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 1α-position in the kidney, producing the biologically active 1α,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).https://www.ncbi.nlm.nih.gov/books/NBK218749/