Vitamin D is a group of structurally related, fat-soluble compounds responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, along with numerous other biological functions. In humans, the most important compounds within this group are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Unlike the other twelve vitamins, vitamin D is only conditionally essential - in a preindustrial society people had adequate exposure to sunlight and the vitamin was a hormone, as the primary natural source of vitamin D was the synthesis of cholecalciferol in the lower layers of the skin's epidermis, triggered by a photochemical reaction with ultraviolet B (UVB) radiation from sunlight...