FATS

FATS. Fat is a generic term for triacylglycerols, which are a class of structurally similar chemical compounds that contain three fatty acid molecules that are linked or chemically esterified to one glycerol molecule (Figure 1). Mammals store triacylglycerol as lipid droplets in specialized cells referred to as adipocytes, which compose the white or yellow tissue known as adipose or neutral fat tissue. White fat has several functions in mammals. It is a reservoir for storing excess energy obtained from the diet. Fatty acids are a dense storage form of chemical energy in mammals. Adipose tissue also has an important role in padding and thereby protecting various organs throughout the body from temperature extremes and physical impact or trauma. Triacylglycerol, when converted to phospholipid, is a primary constituent of cell membranes and therefore is critical for all forms of life. Mammals also contain brown fat in various locations throughout the body including the neck, thorax, and abdomen. Brown fat functions to generate body heat and therefore it is an important tissue for energy expenditure, otherwise referred to as "burning calories." Brown fat can generate heat because it contains mitochondria with a unique and specialized function. In most other cells, mitochondria are the energy-producing compartments in the cell that generate adenosine triphosphate (ATP), which is a chemical form of cellular energy that is required for numerous cellular chemical reactions. In brown fat, the energy generated by mitochondria is used to generate heat. Brown fat cells contain an "uncoupling" protein that diverts energy away from ATP synthesis and toward heat production. Energy utilization by brown fat is tightly regulated by signals it receives from the sympathetic nervous system. Animals that are adapted to cold temperatures display increased heat production from brown fat, and brown fat is proportionally more abundant in infants than in adults.

Classes of Fatty Acids

Fatty acids are a diverse family of structurally similar carbon chains that contain a single carboxylic acid group (see Figure 1). Fatty acids differ from one another by their carbon chain length, which is usually an even number of carbons that can exceed twenty carbon atoms. Fatty acids are often categorized as short-chain, medium-chain, or long-chain fatty acids because each of these groups displays distinct physical properties. Short-chain fatty acids contain up to seven carbon molecules and are liquids even at cold temperatures. Medium-chain fatty acids, which contain between eight and twelve carbons, are liquids at room temperature but solidify when refrigerated. Long-chain fatty acids contain greater than twelve carbons and are solids at room temperature, but liquefy at elevated temperatures. Long-chain fatty acids are the most abundant fatty acids in plant and animal foods. Short-chain fatty acids are found in whole cow's milk, and medium-chain fatty acids are abundant in coconut milk.

Fatty acids also differ by the number and location of carbon–carbon double bonds, otherwise called the degree of saturation. Saturated fatty acids do not contain any carbon–carbon double bonds because all carbon molecules are "saturated" with hydrogen molecules. The most abundant saturated dietary fatty acids are palmitic and stearic acids, which are long-chain fatty acids found in foods derived from animals and are abundant in meat and dairy products (Table 1; see Figure 1). Monounsaturated fatty acids contain a single carbon–carbon double bond (see Figure 1). Oleic acid is a monounsaturated fatty acid and a common dietary component found in canola and olive oil. Polyunsaturated fatty acids contain up to six carbon–carbon double bonds that are always separated by a methylene group (wCH₂w) (Figure 1). Polyunsaturated fatty acids that contain a series of double bonds that begins between the third and fourth carbon from the methyl or omega end of the molecule (see nomenclature system below) are referred to as omega-3 fatty acids. Linolenic, eicosapentaenoic (EPA), and docosahexaenoic (DHA) are omega-3 fatty acids and flaxseed oil, walnut oil, and fatty fish are good sources of omega-3 fatty acids. Omega-6 fatty acids are another class of polyunsaturated fatty acids that includes linoleic acid and arachidonic acid. They contain a series of carbon–carbon double bonds that begin between the sixth and seventh carbon from the omega end of the fatty acid. Linoleic acid is the most common omega-6 fatty acid in Western-style diets and is found in corn, safflower, and soy oils.

The fatty acid composition of triglycerols found in mammals is usually complex and is influenced by the fatty acid consumed in the diet and by the tissue where it resides. The most common fatty acids in humans are 16, 18, and 20 carbons in length, but longer-chain fatty acids are found in the central nervous system. Most diets contain mixtures of all types of fatty acids, but saturated and monounsaturated fatty acids constitute the vast majority of fatty acids that are consumed in a typical Western diet. A single triacylglycerol molecule rarely contains three identical fatty acids.

Essential Fatty Acids

Rodents placed on a fat-restricted diet are growth impaired, infertile, and develop lesions in the skin and kidney. These pathologies are not observed if the diet is supplemented with linolenic (omega-3) and linoleic acid (omega-6). The results of these studies indicated that mammals cannot synthesize these fatty acids and therefore that these fatty acids are essential components of a healthy diet. Human deficiencies of these essential fatty acids are rare but can occur in infants and children or as a result of intestinal absorption disorders. Human essential fatty acid deficiency compromises liver function, results in unhealthy skin, and impairs growth and development in infants including impaired cognitive function, visual acuity, and hearing.

Essential fatty acids are necessary to maintain the architecture of cell membranes and the integrity of the skin. They are also precursors for the synthesis of eicosanoids ("eicosa" meaning twenty carbons in length), which are bioactive, hormone-like compounds derived from linoleic and linolenic acid. The eicosanoids include prostaglandins, which elicit numerous and varied biological responses including induction of labor, regulation of the female reproductive cycle, and modification of pituitary function. Thromboxane is an eicosanoid that functions in platelet aggregation and blood clotting; leukotrienes function in the inflammation and allergic responses. The omega-3 fatty acid alpha-linolenic acid is also a precursor for eicosapentaenoic acid (EPA) and docohexaenoic acid (DHA) synthesis. Both DHA and arachidonic acid are important for nervous system and retina development. DHA may be an essential dietary fatty acid for preterm infants because studies indicate that it is not synthesized in sufficient quantities to meet the infant's needs.

There is no Recommended Dietary Allowance (RDA) for essential fatty acids. The minimal adequate adult intake of omega-6 fatty acids is estimated to be 2 to 4 g/day of linoleic acid. Americans normally consume about 10 to 15 g/day. The minimal adequate adult intake of omega-3 fatty acids is estimated to be 0.2 to 0.4 g/day, but intakes as high as 3 g/day may have added benefit. Omega-3 fatty acid intakes should be increased during pregnancy and lactation. The World Health Organization recommends an omega-6/omega-3 ratio of 4:1 to 10:1.

Fatty Acids Derived from Food Processing

Synthetic or unnatural types of fatty acids are also common components of Western diets and result from food processing. Fats are processed to increase their shelf life and to alter their physical properties. Monounsaturated and unsaturated fatty acids are chemically inert, whereas polyunsaturated fats are susceptible to oxidation. Polyunsaturated fatty acids degrade by oxidation and become rancid, thereby spoiling foods that contain these compounds. Therefore, products containing polyunsaturated fatty acids tend to have a reduced shelf life, but can be stabilized by converting the polyunsaturated fatty acids contained within these products to more stable monounsaturated and saturated fatty acids through the process of chemical hydrogenation. This processes converts carbon–carbon double bonds to single bonds (Reaction 1):

(Reaction 1; Chemical Hydrogenation)

This process not only stabilizes food, but also changes its physical properties. For example, margarine is produced by the chemical hydrogenation of vegetable oils. This process produces a product that is more stable and solid than vegetable oil and mimics the consistency of natural butter. However, chemical hydrogenation of polyunsaturated fatty acids also results in the formation of "unnatural" trans -fatty acids, which are normally found only in trace quantities in foods from natural sources. Trans -fatty acids do not differ from natural fatty acids in their carbon chain length or degree of saturation, but differ in the orientation or stereochemistry of the carbon–carbon double bonds. Carbon–carbon double bonds can exist in both a cis (the hydrogen atoms that are attached to the carbon atoms that flank the double bond reside on a common plane) or trans (hydrogen atoms reside on different planes) conformation; this is a fundamental principle of organic stereochemistry. The double bonds present in fatty acids from natural, unprocessed food sources usually exist in the cis conformation (see Figure 1). Trans -fatty acids are abundant in foods that undergo chemical hydrogenation and their consumption may increase risk for disease.

Nomenclature of Fatty Acids

All fatty acids can be identified by their "trivial" names, such as oleic or linoleic acid, but these names do not contain information that is necessary to infer their structure or physical properties, that is, the length of their carbon chains or the number and location of carbon–carbon double bonds. Therefore, a nomenclature system has been devised that describes the precise chemical structure of the molecule (see Table 1). The carbon atom that constitutes the carboxylic acid of the fatty acid is referred to as the alpha carbon and is designated as carbon number one; the methyl carbon that constitutes the other end of the molecule is referred to as the omega carbon. Fatty acids are named by the number of carbons in the chain and the number and location of carbon–carbon double bonds. For example, oleic acid is referred to as cis -9-octadecenoic acid, or 18:1(9); the 18 refers to the number of carbons in the fatty acid carbon chain, the 1 refers to the number of carbon–carbon double bonds, and the 9 in parentheses refers to the position of the double bond counting from the carboxylate carbon that is in the cis conformation.

Fatty acids as membrane components and emulsifiers. Fatty acids and triglycerols are lipid soluble and therefore

are hydrophobic molecules that do not dissolve readily in water (as evidenced by the appearance of distinct oil and water layers in many oil-based salad dressings). In aqueous environments, fatty acids aggregate and form ordered structures. All life forms have taken advantage of the hydrophobic properties of fatty acids to make cell membranes, which are semipermeable barriers that separate cells from their environment. Membranes delineate the boundaries of the cell, enable cells to retain water, and form specialized internal structures called subcellular organelles that include mitochondria, Golgi apparatus, and lysosomes. Cell membranes are lipid bilayers that are primarily composed of lipid and membrane-bound proteins. Fatty acids present in cell membranes are components of phospholipids, and phosphoglycerides are the most abundant phospholipids in membranes. Phosphoglycerides are similar in structure to triglycerols. They contain two fatty acid molecules and one phosphate molecule esterified to a glycerol molecule. The phosphate molecule has a hydrophilic amino acid or sugar molecule attached to it. Phospholipids are amphipathic molecules because one end of the molecule contains a water-soluble phosphate molecule, and the other end contains a lipid-soluble carbon chain of the fatty acids. Therefore, phospholipids are ideal components of cell membranes because the phosphate end can dissolve in water while the fatty acid end interacts with other lipid molecules to form a barrier that restricts the efflux of water.

The amphipathic properties of phospholipids make them effective emulsifiers, which are chemicals that interact with both water and oils and prevent them from separating and forming two layers. Lecithin is a phospholipid that is synthesized by mammals and is found in high concentrations in eggs. It is also an effective emulsifier and a common food additive in margarine, salad dressings, chocolate, and a variety of baked items. Fatty acids are components of many household products including lubricants, cooking oils, soaps, and detergents.

Dietary and Biosynthetic Sources of Fat

Fatty acids found in mammals are derived from both dietary sources and intracellular biosynthesis. Humans can synthesize all of the necessary fatty acids with the exception of the essential fatty acids. Fatty acids are synthesized in most cells from excess dietary carbohydrate, amino acids, and from other fatty acids. Palmitic acid (16:0) is synthesized by mammals and is a precursor for the synthesis of all other nonessential fatty acids. The carbon chain of palmitic acid is extended by the sequential addition of two carbons to the carboxy terminal end of the molecule. This is an enzyme catalyzed reaction that uses acetyl coenzyme A (CoA) as a source of the two carbon atoms. Mono-and polyunsaturated fatty acids are synthesized by the desaturation of saturated fatty acids. The first double bond is formed between the C–9 and C–10 of palmitate or stearate to form palmitoleic or oleic acid. This is the first step in the synthesis of polyunsatirated fatty acids. This reaction is inhibited by dietary polyunsaturated fatty acids but activated by insulin and thyroid hormone.

Triglycerols are synthesized by most tissues from glycerol 3-phosphate, an intermediate in carbohydrate metabolism, and chemically activated fatty acids known as fatty acyl CoAs. This reaction occurs most frequently in the liver and white adipose tissue. In the liver, triacylglycerol synthesis is necessary for the assembly of lipoproteins, whereas triacylglycerol synthesis in adipose tissue functions to create long-term energy stores for mammals. Although a storage form of energy, fat is a dynamic tissue. Triacylglycerols constantly undergo hydrolysis and resynthesis in adipocytes. Newly synthesized triacylglycerol molecules remain intact for only a few days.

Digestion and Transport

About 90 percent of dietary lipid is in the form of triacylglycerols, and typical adults consume about 60 to 150 g/day. During digestion, dietary lipids aggregate and form water-insoluble particles in the gut that must be disrupted before absorption. Specific enzymes in the stomach, called gastric lipases, and in the intestine, called pancreatic lipases, bind to the lipid droplets and catalyze the hydrolysis or removal of fatty acids from triacylglycerols resulting in the liberation of free fatty acids, diacylglycerols, and monoacylglycerols. Fatty acids are also liberated from phospholipids by pancreatic phospholipases. The products of triglycerol hydrolysis are made soluble by bile acids, which are negatively charged detergents that are synthesized from cholesterol in the liver and secreted into the duodenum. Bile acids form micelles, which are disc-shaped particles with a negatively charged exterior that is water soluble and a hydrophobic center that sequesters fatty acids. During digestion, liberated fatty acids are continuously transferred from lipid droplets to micelles. Virtually all free fatty acids are transported from the micelles into intestinal epithelial cells by passive diffusion. Lipids that cannot be made soluble are not absorbed and are excreted.

Once absorbed into the intestinal cells, short-and medium-chain fatty acids are released directly into blood and taken up by the liver. Long-chain fatty acids are resynthesized into triacylglycerols and complex with apolipoproteins to form lipid globules known as chylomicrons. Chylomicrons travel through the lymphatic system and then through the venous plasma. Most triacylglycerol in chylomicrons is metabolized by lipoprotein lipase that is bound to the surface of adipose and muscle cells.

Metabolism of Fat

Most fat cells are derived in infancy and adolescence except in instances of severe childhood obesity. As fat stores accumulate, adipocytes increase in size but generally not in number. Normal fat stores provide sufficient energy to sustain humans for several weeks during total starvation. During fasting, fatty acids are catabolized or broken down to acetyl-CoA, which is an intermediate in the citric acid cycle. This reaction requires carnitine, a derivative of the amino acid lysine. The oxidative breakdown of fatty acids occurs in mitochondria through a series of reactions known as beta-oxidation. Fatty acids are rich sources of energy; 44 moles of ATP are generated by the complete oxidation of 1 mole of a six-carbon fatty acid, whereas only 38 moles of ATP are generated from 1 mole of glucose, a six-carbon sugar. During starvation, acetyl-CoA can be converted to ketone bodies, which include acetone, acetoacetate and alpha-hydroxybutyrate. These compounds are produced exclusively in the liver but readily enter the circulatory system by passive diffusion. The odor associated with the generation of these ketones becomes apparent in the breath and urine of individuals. Ketone bodies are an alternative energy source for glucose during starvation, and are utilized by the brain and other tissues. Normally, ketones are rapidly metabolized by the peripheral tissues and do not accumulate in blood. However, if the citric acid cycle is depressed by low glucose due to starvation, diabetes mellitus, or a high-fat, low-carbohydrate diet, ketones accumulate in serum and a state of ketosis can result. High concentrations of ketones in blood can lower its pH and result in metabolic acidosis, which can be fatal during diabetic ketosis.

Fatty Acid Regulation of Gene Expression

Polyunsaturated fatty acids and eicosanoids are informational or signaling molecules that can influence the expression of certain genes involved in lipid synthesis, breakdown, and transport. Omega-3 and omega-6 polyunsaturated fatty acids lower the accumulation of triacylglycerol in muscle by inhibiting triacylglycerol synthesis in the liver and accelerating the breakdown of fatty acids in the liver and skeletal muscle. Linoleic and linolenic acid, as well as certain pharmaceuticals, bind to and activate the transcriptional activity of a family of related nuclear receptors known as the peroxisome proliferator–activator receptors (PPARs). These receptors are transcription factors that can directly bind DNA and elevate the transcription of genes. The target genes are involved in the metabolism, storage, and transport of lipids, triacylglycerol, and fatty acids. These receptors also regulate the differentiation of immature adipocytes into mature fat cells.

Individual members of the PPAR family have different functions. In the fasting liver, PPAR-alpha activates genes that encode enzymes that metabolize lipids to ketone bodies and decreases expression of genes involved in fatty acid synthesis. As fatty acids are hydrolyzed from triacyglyceride, PPAR-alpha is further activated. PPAR-alpha activates the expression of genes in fat cells that are necessary for fatty acid uptake, triacylglycerol synthesis, and fat storage.

Determinants of Total Body Fat

Fat is a storage form of energy, and as such only accumulates when energy intake exceeds energy output. Total body fat accumulation is determined by complex interactions among genes, environment, and behavior. The human body can adjust to a wide range of fat intake, but both deficiency and excess are associated with disease. In a normal, healthy individual, fat stores constitute 12 to 18 percent of total body weight in males and 18 to 24 percent in females. Excessive consumption of high-calorie foods and/or a lack of exercise elevate fat stores. In some cases, the genetic background alone can determine total body fat in the absence of strict dietary control. Children with obese parents are at higher risk of becoming obese, and studies of identical twins also indicate that risk for obesity has a strong hereditary component. Furthermore, more than 75 percent of the Pima Indians are obese, again indicating a strong influence of genetics on fat accumulation. Many genes have been identified that control weight gain. The products of these genes regulate energy balance and expenditure and are signaling hormones that regulate appetite and fat metabolism. Some studies indicate that genetic factors, and the metabolic signals they generate, balance energy expenditure and appetite to form an individual's "set point" that specifies body weight. These signals include the satiety hormones such as serotonin and leptin. The neurotransmitter serotonin is responsible for "cravings" that can increase consumption of particular food types. Leptin is a peptide hormone that is secreted by fat cells and signals the hypothalamus. Leptin secretion is proportional to fat cell size, and increased leptin concentrations in blood signal the brain to increase energy expenditure and decrease food intake. Mice lacking the leptin gene or the leptin receptor become obese. Human mutations in the leptin gene are rare but result in obesity.

Dietary Fat and Disease Risk

Lipids constitute about 33 percent of total energy intake in the typical North American diet, whereas Japanese diets have a lower fat intake (11 percent of energy from fat). Western-style diets are deficient in omega-3 fatty acids and contain excess omega-6 fatty acids. Some evidence indicates that prehistoric diets that were consumed through much of human evolution contained an omega-6/omega-3 fatty acid ratio that was near 1.0, whereas this ratio is about 20 in the typical Western diet. Vegetarian diets also tend to contain excess omega-6 fatty acids. Diets deficient in omega-3 fatty acids or diets that contain an elevated omega-6/omega-3 ratio may increase risk for cardiovascular disease and cancer.

Research over the past few decades has indicated that excess consumption of saturated fat increases risk for disease including heart disease (arteriosclerosis), obesity, diabetes, and certain cancers (see "Fat and Heart Disease"). Obesity is a clinical condition defined as having a body weight that is greater than 20 percent above a desirable body weight standard or a body mass index that exceeds 30 kg/m². Obesity occurs in epidemic proportions in the United States and other Western societies, especially in individuals from lower socioeconomic level. Its prevalence is rapidly increasing in developing societies that are adapting Western lifestyles. The combination of increased fat intake and sedentary lifestyle (otherwise referred to as excess energy intake) increases risk for overweight and obesity. Increased body fat, in turn, is an independent risk for heart disease, diabetes, and high blood pressure. Elevated fat intake can also increase risk for cancers of the colon, prostate, and breast. The incidence of cancers of the breast is high in populations with high intakes of either natural saturated fat or trans -fatty acids, but not diets rich in olive oil, which contains high levels of monounsaturated fatty acids. High polyunsaturated fat intake in the form of linoleic acid (omega-6) increases risk for breast cancer incidence in mice, compared to diets high in omega-3 fatty acids.

Cultures in which traditional foods have high concentrations of monounsaturated fats, products that include olive oil and fish, have lower incidence of heart disease compared to the United States. The prevalence of heart disease in Mediterranean countries is only 50 percent of that found in the United States, even when fat represents almost 40 percent of total energy intake. However, the decreased rates of heart diseases in these countries also reflects other dietary patterns including a high consumption of fresh fruits and vegetables and other lifestyle differences.

See also Assessment of Nutritional Status; Body Composition; Cholesterol; Gene Expression, Nutrient Regulation of; Mediterranean Diet; Nutrition.

BIBLIOGRAPHY

Berdanier, Carolyn D., and James L. Hargrove. "Nutrient Receptors and Gene Expression." In Nutrition and Gene Expression, edited by Carolyn D. Berdanier and James L. Hargrove, pp. 207–226. Boca Raton, Fla.: CRC Press, 1993.

Devlin, Thomas M. Biochemistry, 5th ed. New York: Wiley-Liss, 2002.

Kersten, Sander, Beatrice Desvergne, and Walter Wahli. "Roles of PPARs in Health and Disease." Nature 405 (2000): 421–424.

Simopoulos, Artemis P. "The Mediterranean Diets: What Is So Special About the Diet of Greece?" Journal of Nutrition 131 (2001): 3065S–3073S.

Smolin, Lori A., and Mary B. Grosvenor. Nutrition, Science and Application. Philadelphia: Saunders College Publishing, 2000.

Stipanuk, Martha H. Biochemical and Physiological Aspects of Human Nutrition. Philadelphia: W. B. Saunders, 2000.

Patrick J. Stover

Fat and Heart Disease

Risk for heart disease results from excess fat consumption and the type of fat that is present in the diet. Diets high in saturated fatty acids, especially those found in animal fat, increase the concentration of lowdensity lipoprotein (LDL) cholesterol or "bad" cholesterol. Elevations in serum LDL concentrations increase risk for arteriosclerosis. Consumption of trans- fatty acids, although only representing between 2 and 4 percent of calories in Western diets, also increases risk for heart disease, but the pathogenic mechanisms are not certain. Trans -fatty acids may be as efficient as natural saturated fat in increasing serum LDL concentrations, and their consumption replaces foods that contain beneficial unsaturated fatty acids. Consumption of omega-3 and omega-6 fatty acids, especially when they replace consumption of saturated fat, decreases risk for heart disease, in part by lowering LDL cholesterol levels. Omega-3 fatty acids are more protective than omega-6 fatty acids. Omega-6 fatty acids may lower serum HDL cholesterol, which is harmful because HDL protects the heart from disease. Omega-3 fatty acids may prevent heart disease by improving immune function, lowering blood pressure, and inhibiting the growth of plaques on blood vessel walls. Omega-3 fatty acids obtained from whole food sources such as fatty fish seems to be more beneficial than dietary supplements.

Pharmaceuticals That Target F>at Metabolism

Many of the most prevalent diseases in Western cultures are related to excessive caloric intake and sedentary lifestyles, diseases that include obesity, hyperlipidemia, diabetes, and arteriosclerosis. These states often occur in combination, and are diagnosed as syndrome x. Pharmaceutical have been developed to manage these disorders. These agents either inhibit intestinal fat absorption or affect fat metabolism by manipulating the activity of PPARs.

Fibrates (gemfibrozil, bezafibrate, fenofibrate) are pharmaceuticals that target and inhibit the function of PPAR-alpha. Thiazolidinediones target PPAR-alpha. Fibrates are effective in the treatment of cardiovascular disease. They function to elevate HDL levels by increasing the expression of proteins necessary for its structure, and decreasing plasma triglyceride by accelerating fatty acid oxidation in the liver. TZDs are effective in the treatment of Type 2 diabetes because they have a hypolipidemic and hypoglycemic effect.

Nondigestible commercial lipids have also been developed to limit total fat intake. One product, Olestra, contains fatty acids linked to the sugar sucrose. These products replace natural fat in foods, and were designed to taste like natural fat. However, they cannot be hydrolyzed in the gut and therefore are not absorbed. Other pharmaceuticals target and inhibit pancreatic lipase, such that natural dietary lipids are not broken down to fatty acids and therefore are not absorbed.

Fats

Definition

Purpose

Description

Precautions

Parental concerns

Resources

Definition

Fats are also known as lipids. A lipid is a substance that is poorly soluble or insoluble in water The term ‘dietary fat’ encompasses many different types of fat. Over 90% of dietary fats are called triacylglycerols or triglycerides Other dietary fats include cholesterol.

Triacylglycerols contain three fatty acids attached to a glycerol molecule. Fatty acids vary according to their length, which is composed of carbon and hydrogen atoms joined together to form a hydrocarbon chain. The number of double bonds that occur between the carbon molecules also varies. The chemical structure of each type of fatty acid determines its physical characteristics and its nutritional and physiological function. Regardless of the type of fatty acid present, all triacylglycerols provide 9 kcal (37 KJ) per gram; this makes fat the most concentrated source of energy in the diet. Fatty acids should provide no more than 30–35% of dietary energy or approximately no more than 70 g aday for women and no more than 90 g a day for men.

Typical high sources of fat in the diet include cooking fats and oils, fried food, fatty and processed meats. These should form a very small part of the diet. Care should be taken to reduce fried foods; avoid adding fats and oils during cooking; to grill food, which allows fat to drip out; and to choose lean meats and low fat dairy products. A product is thought to be low in fat if it contains less than 3 g fat per 100 g and high in fat if it contains more than 20 g fat per 100 g or 21 g fat per serving.

Purpose

Some types of fatty acids are essential nutrients. They must be consumed in the diet for the body to function properly. Fats form the structure of cell membranes, they are involved in the transport, breakdown and excretion of cholesterol and they are the building blocks for many important compounds such as hormones, blood clotting agents, and compounds involved in immune and inflammatory responses. Fats also transport fat soluble vitamins and antioxidants; provide the body with insulation and form a protective layer around organs; are a structural component of the brain and nervous system; and provide a reserve supply of energy in the form of adipose tissue (body fat). Excess amounts of adipose tissue defines obesity and may lead to health problems such as diabetes, cancer and heart disease.

Description

Saturated fatty acids

Saturated fatty acids have a hydrocarbon chain where each carbon atom carries its maximum number of hydrogen atoms except for the end carboxyl group and they do not have any double bonds. The molecules are straight, allowing them to pack closely together. For this reason, they are solid at room temperature with a high melting point. Saturated fatty acids are chemically stable both within the body and in food.

Saturated fatty acids are named according to the number of carbon atoms they contain. Each one has a common name (e.g., stearic acid), a systematic name (e.g., octadecanoic acid because stearic acid has 18 carbon atoms), and a notational name (e.g., 18:0 as stearic acid has 18 carbon atoms but no double bonds).

Animal products such as meat fat, dripping, lard, milk, butter, cheese and cream are the primary sources of saturated fatty acids. Most plant products have a lower amount of saturated fat with the exception of coconut and palm oil.

SATURATED FATTY ACIDS AND HEALTH Saturated fatty acids increase the body’ levels of cholesterol, including low density lipoprotein (LDL) cholesterol. LDL cholesterol is commonly known as ‘bad’ cholesterol. High levels of LDL cholesterol in the blood increase the risk of cardiovascular disease. LDL cholesterol transports excess cholesterol through the bloodstream where it can become deposited in the walls of the arteries and form a hardened plaque. This is called atherosclerosis. This thickening of the artery walls reduces the flow of blood supplying the heart, brain, and other organs. A heart attack or stroke is caused by a blood clot blocking these narrowed arteries. Saturated fatty acids also contribute to production of these blood clots as they are converted into substances that can increase the stickiness of the blood and increase its tendency to clot. For this reason dietary guidelines recommend that no more than 10% of dietary energy should come from saturated fatty acids. This means that on a daily basis approximately no more than 22 g saturated fat should be consumed.

source: Division of Nutrition Research Coordination, National Institutes of Health, U.S. Department of Health and Human Services.

(Illustration by GGS Information Services/Thomson Gale).

by a woman consuming 2,000 calories a day and no more than 28 g saturated fat should be consumed by a man consuming 2,500 calories a day. A product is considered low in saturated fat if it contains less than 1.5 g per 100 g and high in saturated fat if it contains more than 5 g of fat per 100 g.

Monounsaturated fatty acids

Monounsaturated fatty acids have a hydrocarbon chain that contains one unsaturated carbon bond that is not fully saturated with hydrogen atoms. Instead, it has a double bond to the adjoining carbon atom. Double bonds are either in a cis or trans formation. In the cis formation the hydrogen atoms bonded to the carbon atoms in the double bond are positioned on the same side of the double bond. This creates a kink in the hydrocarbon chain. There is also a free electron or slightly negative charge surrounding the double bond causing them to repel each other. The molecules are not packed closely together and become liquid (oil) at room temperature. In the trans formation the hydrogen atoms are on opposite sides of the carbon-carbon double bond resembling the characteristics of a saturated fatty acid. There is less kinking of the hydrocarbon chain and the fat is more solid at room temperature. trans bonds are rarely seen in nature.

Monounsaturated fatty acids are named according to the number of carbons they contain and the position of their double bond. Like saturated fatty acids, they each have a common name, a systematic name, and a notational name. Fatty acids with double bonds in the ninth position are sometimes called n-9s or omega-9s.

The most concentrated sources of monounsaturated fatty acids in the diet are olive oil and rapeseed oil. They are present in many other foods including nuts and seeds, avocados, eggs, fish and meat fat.

MONOUNSATURATED FATTY ACIDS AND HEALTH Monounsaturated fatty acids reduce the level of total and LDL cholesterol. It also has a significant effect on increasing and maintaining the body’ level of high density lipoprotein (HDL) cholesterol. HDL cholesterol is commonly known as ‘good’ cholesterol because it removes cholesterol from the blood transferring it to body tissues where it is used to make hormones and other substances the body needs. Therefore, higher levels of HDL cholesterol are associated with a reduction in the risk of cardiovascular disease. Between 10–20% of dietary energy should come from monounsaturated fat.

Polyunsaturated fatty acids

Polyunsaturated fatty acids have a hydrocarbon chain containing two or more double bonds not fully saturated with hydrogen atoms. The double bonds may either be in the cis or trans formation. The majority of naturally occurring polyunsaturated fats are in the cis form. In this form the hydrogen atoms bonded to the carbon atoms in the double bond are positioned on the same side of the double bond. This creates a kink in the hydrocarbon chain. There is also a free electron or slightly negative charge surrounding the double bond causing them to repel each other. The molecules are not packed closely together and become liquid (oil) at room temperature. The presence of one or more double bonds with free electrons and a negative charge makes them unstable molecules ready to.

KEY TERMS

Antioxidant— A chemical that has the ability to neutralize free radicals and prevent damage that would otherwise occur through oxidation.

Atherosclerosis— A thickening of the artery walls that impedes the flow of blood supplying the heart, brain, and other organs.

Bile acids— Produced by the liver, from cholesterol, for the digestion and absorption of fat.

Carboxyl group— The carbon atom at the end of a fatty acid hydrocarbon chain is attached by a double bond to oxygen and by a single bond to hydrogen forming the chemical structure carboxyl.

cis formation— The arrangement of atoms where hydrogen atoms sit on the same side of the carbon to carbon double bond.

Electron— A component of an atom or molecule. It has a negative charge when a free or unpaired electron exists making it chemically unstable and likely to initiate chemical reactions.

Essential fatty acid— A molecule that cannot be made by the body and must be supplied by food in order to prevent deficiency.

Fatty acid— A molecule consisting of mainly carbon atoms joined together to form a carbon chain to which hydrogen atoms are attached. Fatty acids vary according to their degree of saturation (i.e., the number of hydrogen atoms attached and the length of the hydrocarbon chain).

High density lipoprotein (HDL)— One of several proteins in the blood that transports cholesterol to the liver and away from the arteries.

Hydrogenated— Usually refers to partial hydrogenation of oil, a process where hydrogen is added to oils to reduce the degree of unsaturation. This converts fatty acids from a cis to trans fatty acids.

Omega-3— Polyunsaturated fatty acid where the first double bond occurs on the third carbon-to-carbon double bond from the methyl end of the hydrocarbon chain.

Omega-6— Polyunsaturated fatty acid where the first double bond occurs on the sixth carbon-to-carbon double bond from the methyl end of the hydrocarbon chain.

Omega-9— Polyunsaturated fatty acids where the first double bond occurs on the ninth carbon-to-carbon double bond from the methyl end of the hydrocarbon chain.

Oxidation— A chemical reaction in which electrons are lost from a molecule or atom. In the body these reactions can damage cells, tissues, and deoxyribo-nucleic acid (DNA) leading to cardiovascular disease or cancer.

trans fatty acids— Monounsaturated or polyunsaturated fats where the double bonds create a linear formation. They are formed largely by the manufacture of partial hydrogenation of oils, which converts much of the oil into trans fat. Hydrogenated fats and trans fats are often used interchangably.

react with other chemicals. Polyunsaturated fatty acids are susceptible to chemical changes or oxidation within food leading to cell damage in the body.

Polyunsaturated fatty acids are named similar to other fatty acids. They have a common name, a systematic name, and a notational name. Fatty acids with double bonds starting in the sixth position are commonly known as n-6s or omega-6s.

POLYUNSATURATED FATTY ACIDS AND HEALTH Polyunsaturated fatty acids are divided into two groups, omega-6s and omega-3s. There is one essential fatty acid in each of these groups from which all other fatty acids can be made in the human body. These essential fatty acids cannot be made by the body and must be obtained from the diet. They are a necessary component of the diet; without them deficiency symptoms and poor health would result. Linoleic acid (omega-6) and alpha-linolenic acid (omega-3) are the essential fatty acids. Linoleic acid should provide at least 1% of dietary energy and alpha-linoleic acid should provide 0.2% dietary energy. These essential fatty acids are converted into longer chain fatty acids that form important substances in the body such as hormones, blood clotting agents, and compounds involved in immune and inflammatory responses.

These long chain fatty acids are not technically essential, but they have an important role in the body. Examples of long chain fatty acids include aracha-donic acid (AA ), eicosapentaenoic acid (EPA ), and docosahexanoic acid (DHA ). Long chain omega-3

fatty acids become essential if there is insufficient linoleic and aplha-linolenic acid available in the diet. These fats play a significant role in development of the brain, nervous system, and retina in fetal development and early life.

OMEGA-6 The most concentrated sources of omega-6 in the diet is vegetable oils, such as sunflower, safflower, corn, cottonseed, canola, and soya oils. They are also present in plant seeds, nuts, vegetables, fruit and cereals. In addition to being a source of linoleic acid, omega-6s have been shown to have a lowering effect on both LDL and HDL cholesterol. However, there are health concerns with excessive omega-6 intakes. Omega-6 fats are susceptible to oxidation within the body and may contribute to tissue damage that leads to atherosclerosis and cancer. Omega-6 fats should contribute no more than 10% of dietary energy. Antioxidant nutrients such as vitamin E are required to reduce this oxidation with higher intakes of omega-6 fats. Omega-6s compete with the more beneficial omega-3 fatty acids, so it is recommended that the omega-6:omega-3 ratio is reduced to 4:1.

OMEGA-3 Short chain omega-3 fats are found in flaxseed or linseed oil, walnut oil, canola oil, and rapeseed oil. The best sources of long chain omega-3s are fish and fish oil.

Evidence suggests that consuming long chain omega-3 fats has cadiovascular health benefits. This believed to be the result of their anti-clotting effect. Growing evidence also suggests that consuming long chain omega-3s has benefits beyond those achieved when consuming shorter chain fatty acids. The United Kingdom’ government Food Standards Agency recommends that oily fish be consumed at least once a week.

There has been much interest in the effect of EPA and DHA deficiency and supplementation on behavior in children, particularly those with learning difficulties. Although there is some evidence of benefit with EPA, in 2006 the U.K. Food Standards Agency concluded that there was insufficient evidence to reach a firm conclusion and additional clinical trials were needed.

There is also interest in the anti-inflammatory properties of long chain omega-3s in inflammatory conditions such as Crohn’ disease and rheumatoid arthritis. The role of omega-3s has been evaluated in treatment of depression and prevention of cognitive decline but more research is needed to confirm these benefits.

Omega-3 fats have been shown to reduce blood pressure and triglyceride levels (another fat in the blood that contributes to raising the risk of cardiovas cular disease). To achieve these benefits, omega-3s must be taken in pharmacological doses and there are small risks associated with these high doses such as raised LDL cholesterol, poor control of diabetes, and increased risk of bleeding. Large doses of omega-3s should only be taken under the supervision of a qualified medical doctor.

Trans fatty acids

Trans fatty acids are monounsaturated or polyun-saturated fatty acids where the double bond is in the trans rather than cis formation. They occur naturally in small amounts in lamb, beef, milk, and cheese as they are created in the rumen of cows and sheep. The majority of trans fat in the diet comes from the partial hydrogenation of vegetable oils. This is a process in food manufacture that adds hydrogen atoms to unsaturated fatty acids so that oils become more hardened at room temperature. The process results in some of the double bonds of the fatty acid molecules becoming saturated and some of the remaining double bonds changing from a cis to a trans formation. For example, when partially hydrogenated oleic acid becomes elaidic acid or 9 trans-octadecenoic acid. Trans fats are semi-solid at room temperature and more stable within food. Partial hydrogenation of oils has traditionally been used to develop spreading fats and margarines, for fast food, and in cakes and biscuits. Manufacturers are using it less because of the health problems associated with it. In 2006, New York City adopted the United States’ first major ban on all but trace amounts of artificial trans fats in restaurant cooking. As of July 2008, a serving of food must not contain more than half a gram of trans fat. Food legislation in the United States and the European Union states that hydrogen-ated or partially hydrogenated fats must be labeled in the ingredients of food and in some cases the amounts of trans fat must also be labeled.

TRANS FATTY ACIDS AND HEALTH Trans fat raises LDL cholesterol in a similar way to saturated fat and it reduces HDL cholesterol. It may also raise blood triglyceride levels. The combination of both these effects means that it is most likely to increase cardiovascular risk. The World Health Organization recommends phasing out trans fat in food manufacture and reducing trans fat consumption to no more than 1% of dietary energy or 2.5 g per day.

Cholesterol

Cholesterol is essential to the structure of cell membranes and production of bile acids for digestion, steroid hormones, and vitamin D. Dietary cholesterol

QUESTIONS TO ASK YOUR DOCTOR

• Why or why wouldn’t you recommend a full lipid profile for me?
• What is my cholesterol level? Is it within a normal range?
• What is my risk of heart disease or stroke?

has little effect on blood cholesterol levels because an increased dietary intake reduces the amount the body produces. Only extreme dietary levels of cholesterol need to be restricted. For most individuals, dietary measures that reduce saturated fat also avoid excessive cholesterol consumption. However, individuals with familial hypercholesterolemia may need to consume less than 300 mg/day, which requires avoidance of most animal products.

The most concentrated dietary sources of cholesterol include liver, offal, and products made from egg yolk, mayonnaise, fish roes, and shellfish.

Precautions

The guidelines for the recommended levels of dietary fat is not appropriate for those under two years of age, for those who are ill or malnourished, or for those diagnosed with anorexia nervosa

Parental concerns

A study conducted by the International Study of Asthma and Allergies in Childhood (ISAAC) found a strong correlation between trans fatty acids and increased occurrence of allergies in adolescents. Parents should provide healthy alternatives to foods containing fatty acids and monitor the amount and type of fats consumed in their diet. These preventative measures help avoid serious health problems including heart disease and stroke that can result from high levels of fatty acids in the diet.

Resources

BOOKS

Hark, L., and D. Deen. Nutrition for Life London: Dorling Kindersley, 2005. .

Thomas, Cardiovascular Disease: General Aspects. Manual of Dietetic Practice Oxford: Blackwell Science, 2001. Thomas, B. Hyperlipidaemia. Manual of Dietetic Practice Oxford: Blackwell Science, 2001.

Webster-Gandy, J., A. Madden, and M. Holdsworth. Oxford Handbook of Nutrition and Dietetics Oxford: Oxford University Press, 2006.

PERIODICALS

Chavarro, Jorge E., Janet W. Rich-Edwards, Bernard A. Rosner, and Walter C. Willett. ‘Dietary Fatty Acid Intakes and the Risk of Ovulatory Infertility.’ American Journal of Clinical Nutrition 85, no. 1 (January 2007): 231-237.

Hooper, L., C. Summerbell, J. Higgins, et al. ‘Dietary Fat Intake and Prevention of Cardiovascular Disease: Systematic Review.’ British Medical Journal 322 (March2001): 757-763.

Mead, A., G. Atkinson, D. Albin, et al. ‘Dietetic Guidelines on Food and Nutrition in the Secondary Prevention of Cardiovascular Disease—Evidence from Systematic Reviews of Randomized Controlled Trials.’ Journal of Human Nutrition and Dietetics 19 (January 2007): 401-409.

Mozaffarian, D., M. B. Katan, A. Ascherio, M. J. Stampfer, and W. C. Willett ‘Trans Fatty Acids and Cardiovascular Disease.’ The New England Journal of Medicine. 354, no. 15 (April 13, 2006): 1601-1613.

Nutrition Sub-Committee of the Diabetes Care Advisory Committee of Diabetes UK. ‘The Dietitians Challenge: The Implementation of Nutritional Advice for People with Diabetes.’ Journal of Human Nutrition & Dietetics 16, no. 6, (2003): 421-452.

ORGANIZATIONS

American Dietetic Association (ADA). 120 South Riverside Plaza, Suite 2000, Chicago, IL 60606-6995. Telephone: (800): 877-1600. Website: <http://www.eatright.org>.

American Heart Association. 7272 Greenville Avenue, Dallas, TX 75231. Telephone: (800) 242-8721. Website: <http://americanheart.org>.

British Dietetic Association. 5th Floor, Charles House, 148/9 Great Charles Street, Queensway, Birmingham, B₃ 3HT. Telephone: 0121 200 8080. Website: <www.bda.uk.com>.

British Heart Foundation. 14 Fitzgerald Street, London W1H 6DH. Telephone: 020 7935 0185. Website: <www.bhf.org.uk>.

British Nutrition Foundation. High Holborn House, 52-54 High Holborn, London WC1V 6RQ. Telephone: 020 7404 6504. Website: <www.nutrition.org.uk>. Food Standards Agency. Aviation House, 125 Kingsway, London WC2B 6NH. Telephone: 020 7276 8000. Web sites: <www.food.gov.uk>

OTHER

Fish and Omega-3 Fatty Acids American Heart Association. [cited May 7, 2007]. <http://www.americanheart.org/presenter.jhtml?identifier=4632> .

Food and Drug Administration. ‘Food Labeling; Trans Fatty Acids in Nutrition Labeling; Consumer Research to Consider Nutrient Content and Health Claims and Possible Footnote or Disclosure Statements; Final Rule and Proposed Rule.’ United States Department of Health and Human Services. Federal Register68, no. 133 (July 11, 2003). [cited May 7, 2007]. <http://www.cfsan.fda.gov/~acrobat/fr03711a.pdf>

Higdon, Jane. ‘Essential Fatty Acids.’ Micronutrient Information CenterLinus Pauling Institute, Oregon State University. December 7, 2005. [cited May 7, 2007]. <http://lpi.oregonstate.edu/infocenter/othernuts/omega3fa/>

Larsen, Joanne. ‘Fatty Acids’. Ask the Dietitian [cited May 7, 2007]. <http://www.dietitian.com/fattyaci.html>

Deborah Lycett, BSc(Hons) RD MBDA.

Feingold diet seeDr. Feingold diet

Fats

Lipids are organic substances consisting mostly of carbons and hydrogen atoms . They are hydrophobic, which means that they have little or no affinity to water. All lipids are soluble (or dissolvable) in nonpolar solvents, such as ether, alcohol, and gasoline. There are three families of lipids: (1) fats, (2) phospholipids, and (3) steroids.

Fatty acids and glycerol make up the larger molecule of fats. A fatty acid consists of a long carbon skeleton of 16 or 18 carbon atoms, though some are even longer. The carbonyl group, which is a carbon atom double-bonded to an oxygen atom and single-bonded to an oxygen attached to a hydrogen (OH-C=O), is the acidic group of the fatty acids. The acidic property is determined by the ability of the hydrogen to dissociate, or break away, from the oxygen atom. The carbonyl group is followed by a long chain of carbon atoms bonded to hydrogen, which is referred to as the hydrocarbon "tail." The long hydrocarbon tail gives fatty acids their hydrophobic, or "water-fearing" property. Fats cannot be dissolved in water because fats are nonpolar (an equal distribution of electrons) and water is polar (an unequal distribution of electrons). The polarity of water is unable to form bonds and break down the nonpolar fatty acid molecule.

There are different types of fatty acids, which vary in length and the number of bonds. Saturated fatty acids have single bonds between the carbon atoms that make up the tail. The carbon atoms are "full" or saturated, and therefore cannot take up any more hydrogen. Most animal fat, such as butter, milk, cheese, and coconut oil, are saturated. Unsaturated fatty acids have one or more double bonds between carbon atoms. A double bond is the sharing of four electrons between atoms, while a single bond is the sharing of two electrons. The double bond has the ability to lend its extra two electrons to another atom, thereby forming another bond. Monounsaturated fatty acids contain only one double bond, such that each of the carbon atoms of the double bond can bond with a hydrogen atom. An example of monounsaturated fatty acids is oleic acid, which is found in olive oil. Polyunsaturated fatty acids contain two or more double bonds, such that four or more carbon atoms can bond with hydrogen atoms. Most vegetable fats are polyunsaturated fatty acids. The double bonds change the structure of the fatty acid, in that there is a slight bend where the double bond is located.

Foods high in saturated fatty acids include whole milk, cream, cheese, egg yolk, fatty meats (e.g., beef, lamb, pork, ham), coconut oil, regular margarine, and chocolate. Foods high in polyunsaturated fatty acids include vegetable oils (e.g., safflower, corn, cottonseed, soybean, sesame, sunflower), salad dressing made from vegetable oils, and fish such as salmon, tuna, and herring.

Triglycerides are the basic unit of fat and are composed of three ("tri-") fatty acids individually bonded to each of the three carbons of glycerol. Fatty acids rarely exist in a free form in nature because they are highly reactive, and therefore make bonds spontaneously.

Fat Function, Metabolism, and Storage

Fats and lipids play critical roles in the overall functioning of the body, such as in digestion and energy metabolism. Usually, 95 percent of the fat in food is digested and absorbed into adipose, or fatty, tissue. Fats are the body's energy provider and energy reserve, which helps the body maintain a constant temperature. Fats and lipids are also involved in the production and regulation of steroid hormones , which are hydrophobic (or "water-fearing") molecules made from cholesterol in the smooth endoplasmic reticulum, a compartment within a cell in which lipids, hormones, and proteins are made. Steroid hormones are essential in regulating sexuality, reproduction, and development of the human sex organs, as well as in regulating the water balance in the body. Steroid hormones can also freely flow in and out of cells, and they modify the transcription process, which is the first step in protein synthesis, where segments of the cell's DNA , or the genetic code, is copied.

Fats and lipids also have important structural roles in maintaining nerve impulse transmission, memory storage, and tissue structure. Lipids are the major component of cell membranes. The three most common lipids in the membranes of eukaryots, or nucleus-containing cells, are phospholipids, glycolipids, and cholesterol. A phospholipid has two parts: (1) the hydrophilic ("water-loving") head, which consists of choline, phosphate, and glycerol, and (2) the hydrophobic ("water-fearing") fatty acid tail, which consists of carbon and hydrogen. The hydrophilic head is the part of the phospholipids that is in contact with water, since it shares similar chemical properties with water molecules. The hydrophobic tail of the phospholipids faces inward, and therefore is able to avoid any contact with water. In this particular arrangement, the phospholipids arrange themselves in a bilayer (double layer) alignment in aqueous solution.

Fats are metabolized primarily in the small intestines because the enzymes of the stomach cannot break down fat molecules due to their hydrophobicity. In the small intestines, fat molecules stimulate the release of cholecystokinin (CCK), a small-intestine hormone, into the bloodstream. The CCK in the blood triggers the pancreas to release digestive enzymes that can break down lipids. The gallbladder is also stimulated to secrete bile into the small intestines. Bile acids coat the fat molecules, which results in the formation of small fat globules, which are called micelles. The coating prevents the small fat globules from fusing together to form larger fat molecules, and therefore the small fat globules are more easily absorbed. The pancreatic enzymes can also break down triglycerides into monoglycerides and fatty acids. Once this occurs, the broken-down fat molecules are able to diffuse into the intestinal cells, in which they are converted back to triglycerides, and finally into chylomicrons.

Chylomicrons, which are composed of fat and protein, are macromolecules that travel through the bloodstream into the lymphatic capillaries called lacteals. The lymphatic system is a special system of vessels that carries a clear fluid called lymph, in which lost fluid and proteins are returned to the blood. The lacteals absorb the fat molecules and transport them from the digestive tract to the circulatory system, dumping chylomicrons in the bloodstream. The adipose and liver tissues, which release enzymes called lipoprotein lipase, break down chylomicrons into monoglycerides and fatty acids. These molecules diffuse into the adipose and liver cells, where they are converted back to triglycerides and stored as the body's supply of energy.

Fat Nutrition

The energy value of fats is 9 kcal/gram (kilocalories per gram), which supplies the body with important sources of calories. Calories are units of energy. The breaking of bonds within fat molecules releases energy that the body uses. A kilocalorie is the unit used to measure the energy in foods. It is the equivalent of "calories" listed on Nutrition Facts labels on food packaging.

Some of the foods known to contain large amounts of fat include the obvious examples, such as butter on toast, fried foods, and hamburgers. But many of the foods that people consume on a daily basis have hidden sources of fat that may not be obvious to the person eating them. These foods include cookies and cakes, cheese, ice cream, potato chips, and hot dogs. One way to avoid foods that contain high amounts of fat is to look at the Nutrition Facts label located on the packages of most foods, where the total fat content of the food is listed.

Actual intake of fat can vary from 10 percent to 40 percent of the calories consumed daily, depending on personal or cultural regimens. Limiting one's daily fat intake to less than 30 percent of total calorie intake and increasing consumption of polyunsaturated fatty acids have been shown to be beneficial in maintaining a healthful diet .

Effects of Excess Dietary-Fat Intake

The recommended intake of fats in the American diet is to limit fats to below 30 percent of the total daily caloric intake. One-third of fats should come from saturated fats, with the other two-thirds split evenly between monounsaturated and polyunsaturated fat. It is estimated that in the average American diet (as of 2002), fats make up 42 percent of calories, with saturated fat making up between a third and a half of that amount.

The effects of this excess intake of dietary fat has some well-established implications for the health of overweight Americans. For instance, the consumption of excess amounts of saturated fats has been recognized as the most important dietary factor to increase levels of cholesterol. A high cholesterol level is detrimental to health and leads to a condition known as atherosclerosis. Atherosclerosis is the build-up of cholesterol on the walls of arteries , which may eventually result in the blocking of blood flow. When this occurs in the arteries of the heart, it is called coronary artery disease. When this process occurs in the heart, a myocardial infarction, or heart attack , may occur.

Besides the cholesterol implications due to high fat intake, obesity is a factor in the causation of disease. Being overweight or obese is highly associated with increasing the risk of type II diabetes , gallbladder disease, cardiovascular disease, hypertension , and osteoarthritis .

Fat-Replacement Strategies

The purpose of fat-replacement strategies is to reduce the percentage of fat in various foods, without taking away the appealing taste of the food. There are three broad categories of fat-replacement strategies: (1) adding water, starch derivatives, and gums to foods, (2) using protein-derived fat replacements, and (3) using engineered fats.

The addition of water to foods lowers the quantity of fat per serving in the selected food item. When starch derivatives are added to food, they bind to the water in the food, thus providing a thicker product that simulates the taste and texture of fat in the mouth. Examples of specific starch derivatives include cellulose , Z-trim, maltrin, stellar, and oatrim. The problem with starch derivatives, however, is their limitations as a fat replacement in foods that require frying.

Protein-derived fat replacements are made from egg and milk proteins, which are made into a microscopic globule of protein. They give the sensation of fat in the mouth, although they contain no fatty acids. One such product is Simplesse, which is used mostly in frozen desserts. Because its chemical structure is easily destroyed by cooking or frying, its use is limited in most other foods.

The third fat-replacement strategy includes the use of engineered fats, which are made by putting together various food substances. One popular engineered fat is olestra, which is made by adding fatty acids to regular table sugar molecules (sucrose). This process results in a product that can neither be broken down in the digestive tract nor absorbed. It therefore cannot provide energy, in terms of carbohydrates or fatty acids, to the body. Olestra is the first engineered fat to be used in fried foods. It does have its drawbacks, however. Olestra can cause abdominal cramping, loose stools, and it can bind beneficial substances that are normally absorbed, such as the fat-soluble vitamins (vitamins A, D, E, and K) and carotenoids .

In addition to fat-replacement strategies, there are low-fat or fat-free versions of many foods on the market. Some products made to be low-fat or fat-free include milk, yogurt, some cheeses, and deli meats. As a general rule, products that claim to have reduced amounts of fat should conform to the following stipulations: (1) a product labeled "reduced-fat" must have at least 25 percent less fat than the normal product, (2) a "low-fat" product can have no more than three grams of fat per serving, and (3) a "fat-free" product most have less than 0.5 grams of fat per serving. But one does not always need to look for foods made to contain less fat than normal, as there are plenty of natural foods that contain very little fat, or no fat at all, including most fruits and vegetables. Other foods that fit into the category of low-fat or nonfat foods include egg whites, tuna in water, skinless chicken, and pasta.

Foods that are low in fat are important for a healthful diet. While fats are essential components for bodily function, excess consumption of fats can lead to health problems such as obesity and heart disease . A healthful diet therefore consists of balanced proportions of proteins, fats, and carbohydrates.

see also Fat Substitutes; Lipid Profile; Omega-3 and Omega-6 Fatty Acids.

Jeffrey Radecki

Susan Kim

Bibliography

Campbell, Neil A., et al. (2000). Biology, 4th edition. San Francisco: Benjamin/Cummings.

Must, A., et al. (1999). "The Disease Burden Associated with Overweight and Obesity." Journal of the American Medical Association 282: 1523.

Robinson, Corinne H.; Weigley, Emma S.; and Mueller, Donna H. (1993). Basic Nutrition and Diet Therapy, 7th edition. New York: Macmillan.

Wardlaw, Gordon M., and Kessel, Margaret (2002). Perspectives in Nutrition, 5th edition. Boston: McGraw-Hill.

fats or, more technically, lipids, together with carbohydrates and proteins, are one of the three staples of the diet. As the seventeenth-century nursery rhyme relates, ‘Jack Sprat he ate no fat/His wife she ate no lean’. This does not mean, however, that Jack Sprat could not get fat, because excess carbohydrates and proteins can be broken down in the body and the fragments synthesized into fat. But had he eaten no fat whatsoever he would have been deprived of certain essential fatty acids, and of the fat-soluble vitamins. Fats serve many purposes: they not only provide thermal insulation and a source of energy, with stores in reserve, but also are involved in important functions, providing for example the materials for components of cell membranes, of the myelin sheaths that electrically insulate nerve fibres.

Chemically, fats are esters — that is, formed by combination of an alcohol with an acid. In this case the alcohol is glycerol, which has three alcohol groups, allowing it to combine with three acidic groups. The acids are fatty acids: long chains of carbon atoms linked to a carboxyl (acidic) group. Three fatty acids therefore combine with one glycerol molecule to form a triglyceride or neutral fat molecule.

Triglycerides are the main fats in the diet. After breakdown and absorption in the alimentary system, they are resynthesized and stored in special cells in which fat globules coalesce to form a large droplet, almost filling the cell. Aggregations of fat cells form adipose tissue (as in the white fat of uncooked meat). White fat can be deposited in a variety of tissues, especially beneath the skin and in muscles — but a great deal of human endeavour is today devoted to getting rid of adipose tissue collected around the waist, even to the extent of having liposuction. Fat acts as a rich fuel reservoir that can be utilized during starvation. Oxidation of fat yields about twice the energy that can be derived from equal amounts of carbohydrate or protein. However, white fat is not easily got rid of physiologically, since it is poorly perfused with blood. Hence hormones that mobilize fat do not reach the target in high concentration, nor is there a high flow rate to carry the energy-giving molecules in the blood to the tissues, such as muscles, where they can be burned.

There is a second type of fat deposit, namely brown fat, found at the base of the neck and between the shoulder blades. This tissue is specialized for thermogenesis, i.e. the rapid mobilization of fat to generate heat. The cells contain many small lipid droplets, a pigment, and many mitochondria, the latter essential for breaking down the lipids into simple two carbon fragments from which energy can be rapidly generated. Furthermore the tissue is well perfused with blood and has a rich innervation by nerves liberating noradrenaline, one of the hormones that can rapidly mobilize the breakdown of fats. Brown fat is important for heat production in infants, and is retained variably into later life. Those who retain the most brown fat into adulthood find it easier to avoid putting on weight. Hibernating animals lay down large amounts of brown fat to see them through the dormant period.

Many different fatty acids are found among the lipids. They vary in the number of carbon atoms in the chain, which in some cases are branched, and also in the number of double bonds they contain, if any. Those with double bonds are known as unsaturated, and those without as saturated. For example palmitic acid and palmitoleic acid both have 16 carbon atoms, but the latter has one double bond (monounsaturated). Oleic, linoleic, and arachidonic acid have respectively 16, 18, and 20 carbon atoms in the chain and 1, 2, and 4 double bonds. Oleic and linoleic acids are essential fatty acids, that is they cannot be synthesized in the body and are therefore essential dietary constituents. Readers will be familiar with the term ‘rich in polyunsaturates’ applied to many supermarket products like margarines, sunflower oils, etc. There is evidence that diets rich in polyunsaturated fats are less likely to cause atherosclerosis than ones rich in animal fats, with their predominance of saturated fats.

Fatty acids are essential constituents of phospholipids and glycolipids. In phospholipids the glycerol moeity is combined with only two fatty acids, the remaining alcohol group being combined with a phosphate group linked to an alcohol (e.g. to serine, choline, or inositol, to give the phospholipids phosphatidylserine, phosphatidylcholine, and phophatidylinositol). These compounds are ‘amphipathic’, that is, they have a polar head group, which is compatible with an aqueous environment, and a non-polar tail, which is not. Such molecules can form films (as does oil spread on the surface of water) in which the hydrophobic tails interact with each other, projecting out of the aqueous surface, while the polar head groups remain in the water. Now imagine such films are brought together so that the hydrophobic tails of one film are opposed to those of the second. This is a very close approximation to the structure of all cell membranes, where the polar head of one lipid layer contacts the extracellular environment, while the head groups of the other layer contact the aqueous environment within the cell. These structures, so-called lipid bilayers, form flexible membranes, which are very impermeable to the movement of substances across them, whilst particular permeability properties are provided by the inclusion of protein molecules in the membrane. Some membrane fatty acids can be mobilized as autacoids (released to affect other cells) and as intracellular messengers. For example, arachidonic acid gives rise to prostaglandins in cell membranes, and phosphatidylinositol is the source of the important ‘second messenger’ inositol triphosphate, implementing an internal response to a chemical message from outside the cell.

Alan W. Cuthbert

fat / fat/ • n. a natural oily or greasy substance occurring in animal bodies, esp. when deposited as a layer under the skin or around certain organs. ∎  a substance of this type, or a similar one made from plant products, used in cooking. ∎  the presence of an excessive amount of such a substance in a person or animal, causing them to appear corpulent. ∎  Chem. any of a group of natural esters of glycerol and various fatty acids, which are solid at room temperature and are the main constituents of animal and vegetable fat. ∎  something excessive or unnecessary: fat in the state budget.• adj. (fat·ter , fat·test ) (of a person or animal) having a large amount of excess flesh: the driver was a fat, wheezing man. ∎  (of an animal bred for food) made plump for slaughter. ∎  containing much fat: fat bacon. ∎  large in bulk or circumference: a fat cigarette. ∎ inf. (of an asset or opportunity) financially substantial or desirable: a fat profit. ∎  inf. used ironically to express the belief that there is none or very little of something: fat chance she had of influencing him a fat lot of good that'll do him. ∎  ∎  (of wood) containing a high proportion of resin: fat pine.• v. (fat·ted, fat·ting) archaic make or become fat: [as adj.] (fatted) a fatted duck. PHRASES: live off (or on) the fat of the land have the best of everything.DERIVATIVES: fat·ness n.fat·tish adj.