- Vitamin D
Vitamin D designates a group of fat-soluble compounds that were rst recognized for their antirachitic function. Vitamins D2 (ergocalciferol) and D3 (cholecalciferol) are the major forms relevant to human health. Vitamin D2 is derived from the plant steroid, ergosterol. Vitamin D3 is made from its precursor, 7-dehydrocholesterol, in human skin when exposed to the ultraviolet (UV) irradiation in the sunlight. Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylations in the body for activation [1-4]. Because vitamin D can be produced endogenously, it does not meet the classical criterion of a vitamin. It is classied as a hormone  with an endocrine system that produces the active form of the hormone 1,25-dihydroxyvitamin D [1,25(OH)2D] primarily in the kidney but which acts in distant sites. However, since it was rst identied as an essential nutrient and dietary intake is required in certain circumstances, it is classied as a vitamin.