Vitamin D is a fat-soluble vitamin that plays a role in many important body functions. It is best known for working with calcium in your body to help build and maintain strong bones. Vitamin D is also involved in regulating the immune system and cells, where it may help prevent cancer.
Your body stores vitamin D and can make it when your skin is exposed to sunlight. Vitamin D is also found in some foods, mostly ones like milk that have been fortified with vitamin D. There are two forms of vitamin D: ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Some research suggests that cholecalciferol is better at raising levels of vitamin D in the blood.