fat

(fat )

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1. Body tissue that serves as an energy reserve and as a source of inflammatory cytokines. Excessive amounts of fat in the body are found in obesity.
SEE: adipocyte; SEE: heart; SEE: obesity
fatty (fat′ē)
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, adj.
2. In chemistry, a triglyceride ester of fatty acids; any of a group of organic compounds closely associated in nature with the phosphatides, cerebrosides, and sterols. The term lipid is applied in general to a fat or fatlike substance. Fats are insoluble in water but soluble in ether, chloroform, benzene, and other fat solvents. During hydrolysis, fats break down into fatty acids and glycerol (an alcohol). Fats are hydrolyzed by the action of acids, alkalies, lipases (fat-splitting enzymes), and superheated steam.

CHEMICAL STRUCTURE
In the fat molecule, one molecule of glycerol is combined with three of fatty acids. Three fatty acids, oleic acid (C18H34O2), stearic acid (C18H36O2), and palmitic acid (C16H32O2), constitute the bulk of fatty acids in neutral fats found in body tissues. According to the fatty acid with which the glycerol is combined, corresponding fats are triolein, tristearin, and tripalmitin. These three fats are the principal fats present in foods.

PHYSIOLOGY
The most important function of fats is as a form of stored or potential energy. In conjunction with carbohydrates, fats are protein sparers: dietary or body protein need not be used for energy production. Glycogen storage is sufficient to supply energy needs for about 12 hr, but in a 70-kg man of average build, 12 kg of stored fat (as triglycerides) can supply energy needs for as long as 8 weeks. Subcutaneous fat provides a small amount of insulation against heat loss, and some organs such as the eyes and kidneys are cushioned by fat. The diglyceride phospholipids are part of all cell membranes. Dietary fat provides the essential fatty acids needed for normal growth.

Because certain fatty acids (linoleic, d-linolenic, and arachidonic) are necessary for formation of other products and because the body does not synthesize these fatty acids, they are classed as essential fatty acids. Linolenic acid can be converted into other fatty acids including arachidonic acid.

Animals fed a fat-free diet develop dermatitis and fail to grow; the liver becomes fatty, and there are neurological disturbances. These changes can be prevented or reversed by the addition of linoleic and linolenic acids to the diet. The human diet should consist of about 4% of calories from linoleic and 1% from linolenic acids.

DIGESTION AND ABSORPTION
In the stomach, emulsified fats such as cream or egg yolk are acted on by gastric lipase. Most fats undergo digestion in the intestine, where a pancreatic lipase hydrolyzes them to fatty acids and glycerol. The salts in bile are not enzymes; they emulsify fats and permit pancreatic lipase to digest them. Bile salts then make fatty acids soluble in water so that they may be readily absorbed. In the intestinal mucosa, fatty acids and glycerol combine to form neutral fats, then join to proteins to form chylomicrons, which enter the lacteals. In this form, they are carried in the lymph through the lymph vessels to the thoracic duct, which empties lymph into the blood.

METABOLISM
Absorbed fats are oxidized to carbon dioxide and water to produce energy; stored in adipose tissue for energy production later; changed to phospholipids for cell membranes; converted to acetyl groups for the synthesis of cholesterol, from which other steroids are made; and used to make secretions such as sebum.

Intermediary metabolism: In the oxidation of fat to carbon dioxide and water, several ketones are formed, esp. acetoacetic acid, betahydroxybutyric acid, and acetone. Excessive production of ketone bodies, when fats are incompletely oxidized, is called ketosis.

SOURCES
In addition to fat being absorbed from the intestine, body fat may arise from the conversion of carbohydrates (glucose) or excess amino acids into fat. Fatty acids cannot be converted directly to glucose, but they are split into two-carbon acetyl groups that enter the Krebs cycle and thereby have the same energy-producing function as carbohydrates.

NUTRITION
Fats have a high caloric value, yielding about 9 kcal per gram as compared with about 4 kcal per gram for carbohydrates and proteins. The average American diet of 3000 kcal may derive 40% of the caloric value from fats. Nutritionists and epidemiologists believe that decreasing dietary fat to 30% would decrease the risk of developing cancer, esp. of the colon, breast, and prostate.

Fats improve the taste and smell of foods, provide a feeling of satiety, and, because of their high caloric content, are important in high-calorie diets. Fat-free fat substitutes that have been termed designer fats have been investigated for several decades. Whether they will play a major role in providing foods with fewer calories from fat has not been determined. SEE TABLE: Food Sources of Saturated and Trans Fats

CONTRAINDICATIONS
Fat intake should be reduced in certain diseases such as hepatitis and in low-calorie diets.

SEE: trans-fatty acid
Food Sources of Saturated and Trans Fats
Meat productsVisible fat and marbling in beef, pork, and lamb, esp.in prime-grade and ground meats, lard, suet, salt pork
Processed meatsFrankfurters, luncheon meats such as bologna, corned beef, liverwurst, pastrami, and salami
Bacon and sausage
Poultry and fowlChicken and turkey (mostly beneath the skin), cornish hens, duck, and goose
Whole milk and whole-milk productsCheeses made with whole milk or cream, condensed milk, ice cream, whole-milk yogurt, all creams (sour, half-and-half, whipped)
Plant productsCoconut oil, palm-kernel oil, cocoa butter
MiscellaneousFully hydrogenated shortening and margarine, many cakes, pies, cookies, and mixes

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