Vitamin D is a group of fat-soluble prohormones, the two major forms of which are vitamin D2 (or ergocalciferol) and vitamin D3 (or cholecalciferol). The term vitamin D also refers to metabolites and other analogues of these substances. Vitamin D3 is produced in skin exposed to sunlight, specifically ultraviolet B radiation.Vitamin D plays an important role in the maintenance of organ systems.Vitamin D regulates the calcium and phosphorus levels in the blood by promoting their absorption from food in the intestines, and by promoting re-absorption of calcium in the kidneys.It promotes bone formation and mineralization and is essential in the development of an intact and strong skeleton. Although, at very high levels it will promote the resorption of bone.It inhibits parathyroid hormone secretion from the parathyroid gland.Vitamin D affects the immune system by promoting immunosuppression, phagocytosis, and anti-tumor activity.Vitamin D deficiency can result from inadequate intake coupled with inadequate sunlight exposure, disorders that limit its absorption, conditions that impair conversion of vitamin D into active metabolites, such as liver or kidney disorders, or, rarely, by a number of hereditary disorders. Deficiency results in impaired bone mineralization, and leads to bone softening diseases, rickets in children and osteomalacia in adults, and possibly contributes to osteoporosis. Research has indicated that vitamin D deficiency is linked to colon cancer; conflicting evidence links vitamin D deficiency to other forms of cancer.