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n.

1. 1. The ester of glycerol and one, two, or three fatty acids.
   2. Any of various soft, solid, or semisolid organic compounds constituting the esters of glycerol and fatty acids and their associated organic groups.
   3. A mixture of such compounds occurring widely in organic tissue, especially in the adipose tissue of animals and in the seeds, nuts, and fruits of plants.
   4. Animal tissue containing such substances.
   5. A solidified animal or vegetable oil.
2. Obesity; corpulence.
3. The best or richest part: living off the fat of the land.
4. Unnecessary excess: “would drain the appropriation's fat without cutting into education's muscle” (New York Times).

adj., fat·ter, fat·test.

1. Having much or too much fat or flesh; plump or obese.
2. Full of fat or oil; greasy.
3. Abounding in desirable elements.
4. Fertile or productive; rich: “It was a fine, green, fat landscape” (Robert Louis Stevenson).
5. Having an abundance or amplitude; well-stocked: a fat larder.
6. 1. Yielding profit or plenty; lucrative or rewarding: a fat promotion.
   2. Prosperous; wealthy: grew fat on illegal profits.
7. 1. Thick; large: a fat book.
   2. Puffed up; swollen: a fat lip.

tr. & intr.v., fat·ted, fat·ting, fats.

To make or become fat; fatten.

idioms:

a fat lot Slang.

1. Very little or none at all: a fat lot of good it will do him.

fat chance Slang.

1. Very little or no chance.

[Middle English, from Old English fǣtt, fatted.]

fatly fat'ly adv.
fatness fat'ness n.

SYNONYMS  fat, obese, corpulent, fleshy, portly, stout, pudgy, rotund, plump, chubby. These adjectives mean having an abundance and often an excess of flesh. Fat implies excessive weight and generally has negative connotations: was getting fat and decided to exercise. Obese and corpulent imply gross overweight: “a woman of robust frame . . . though stout, not obese” (Charlotte Brontë). The dancer was corpulent but surprisingly graceful. Fleshy implies a not necessarily excessive abundance of flesh: firm, fleshy arms. Portly refers to bulk combined with a stately or imposing bearing: “a portly, rubicund man of middle age” (Winston Churchill). Stout denotes a thickset, bulky figure: a painting of stout peasants. Pudgy means short and fat: pudgy fingers. Rotund suggests roundness of figure, often in a squat person: “this pink-faced rotund specimen of prosperity” (George Eliot). Plump and chubby apply to a pleasing fullness of figure: a plump little toddler; chubby cheeks.

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

See also body weight; cell membrane; metabolism; obesity.

1. Chemically fats (or lipids) are substances that are insoluble in water but soluble in organic solvents such as ether, chloroform, and benzene, and are actual or potential esters of fatty acids. The term includes triacylglycerols (triglycerides), phospholipids, waxes, and sterols.

2. In more general use the term ‘fats’ refers to the neutral fats which are triacylglycerols, mixed esters of fatty acids with glycerol.

neutral fat; triglyceride

True fats, neutral fats, or triglycerides (also called triacylglycerols) are lipids formed by the combination of glycerol and three fatty acids. Triglycerides are sometimes distinguished by their physical state: those which are liquid at room temperature (20°C) are called oils; those which are solid at room temperature are called fats. Often, however, fats and oils are both referred to as fats.

Fats contain carbon, hydrogen, and oxygen. They have a high proportion of hydrogen which makes them a more concentrated form of energy than either carbohydrates or protein. Each gram of fat or oil produces about 9 Calories of energy. Fats are the primary source of energy during prolonged aerobic exercise. The release of energy from fat requires more oxygen than the release of the same amount of energy from carbohydrates. Fat metabolism therefore puts a greater strain on the oxygen transport system.

Excess dietary fat is stored as body fat in adipose tissue. Excess dietary carbohydrates and proteins may also be converted into fat and stored in adipose tissue to provide energy (e.g. during the fasting state between meals), heat insulation, cushioning, and buoyancy.

There is a lot of confusion over the part dietary fat plays in causing disease. The confusion stems from the bewildering number of types of fat, and there is disagreement about how harmful or beneficial each type is (see saturated fat and unsaturated fat). Nevertheless, it is beyond dispute that high intakes of dietary fat are linked to obesity and coronary heart disease. Doctors speak with one voice when they say that we need to restrict our total fat so that it contributes no more than 35 per cent of the total calorific intake in the diet. Only 10 per cent of calorific intake should be saturated fat, the form of fat most clearly linked to disease. The average person in North America and the UK derives 40-45 per cent of his or her calories from fat.

Apart from the health risks, high fat diets may also impair physical performance. Athletes are usually advised to avoid fatty foods before competition because they can take 3-4 hours to digest. It seems that a high fat meal may also cause early fatigue by increasing the level of fatty acids in the blood.

It is wrong to try to eliminate fat from the diet completely. It is needed to absorb vitamins A, D, E, and K, and a little fat in the digestive system helps to delay hunger pangs. Some fatty acids are essential for the manufacture of other lipids, such as steroids. Other fatty acids appear to actually reduce the risk of coronary heart disease (see omega-3 fatty acids). Also, a modest intake of fat helps to maintain optimum levels of triglycerides in the muscles. These act as energy stores for aerobic activities.

noun

1. Adipose tissue: suet. See fat/thin.
2. An amount or quantity beyond what is needed, desired, or appropriate: excess, glut, overage, overflow, overmuch, overrun, overstock, oversupply, superfluity, surplus, surplusage. See excess/insufficiency/enough.

adjective

1. Having too much flesh: corpulent, fatty, fleshy, gross, obese, overblown, overweight, porcine, portly, stout, weighty. See fat/thin.
2. Having the qualities of fat: adipose, fatty, greasy, oily, oleaginous, unctuous. See fat/thin.
3. Affording profit: advantageous, lucrative, moneymaking, profitable, remunerative, rewarding. See get/lose.
4. Relatively great in extent from one surface to the opposite: thick. See thick/thin.

Idioms beginning with fat:
fat cat
fat lot
fat city
fate
fate worse than death, a
fat farm
fat is in the fire, the
fat of the land, the

See also chew the fat; kill the fatted calf.

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adj

Definition: containing oily substance
Antonyms: lean

adj

Definition: overweight
Antonyms: skinny, slender, slight, slim, thin

adj

Definition: productive, rich
Antonyms: impoverished, poor, unproductive

n

Definition: overweight
Antonyms: skinniness, thinness

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n

1. a substance composed of lipids or fatty acids and occurring in various forms or consistencies ranging from oil to tallow. 2. a type of connective tissue containing stored lipids.

Fats, or lipids, are a group of chemical substances in food that are generally insoluble in water. There are several classes of fats. The triglycerides (triacylglycerols) are the predominant constituent of vegetable and animal fats and oils. They are composed of a 3-carbon glycerol backbone and three fatty acids of various types. The character of the fat is determined by these fatty acids. Saturated fatty acids tend to make the fat "hard" or solid at room temperature and are associated with an increased risk of heart disease. Monounsaturated fatty acids are prominent in olive and canola oils and do not increase the risk of heart disease. The third group of fatty acids, the polyunsaturated fatty acids, are important components of omega-3 fish and plant oils. They play a role in blood clotting and in inflammatory responses in the body.

Phospholipids are closely related to triacylglycerols, except that one of the carbons on the glycerol contains one of several phosphate groups; the other two carbons have fatty acids. A third group, related to the first two, is the sphingomyelins and other complex brain lipids.

Cholesterol and its precursors are another group of fats that are essential for membranes; these are chemically composed of several 5-and 6-membered carbon rings. Steroids make up a fifth group of fats. Steroids are derived from cholesterol and include the androgens and estrogens, among others.

(SEE ALSO: Blood Lipids; HDL Cholesterol; LDL Cholesterol; Lipoproteins; Triglycerides; VLDL Cholesterol)

— GEORGE A. BRAY

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For more information on fat, visit Britannica.com.

1. Material accumulating on a trowel during smooth troweling; used to fill in small imperfections.
2. See fat concrete, fat lime, etc.

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A type of lipid. A true fat, or neutral fat, is a triacylglycerol (triglyceride) made of glycerol and three fatty acids. It provides energy, heat insulation, mechanical cushioning, and buoyancy. Fat can also be used in the body to make other chemicals, such as steroids. Fat is one of the basic nutrients and it is the body's most concentrated source of energy. Each gram produces about 9 kcal of energy at a cost of 2.031 l of oxygen (i.e. 0.255 l oxygen cal⁻¹) and the fat stores in the body exceed 70, 000 kcal. Fat is preferentially metabolized during low intensity, long duration exercise. However, it has to be converted to its basic components (glycerol and free fatty acids) before it can be used to make adenosine triphosphate and this conversion is too slow to meet all of the energy demands of intense physical activity. High fat diets are associated with obesity and heat disease, therefore it is generally recommended that fat should contribute less than 35% of the total calorific intake in the diet. See also essential fatty acids.

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.

Table 1

Classes of fatty acids
Trivial name	Systematic name	Numerical symbol
Saturated fatty acids
Lauric acid	Dodecanoic	12:0
Myristic acid	Tetradecanoic	14:0
Palmitic acid	Hexadecanoic	16:0
Stearic acid	Octadecanoic	18:0
Monounsaturated fatty acids
Palmitoleic acid	cis-9-hexadecenoic	16:1(9)
Oleic acid	cis-9-octadecenoic	18:1(9)
Polyunsaturated fatty acids (omega-6)
Linoleic acid	cis, cis-9, 12-octadecadienoic	18:2 (9,12)
Arachidonic acid	All cis-5,8,11, 14-eicosatetraenoic	20:4 (5,8,11,14)
Polyunsaturated fatty acids (omega-3)
Linolenic acid	All cis-9-12-15-octadecatrienoic	18:3 (9,12,15)
EPA	All cis-5,8,11,14, 17-Eicosapentaenoic	20:5 (5,8,11,14,17)
DHA	All cis-4,7,10,13,16, 19-Docosahexaenoic	22:6 (4,7,10,13,16,19)

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.

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.

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

Organic compounds that serve as a reserve of energy for the body. Fat is stored in the body's fat tissues, which provide support, protection, and insulation for the body and its organs. A balanced diet must include some fats because, in addition to providing energy for the body, they are necessary for the absorption of certain vitamins.

Many people consume too much fat in their diet; this imbalance can contribute to various diseases (such as disorders of the heart). Some fats, called saturated fats, have been found to raise the level of cholesterol in the blood, whereas other fats, called unsaturated fats, may help reduce blood cholesterol levels.

1. the adipose or fatty tissue of the body.
2. neutral fat; a triglyceride (or triacylglycerol), which is an ester of fatty acids and glycerol (a trihydric alcohol). Each fat molecule contains one glycerol residue connected by ester linkages to three fatty acid residues, which may be the same or different. The fatty acids may have no double bonds in the carbon chain (saturated fatty acids), one double bond (monounsaturated), or two or more double bonds (polyunsaturated).

• f. absorption test — assesses the absorptive capacity of the small intestine, quantitatively by measuring serum lipid levels or qualitatively by plasma turbidity, at timed intervals after the oral administration of fats.
• animal f. — a most important abattoir by-product providing edible fat for the human food chain. Products include oleo oil and oleo stearin used in margarine manufacture and dripping for commercial baking. Nonedible fats go to leather dressings, glycerol manufacture and lubricants. Beef and pork fat are the valuable ones, mutton fat having too strong a flavor for edible fat.
• boiling (burning) f. — see acrolein poisoning.
• f. cattle — a class of beef cattle of any age but usually greater than one year, well-covered and judged ready for slaughter to provide prime cuts of beef.
• f. cow syndrome — a syndrome of anorexia and ketonuria that occurs in overfat cows at calving. Precipitated by events that interfere with the cow's feed intake for even short periods. A poor response to treatment and many cows die.

  Fatty liver in fat cow syndrome. By permission from Blowey RW, Weaver AD, Diseases and Disorders of Cattle, Mosby, 1997

• crude f. — that part of a feed that is extractable by ether. Includes fat, oil, wax, resin and some pigments.
• dietary f. — a rich source of energy for carnivores and omnivores and to a limited extent ruminants. Are usually too expensive for widespread use other than as excipients. They aid in the formation of pellets and in reducing dustiness. Their problem is a tendency to rancidification unless an antioxidant is added.
• f. embolism — lesion created by a fat embolus.
• f. embolus — globules of fat, sufficient to act as emboli occur usually after trauma or surgery, but can also occur in hyperlipemia, myositis and atherosclerosis.
• f. ewe pregnancy toxemia — occurs when there is a voluntary restriction of food intake in late pregnancy associated with lack of ruminal expansion potential caused by excess abdominal fat and multiple fetuses. It is common in hobby sheep farms where it is thought that ewes should lamb with body condition scores greater than 4 rather than less than 3.5.
• leaf f. — the best edible fat from a pig carcass, from under the peritoneum.
• f. marbling — deposition of fat between muscle fibers. A highly desirable characteristic in beef. Is a guarantee of a carcass from a young animal.
• f. necrosis — necrosis in which fat is broken down into fatty acids and glycerol, usually occurring in subcutaneous tissue as a result of trauma. See also lipomatosis.
• orbital f. — fat located deep to the eyeball; substantial amounts provide good shock-absorbent surroundings.
• perivaginal f. prolapse — during a difficult parturition in a fat cow or heifer perivaginal fat is pushed caudally and bursts through the vaginal wall into the vagina.
• f. phanaerosis — conversion in the tissues of invisible fatty substances into fat which can be stained and thus become visible.
• f. prolapse — see perivaginal fat prolapse (above).
• f. sheep — a class of meat sheep of any age but usually greater than one year, well-covered and judged ready for slaughter to provide prime cuts of mutton.

Description	Quantity	Energy (calories)	Carbs (grams)	Protein (grams)	Cholesterol (milligrams)	Weight (grams)	Fat (grams)	Saturated Fat (grams)
cooking/vegetable shortening	1 cup	1810	0	0	0	205	205	51.3
cooking/vegetable shortening	1 tbsp	115	0	0	0	13	13	3.3

IN BRIEF: Excess body weight.

The vet said that our dog was too fat, so we put him on a diet.

Tutor's tip: Being "fat" (overweight) should not hinder you from publishing the "phat." (matter that can be set easily into type)