Vitamin D refers to a group of fat-soluble
secosteroids responsible for enhancing intestinal absorption of
calcium, iron,
magnesium,
phosphate, and
zinc. In humans, the most important compounds in this group are vitamin D
3 (also known as
cholecalciferol) and vitamin D
2 (
ergocalciferol). Cholecalciferol and ergocalciferol can be ingested from the diet and from supplements. Very few foods contain vitamin D; synthesis of vitamin D (specifically cholecalciferol) in the skin is the major natural source of the vitamin. Dermal synthesis of vitamin D from cholesterol is dependent on sun exposure (specifically
UVB radiation).