The major source of vitamin D is the skin, where it is produced by the action of ultraviolet light on steroid precursors. Vitamin D is also present in a limited number of foods, and the dietary sources of the vitamin can be important under circumstances of decreased sunlight exposure. Vitamin D is not a true vitamin, but a prosteroid hormone that is biologically inert until metabolized. In the liver, vitamin D is metabolized to 25(OH)D, which functions as the major storage form by virtue of its
long half-life.