over 64 years of age and for children under 8 than for other age groups. People above poverty levels were more apt to reach the RDA for vitamin A than were those below. Intakes were highest in the western United States and lowest in the South (DHHS-USDA, 1986). According to the 1976-1980 National Health and Nutrition Examination Survey (NHANES II), total mean dietary intake of vitamin A in the U.S. population, excluding infants, was approximately 1,000 RE. Carotenoids and preformed vitamin A contributed 25 and 75%, respectively, of the total intake. Mean serum vitamin A levels measured in NHANES II were within normal ranges for all race, sex, and economic groups.
The active form of vitamin D promotes intestinal absorption of calcium and phosphorus and influences bone mineralization. Vitamin D occurs in two forms that are equally well utilized in the body. Vitamin D2 (ergocalciferol) is produced commercially by ultraviolet (UV) irradiation of the plant sterol ergosterol; vitamin D3 (cholecalciferol) is formed by the action of sunlight on the precursor 7-dehydrocholesterol in the skin. The human body utilizes both forms of vitamin D by hydroxylating first the 25-position in the liver and then the 1a-position in the kidney, producing the biologically active 1a,25-dihydroxycalciferols.
Vitamin D occurs naturally only in animal foods such as liver, butter, fatty fish (fish containing high levels of cholesterol or fatty acids as glycerides), and egg yolks. Because natural milk is a poor source, it is fortified with vitamin D to provide 10 µg (400 IU) per quart. The amount of vitamin D formed by exposure of skin to sunlight depends upon the length of the UV irradiation, the intensity, which can be diminished by atmospheric pollution, and skin pigmentation. Aging skin may have diminished capacity to synthesize vitamin D (MacLaughlin and Holick, 1985).
The 1980 RDAs for vitamin D are set at 10 µg (400 IU) of cholecalciferol per day during periods of growth (childhood, pregnancy, lactation) and 5 µg (200 IU) per day for nonpregnant, nonlactating adults. National surveys in the United States have never monitored vitamin D intake or nutritional status. Recent studies suggest that some elderly people may exhibit poor vitamin D status (Omdahl et al., 1982; Parfitt et al., 1982).
Vitamin E is an important antioxidant that is thought to protect polyunsaturated fatty acids from oxidative destruction in cell membranes. Vitamin E activity in foods is due to the presence of tocopherols and tocotrienolscompounds of plant origin. The most important of these is a-tocopherol; less active are b-tocopherol, g-tocopherol, and a-tocotrienol. Vegetable oils are the richest source of vitamin E. Other good sources include nuts, seeds, whole grains, and wheat germ. The vitamin E content of animal foods is generally low.
The RDA for adults is 8 mg of a-tocopherol equivalents (a-TE) or 12 IU for females age 11 and older and 10 mg of a-TE (15 IU) for males age 15 or older. The need for vitamin E is increased if the polyunsaturated fat intake is high, but in the U.S. food supply, foods with high levels of polyunsaturated fatty acids also have a high vitamin E content.
Vitamin E was not included in national surveys until 1985, when the Continuing Survey of Food Intakes of Individuals was initiated by USDA. In 1985, this survey indicated that on the average, women 19 to 50 years of age consumed 97% of the RDA for vitamin E (USDA, 1987).
Vitamin K is needed in the liver for formation of several blood clotting factors. Vitamin K1, (phylloquinone) is synthesized by plants, whereas vitamin K2 homologs (menoquinones) are synthesized by bacteria. The human body can obtain vitamin K from dietary sources as well as through synthesis by the gut microflora.
Larger amounts of vitamin K are present in dark-green leafy vegetables; lower levels are found in cereals, dairy products, meats, and fruits. A committee of the Food and Nutrition Board estimated the safe and adequate intake range for adults to be 70 to 140 µg per day. The lower end of that range was based on the assumption that half the daily vitamin K intake is supplied by the diet and that half comes from intestinal synthesis. The higher end of the range represents intake derived entirely from diet. The usual diet of the U.S. population contains 300 to 500 µg per day, and there are no reports of vitamin K deficiency or toxicity in the general population; thus, it has been assumed that dietary intake of vitamin K does not need to be monitored (NRC, 1980).