fat

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A mixture of lipids, chiefly triglycerides, that is solid at normal body temperatures. Fats occur widely in plants and animals as a means of storing food energy, having twice the calorific value of carbohydrates. In mammals, fat is deposited in a layer beneath the skin (subcutaneous fat) and deep within the body as a specialized adipose tissue.

Fats derived from plants and fish generally have a greater proportion of unsaturated fatty acids than those from mammals. Their melting points thus tend to be lower, causing a softer consistency at room temperatures. Highly unsaturated fats are liquid at room temperatures and are therefore more properly called oils.

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In food contexts, "fats" generally refers to liquid or solid substances used for cooking foods as well as seasoning, binding, emulsifying and preserving them. These fats can be of animal origin—butter, pork fat (lardo and lard), beef tallow, suet, goose fat, etc.—or plant origin—vegetable fat, most margarines, corn oil, sunflower oil and nut oils, etc., or as shortening, which is a fat based on several vegetable oils, sometimes with animal fats added and solidified through hydrogenation.

Serving Ideas

For a long time, it was believed that foods high in saturated fats (butter, red meat, cheese, etc.) could be replaced by foods containing mostly unsaturated fats (polyunsaturated oil and margarine, white meat, etc.) without concern for overall fat consumption. It has been discovered, however, that the amount of fat consumed, particularly, the amount of saturated fat, is the primary factor to be taken into consideration. It has also been discovered that polyunsaturated fatty acids are not as beneficial for health as previously believed. Monounsaturated fatty acids have been found to be equally, if not more, beneficial than polyunsaturated fatty acids. Therefore, it could be beneficial to incorporate olive oil, for example, which is high in monounsaturated fats, into the daily diet. For active people following a balanced diet, daily fat intake should not represent more than 35% of a total diet. People with a sedentary lifestyle should not eat more than 30%. It can prove difficult to reduce fat consumption, since the fat can often come from sources that may be "invisible." In effect, in many foods produced by the food industry or prepared in restaurants, a large part of the calories may come from fats.
Here are some tips to reduce fat consumption:

      • Reduce portion sizes of meat, and choose lean meats or remove the fat before cooking: round, flank, rib-eye or sirloin in the case of beef; loin, shoulder or fillet for pork; escalope, chop or thigh for veal as well as for poultry, horse meat and game.

      • Use a nonstick frying pan so that only a small amount or no fat is added.

      • Opt for low-fat dairy products: choose cheeses containing less than 20% fat.

      • Choose cooking methods that do not require much fat (baking, broiling and grilling, steaming or microwaving).

      • Avoid hydrogenated fats: hard margarines, shortenings, deep-fried foods, crackers, cookies, cake mixes, snack foods, imitation cheese products, potato chips. Opt for nonhydrogenated margarine.

      • Vary the types of fat in the diet. Choose a vegetable oil (cold-pressed or not), a small amount of butter and a nonhydrogenated margarine. Avoid foods that are high in tropical oils such as cookies, certain baked goods and certain cereals.

Read the list of ingredients, the nutritional information table and the fat content of the food on the product label carefully. Only foods of animal origin contain cholesterol and the term "light" on a food does not necessarily mean it is lower in fat. A "light" olive oil, for example, will simply have a light taste, and having the words "no cholesterol" on a vegetable oil is rather misleading. On the other hand, "light" butters and margarines genuinely contain half of the calories of the ordinary product, but cannot be used for cooking, as their water content is very high. They are thus reserved for spreading on bread or other uses.


Reducing consumption of "added" fats represents the simplest and most effective way of managing dietary intake of fat in general. A reduction in dietary fat intake should, as a matter of necessity, be accompanied by a reduction in saturated fats, in order to reduce risk factors for coronary illnesses, since saturated fatty acids are the most associated with "high" cholesterol increasing the LDLs ("bad") and reducing HDLs ("good") cholesterol.

Fat substitutes

The use of a fat substitute was approved in the United States and Canada several years ago; it is a mixture of egg protein and milk. Its taste has the unctuous quality typical of fat. On labels, it is indicated by the words "protein microparticles." This substitute only contains 1 or 2 calories per gram, as opposed to 9 for fats. At the moment, this substitute is only used in the commercial food industry, particularly in the ice cream industry and as a thickener and textural modifier for frozen desserts. It does not tolerate very high temperatures, so it cannot be used for deep-frying.

A fat substitute has been created in the United States by manipulating the fat molecule. It is supposed to have the same properties with regard to taste, texture and appearance as fat, and tolerates frying; it does not contain any calories or cholesterol, but inhibits the absorption of vitamin E.

These substitutes could help to reduce the consumption of fats but, unlike fat, they do not contribute to the feeling of fullness that appeases the appetite. Foods that contain these fat substitutes, moreover, do not necessarily contain very many fewer calories, as some of the other additives and ingredients used can increase their calorie value.

Nutritional Information
In nutrition, the term "fats" is often used as a synonym for lipids. Fats are twice as high in energy (9 calories/g) as carbohydrates and proteins (4 calories/g). They provide energy and heat and contribute to the formation of body fat. All of the cells in the body (except the red blood cells and the cells of the central nervous system) use fatty acids directly as a source of energy.

Dietary fats add taste to foods and carry the fat-soluble vitamins (A, D, E and K). Without any fat in the diet, these vitamins would become ineffective in the body. Dietary fats provide two types of fat that the human body cannot produce by itself; they are called essential fatty acids, namely linoleic acid and alpha-linolenic acid. These fatty acids are especially found in polyunsaturated acids. The essential fatty acids enable the circulatory and immune systems to function harmoniously. They also contribute to the development of each cell.

Under the heading of the essential fatty acid alpha-linolenic acid are the omega-3 fatty acids found in fish. This type of fat is supposed to be beneficial for chronic inflammatory diseases, such as rheumatoid arthritis, as well as for the immune system and the incidence of atherosclerosis and cardiovascular diseases. 

Only small quantities of fats are necessary. 

High consumption of fat can raise blood cholesterol levels in susceptible individuals and contribute to the development of obesity.

Fats are metabolized slowly, as they slow down the rate of gastric emptying, which spreads the digestive process over a longer period of time.


Hydrogenation

Hydrogenation is carried out on oils that are mostly constituted from polyunsaturated fatty acids, which are liquid at room temperature. It is a process by which hydrogen is added to the molecular structure of unsaturated fats. The effect of this is to solidify the oils, raise their melting point, delay rancidity and improve the consistency of foods made with hydrogenated fats. All-vegetable shortening is obtained by the complete hydrogenation of oil, whereas pure shortening can contain animal or vegetable fats. On the other hand, hydrogenation changes the configuration of fatty acids. Fatty acids that have become "trans" fatty acids act like saturated fatty acids: they raise blood cholesterol, in particular LDLs, or the "bad" cholesterol, and reduce HDLs, or the "good" cholesterol.

Cholesterol

Cholesterol is a fatty substance normally found in blood and also found in foods of animal origin; it does not provide any energy. Cholesterol plays a role in synthesizing bile, the functioning and development of the nervous system, adrenal hormones and reproduction; it is an important component of the myelin that forms a protective casing around the nerves, and is a precursor of vitamin D, among other things. The human body, mainly the liver, manufactures almost 80% of the cholesterol it needs; the remainder (20%) is derived from foods of animal origin. Reducing consumption of saturated fats and cholesterol is recommended, since it is linked with cardiovascular diseases, obesity and various cancers, including colon and breast cancer. No scientific results allow the claim to be made that the risk of cancer would be reduced simply by limiting fat intake to 30% of total energy intake, or simply by changing the type of fat consumed. Cholesterol and lipids are particularly of concern with regard to the development of atherosclerosis, which consists of an accumulation of fat deposits on the walls of the coronary arteries, causing sclerosis—that is, the thickening and hardening of the arterial tissue, which can seriously restrict blood flow. The factor with the greatest effect on blood cholesterol is the excessive consumption of fat, particularly saturated fats.

Triglycerides are another substance that is found in the blood. They rise as a result of excessive alcohol, fat and sugar in the diet.Their influence as a risk factor for coronary diseases is less significant than that of LDLs.

The notion of "good" and "bad" cholesterol only applies to blood cholesterol. However, foods contain several sorts of fat that influence "good" and "bad" blood cholesterol levels in different ways. The three types of fatty acid, in effect, act on the organism in different ways. 

Foods contain a mixture of these fatty acids in different proportions, but with one or the other predominating. Meats and dairy products, for example, are said to be high in saturated fats, whereas olive oil and almonds are high in monounsaturated fats and sunflower oil contains a large amount of polyunsaturated fats.

Saturated fatty acids tend to raise blood cholesterol levels in susceptible individuals who eat a great deal of them. They are mainly found in foods of animal origin and in exceptional cases in tropical oils (palm oil and coconut oil, for example). This type of fat is solid at room temperature.

Monounsaturated fats offer protection for the "good" cholesterol (HDLs), even to the point of increasing it and reducing the "bad" cholesterol (LDLs). This type of fat is also very high in essential fatty acids; these fats are found in olive and canola oils, almonds, hazelnut oil and avocado. They are liquid at room temperature and more resistant to oxidation than polyunsaturated fatty acids. 

Polyunsaturated fatty acids lower the level of "bad" cholesterol (LDLs) but also that of "good" cholesterol (HDLs). They are mainly found in vegetable oils (corn, soy, wheat germ, safflower, sunflower, sesame, among others). The polyunsaturated fat family also includes the fish fats commonly called omega‑3s, which thin the blood, preventing the formation of clots. It is recommended to eat 2-3 servings of fish high in omega-3 per week, such as mackerel, herring, tuna and salmon. However, there is not yet enough research to recommend the use of fish-oil supplements.

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For more information on fat, visit Britannica.com.

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.

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.

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.

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

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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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.

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.

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.

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.

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What are Fats?

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.

What is the Purpose of Fats?

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.

Type of fat	Dietary source	Effect on cholesterol	How often to choose
Trans fat	•“Hydrogenated” or “partially hydrogenated” oils	Raises LDL	Less often
	• Vegetable shortenings, stick margarine, deep fried foods, some fast foods and snack foods (i.e., cookies and crackers)
Saturated fat	•Tropical oils such as palm and coconut oils, cocoa butter, coconuts and coconut milk	Raises LDL	Less often
	•Red meat, the skin from chicken and other birds, butter, whole milk and milk products (i.e., cheese and ice cream)
Monounsaturated fat	•Avocados, olives, certain nuts	Lowers LDL when	More often
	•Olive, canola, and peanut oils	substituted for saturated fat
Polyunsaturated fat (includes omega-3 and omega-6 fatty acids)	•Plant oils like corn, sunflower, and safflower	Lowers LDL when	More often
	• Fish (especially salmon, trout, and herring)	substituted for saturated fat
	•Flaxseed oil

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

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

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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.

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—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.

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)

LearnThatWord.com is a free vocabulary and spelling program where you only pay for results!

as in: overweight
sign description: The pinky and the thumb of one hand make a rocking motion back and forth on the palm of the opposite hand.

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Being fat in a dream is a straightforward symbol of overindulgence, but it can also represent wealth and opulence. The dreaming mind often literalizes common verbal expressions in an effort to convey something to the conscious mind. Thus, an image of a fat person in a dream can indicate anything from a "fat cat," to "fat chance," to "fathead," to "fat city." (See also Obesity).

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A group of compounds that have the same general structure (i.e., a molecule of glycerine esterified with three fatty acid chains). The nature of the fatty acids determines the nature of the fat. Fatty acids that are unsaturated yield an unsaturated fat. Fatty acids that have two or more sites of unsaturation are called polyunsaturated fats. Fatty acids with no unsaturated sites are saturated and make up totally saturated fats. Fats are a storage of energy in the body, having more than twice the caloric output upon metabolism than carbohydrates. Carbohydrates represent 4 calories per gram, and fats represent 9 calories per gram. Fats are necessary for the storage of fat-soluble vitamins like vitamin A, D, E, and K. Vegetable oils contain mixtures of fatty acid profiles that are each, in turn, attached to the glycerine base. The performance of the products, and the nature of the chemical differences relate to the following:

1. How many of the three glycerine 'arms' are attached to fatty acids (i.e., monoglycerides, diglycerides, triglycerides)?
   
2. The nature of the fatty acids attached to the glycerine in the three glycerine arms (i.e., Are they all different? Are two the same? Are they all the same?).
   
3. The level of saturation and molecular weight of the fatty acid 'arms' on the glycerine molecule (i.e., Are they saturated, mono-unsaturated, polyunsaturated?) All of these parameters relate to the melting point, stability, and viscosity and overall performance of the fats. See Fatty Acids, Unsaturated (Bond).
   

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1. any triacylglycerol or mixture of triacylglycerols that is solid below 20°C; those that are liquid at such temperatures are usually referred to as oils.
2. an alternative name for lipid.
3. an alternative name for adipose tissue; see brown adipose tissue, white adipose tissue.

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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.

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.

For a list of words related to fat, see:

• Tissue, Fiber, and Integumentary System - fat: ester of glycerol and fatty acid; tissue of cells expanded with greasy or oily matter
• Large, tall, or fat
• Ugly, unattractive, or malformed
• Physiology - fat: any of various mixtures of solid or semisolid triglycerides in adipose tissue, insoluble in water
• Types of Food - fat: greasy substance forming most of adipose tissue of animals
• Cooking Fats and Oils - fat: greasy substance forming most of adipose tissue of animals
• Cooking Fats and Oils

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See words rhyming with "fatly."

  See crossword solutions for the clue Fat.

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - fedt, fedme, bastardmørtel, fedekvæg, tønde
adj. - fed, tyk, rigelig
v. tr. - fede, opfede
v. intr. - blive fed

idioms:

• fat cat    bagmand, der spytter i partikassen og trækker i trådene
• fat chance    ja, det er der vist store chancer for!
• grow fat    blive fed

Nederlands (Dutch)
vet, beste deel, overbodigheid, dik, wel bevoorraad, winstgevend, vruchtbaar, dom, machtig (eten), vet maken/ worden

Français (French)
n. - graisse, matières grasses, gras, tissu adipeux, (Chim) corps gras
adj. - gras, adipeux, gros, graisseux
v. tr. - engraisser
v. intr. - s'engraisser, prendre du poids

idioms:

• fat cat    gros richard, richard
• fat chance    aucune chance
• grow fat    prendre de l'embonpoint, s'engraisser

Deutsch (German)
n. - Fett
adj. - fett, dick
v. - fett werden, mästen

idioms:

• fat cat    (slang) reiche und einflußreiche Person
• fat chance    (Sl) herzlich wenig Aussicht
• grow fat    dick werden

Ελληνική (Greek)
n. - λίπος, πάχος, χόνδρος, ξίγκι, λαρδί
adj. - παχύς, χοντρός, λιπαρός
v. - παχαίνω

idioms:

• fat cat    (καθομ.) ματσωμένος, παραλής
• fat chance    πού τέτοια τύχη
• grow fat    παχαίνω

Italiano (Italian)
grasso, grosso

idioms:

• fat cat    riccone
• grow fat    ingrassare

Português (Portuguese)
n. - gordura (f)
adj. - gordo
v. - engordar

idioms:

• fat cat    homem muito rico, grande contribuidor de campanha política
• fat chance    pouca chance
• grow fat    engordar, enriquecer

Русский (Russian)
жир, сало, полнота, толстый, жирный, плодородный, урожайный, обильный, никудышный

idioms:

• fat cat    самодовольный богач
• fat chance    Никогда!
• grow fat    жиреть

Español (Spanish)
n. - grasa, sebo, lo mejor, gordura
adj. - grueso, gordo, graso, voluminoso
v. tr. - engordar
v. intr. - engordar

idioms:

• fat cat    pez gordo
• fat chance    pocas posibilidades
• grow fat    engordar, echar carnes

Svenska (Swedish)
n. - fettämne
adj. - tjock, fet, bördig, givande
v. - göda, bli fet

中文（简体）(Chinese (Simplified))
脂肪, 肥肉, 肥的, 油腻的, 胖的, 养肥, 在...中加入脂肪, 长胖, 长肥

idioms:

• fat cat    有钱有势的人
• fat chance    微小的机会
• grow fat    发福, 发胖

中文（繁體）(Chinese (Traditional))
n. - 脂肪, 肥肉
adj. - 肥的, 油膩的, 胖的
v. tr. - 養肥, 在...中加入脂肪
v. intr. - 長胖, 長肥

idioms:

• fat cat    有錢有勢的人
• fat chance    微小的機會
• grow fat    發福, 發胖

한국어 (Korean)
n. - 지방, 비만
adj. - 뚱뚱한, 기름진, 두꺼운, 풍부한, 둔한
v. tr. - ~을 살찌게 하다
v. intr. - 살이 찌다

日本語 (Japanese)
n. - 脂肪, 脂身, 脂肪太り
v. - 太らせる
adj. - 太った, 脂肪の多い, 豊かな, 肥えた, 分厚い, 有利な, 大きな

idioms:

• fat cat    多額の政治献金をする人, 大物
• fat chance    心細い見込み
• polyunsaturated fat    多価不飽和脂肪

العربيه (Arabic)
‏(الاسم) زبدة , سمنه , بدانه (صفه) سمين (فعل) يسمن‏

עברית (Hebrew)
n. - ‮שומן‬
adj. - ‮שמן, עבה, פורייה (אדמה), דביק‬
v. tr. - ‮השמין‬
v. intr. - ‮השמין‬

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