About vitamin D
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 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 mineralisation, and leads to bone softening diseases, rickets in children and osteomalacia in adults, and possibly contributes to osteoporosis. However, sunlight exposure, to avoid deficiency, carries other risks, including skin cancer; this risk is avoided with dietary absorption, either through diet or as a dietary supplement.
Vitamin D in food
Very few foods are naturally rich in vitamin D, so much vitamin D intake in the industrialised world is from fortified products including milk, soy milk and breakfast cereals or supplements. Natural sources of vitamin D include: