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NUTRITIONAL ROLE OF FOOD FATS Energy and Storage Fat is the most concentrated energy source in the diet. One gram of fat provides approximately 9 calories, compared with 4 calories from 1 gm of protein or carbohydrate. Fat is the only form of energy that the body can store in quantity. For this purpose, both carbohydrate and protein are converted to fat. Adipose tis- sue insulates the body from rapid changes in environmental temperature and cushions organs and the body as a whole against external forces. Palatability of Diets Dietary fat has a palatability role difficult to measure but impor- tant to proper nutrition. An appreciable content of fat is requisite to a diet generally acceptable in our society. Observations of fat-craving are quite common among peoples deprived of their accustomed intake of fat. Satiety from fats in the diet may be in part traceable to the slower emptying of the stomach when a high-fat meal has been ingested. Many of the substances respon- sible for the flavors and aromas of foods are dissolved in fat and associated with the fat in the diet. The distinctiveness and attrac- tiveness of a foodstuff often can be attributed to desirable flavor and texture qualities provided by the fatty constituents. Nutrient Carriers Fats in the diet act as carriers of certain nonfat nutrients. Some fats, such as butterfat and the fats of marine animals, eggs, and margarine (through fortification), provide important amounts of vitamins A and D. Many fats contain vitamin E and some are sources of vitamin K. 21
Dietary Fat and Human Health Essential Fatty Acids The essentiality of certain polyunsaturated fatty acids in the diet was demonstrated first in experiments with rats (27). The essential fatty acids are required by animals for growth, re- production, membrane integrity, and the proper utilization and metabolism of fat. Linoleic and arachidonic acids will cure essential fatty acid deficiency, but linolenic acid and its related polyunsaturated acids are not fully effective (147). The amount of dietary linoleate required by experimental animals to prevent deficiency symptoms and for maintenance of the usual pattern of polyunsaturated acids has been determined to lie between 1 and 2 percent of total calories for rats (98) and swine (36). At lower levels of dietary linoleate, the content of linoleate and arachidonate of tissue and plasma lipids decreases, and eicosotrienoic acid increases. Infantile eczema (32) may sometimes be an expression of essential fatty acid deficiency (83) and the linoleate requirement of infants has been estimated to be between 1 and 3 percent of total calories. Human milk normally is at least three times richer in essential fatty acids than cow's milk. Dietary cholesterol affects essential fatty acid deficiency in the rat (163) and the polyunsaturated acid pattern in the tissues. As essential fatty acids account for a substantial proportion of the fatty acids of cholesterol esters and phospholipids in plasma lipoproteins and of mitochondrial lipoproteins, these acids pre- sumably may have critical roles in membrane structures and in transport processes. 22
