Vitamin D is not just a single compound but a group of fat-soluble secosteroids responsible for enhancing intestinal absorption of calcium, magnesium, and phosphate. The most important compounds in this group are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D can be synthesized in the skin upon exposure to sunlight, specifically ultraviolet B radiation, hence its nickname, the "sunshine vitamin."
