Vitamin D has several different forms. Two of these forms are essential to humans: 1) vitamin D2, which plants make, and 2) vitamin D3, which is made by human skin when exposed to sunlight.
Foods may be fortified with vitamin D2 or D3 to help people reach their daily consumption requirements. Vitamin D (D2 and D3) is considered biologically inactive until it undergoes two enzyme-mediated hydroxylation reactions. The first enzymatic hydroxylation reaction occurs in the liver and is mediated by the 25-hydroxylase, which forms 25-hydroxyvitamin D3 [25(OH)D3]. The second reaction occurs in the kidney, mediated by 1α-hydroxylase, which converts 25(OH)D3 to the biologically active hormone calcitriol. Vitamin D2 and D3 are equally effective when the liver and the kidney convert them into the active form, 1,25 forms. Vitamin D3 is made endogenously (this is usually measured in the laboratory).
