Vitamin D (ergocalciferol-D2, cholecalciferol-D3, alfacalcidol) is a fat-soluble vitamin that helps your body absorb calcium and phosphorus. Having the right amount of vitamin D, calcium, and phosphorus is important for building and keeping strong bones. Vitamin D is used to treat and prevent bone disorders (such as rickets, osteomalacia). Vitamin D is made by the body when skin is exposed to sunlight. Sunscreen, protective clothing, limited exposure to sunlight, dark skin, and age may prevent getting enough vitamin D from the sun.