Vitamin D3 (cholecalciferol) is a steroid hormone that plays a role in regulating calcium and bone metabolism. It is obtained from the diet and produced in the skin by photolysis of 7-dehydrocholesterol and released into the bloodstream. Only a few food sources have significant amounts of vitamins D2 and D3 but many foodstuffs nowadays are fortified with vitamin D. The metabolites of vitamin D3 are carried in the circulation bound to a plasma protein called vitamin D binding protein (VDBP). Vitamin D3 undergoes two subsequent hydroxylations to form the active form of the vitamin, 1,25(OH)2 vitamin D3 (calcitriol). The first hydroxylation takes place in the liver and subsequent transport to the kidney allows the second hydroxylation. Calciferol acts by binding to nuclear vitamin D receptors.