Vitamin D

Vitamin D is the collective name for a family of fat-soluble sterols that function as vitamins, with two members doing the heavy lifting: D2, known as ergocalciferol or calciferol, and D3, known as cholecalciferol. The D2 form arises when ultraviolet light strikes ergosterol, the provitamin found in plants and fungi, while D3 is generated beneath human skin from 7-dehydrocholesterol during sun exposure. Nutritionists also treat vitamin D as a prohormone because of its hormone-like control of calcium and phosphorus handling. The best dietary sources are fish liver oils and foods deliberately fortified with the vitamin; liver, egg yolk, sardines, and salmon contribute smaller amounts. Food manufacturers fortify milk, margarine, cereals, and infant formula with it, and must shield the added vitamin from light, heat, and oxidation to keep label potency through shelf life.