% kcal||—||20% to 35% kcal|
|Saturated fatty acids (SFA)||<10% kcal||Minimize||Minimize||<7% kcal (stay away from tropical oils such as coconut oil, palm oil and palm kernel oils that are high in SFA)||<7% kcal||Limit butter, hard margarines, lard, and shortening|
|Trans fatty acids (TFA)||<1% kcal||Minimize¶¶||Minimize¶¶||>1% kcal||Minimize¶¶||Limit hard margarines, lard, and shortening|
|Omega-6 fatty acids (n-6 PUFA)||14 g/day for males; 11 g/day for females||5% to 10% kcal||Choose monounsaturated fatty acids (MUFA) or PUFA (vegetable oils, margarines with liquid vegetable oil as the first listed ingredient)||Use vegetable oils, such as canola, olive, and soybean|
|α-Linolenic acid||1.6 g/day for males; 1.1 g/day for females||0.6% to 1.2% kcal||No specific recommendation|