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the bond in a trans isomer. Naturally occurring geometric isomers in food are mainly cis isomers, but hydrogenation of oils in the manufacture of margarine and shortening results in formation of some trans isomers. This latter process occurs naturally in the rumen of ruminants.

Phospholipids contain glycerol, fatty acids, phosphate, and, with such exceptions as phosphatidylglycerol and phosphatidylinositol, a nitrogenous component. Lecithin, for example, is made up of glycerol, two fatty acids (one saturated, usually), phosphate, and choline. Phospholipids are important structural components of brain and nervous tissue, of membranes throughout body tissues, and of lipoproteins—the carriers of cholesterol and fats in the blood.

Cholesterol and plant sterols, such as sitosterol, are high-molecular-weight alcohols with a characteristic cyclic nucleus and are unrelated to the structure of fats or phospholipids. Cholesterol frequently exists in foods and body tissues esterified to one fatty acid per molecule. It is a component of membranes in body cells and is required for normal development of the brain and nervous tissue. Furthermore, it is the precursor to bile acids, steroid hormones, and 7-dehydrocholesterol in the skin, which in turn is the precursor to vitamin D.

Cholesterol occurs naturally only in foods of animal origin. The highest concentrations are found in liver and egg yolk, but red meats, poultry (especially the skin), whole milk, and cheese make significant contributions to the diet.

Trends in the Food Supply

Trends in the quantities of lipids present in the food supply have been recorded by the U.S. Department of Agriculture (USDA) since 1909. These data represent amounts of lipids that "disappear" into wholesale and retail markets. No account is taken of amounts wasted, and no effort is made to measure intakes by individuals. Thus, food supply data do not represent amounts of lipids actually consumed and are referred to here as amounts available for consumption (see Chapters 2 and 3). Total amounts available are divided by the U.S. population to obtain amounts per capita. The following data on time trends were obtained from Marston and Raper (1987).

Fat available in the food supply increased from an average of 124 g/day per capita in 1909 to 172 g/day in 1985. Although the chief sources of fat during that time have been fats and oils; meat, poultry, and fish; and dairy products, great changes within each of these groups have occurred. The proportion of animal fat declined from 83 to 58% as butter and lard use declined, whereas the proportion of vegetable fat (in margarines and in salad and cooking oils) rose from 17 to 42%. Pork and beef have been major sources of fat in the food supply since 1909, but supplies of beef have declined somewhat recently from 90.7 lb/year per capita in 1975 to 79 lb/year in 1985. The supply of poultry has increased spectacularly since 1940 and continues to increase. In 1985, 70 lb per capita were available compared with 46 lb per capita from 1967 to 1969. The per-capita availability of whole milk dropped from 232 lb during 1967-1969 to 122 lb in 1985, whereas skim and low-fat milks increased from 44 to 112 lb (see Chapter 3).

Fatty acids available in the food supply have all increased since 1909, but the relative contributions of specific fatty acids have changed. The percentage of calories contributed by linoleic acid to total fat intake increased from 7% during 1909-1913 to about 16% in 1985, whereas the corresponding percentage from  SFAs declined from approximately 42 to 34% (Figure 3-5). In 1985, linoleic acid was available at 7% of total calories, SFAs at 15%, and oleic acid at 17%.

Cholesterol availability reached its lowest levels of 464 mg/day per capita in 1917 and 1935, and its highest level of 596 mg/day in 1945. The supply declined to 480 mg/day per capita during 1977-1979, when it plateaued; the decline was due to diminished use of whole milk, butter, eggs, and lard. Food sources of cholesterol have changed somewhat over the century. In 1909, meat, poultry, and fish furnished 28% of the cholesterol in the food supply; in 1985, they supplied 40%. Fats and oils supplied 12% of the total cholesterol in 1909, but only 5% in 1985. Egg use has declined, but in 1985 still supplied 40% of the cholesterol in the food supply (see Chapter 3).

Lipid Intake: National Surveys

Actual intakes of various lipids have been estimated in national surveys, but the different surveys fail to agree on trends in actual consumption of fat. Data from the USDA's Nationwide Food Consumption Surveys (NFCS) of 1955, 1965, and 1977-1978 show little change in fat levels used by households, but mean individual intakes were lower during 1977-1978 than in 1965 (USDA, 1984). Furthermore, compared with 1977-1978, a decline in fat intake was indicated in the 1985 and 1986 USDA Continuing Survey of Food Intakes of

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