Vitamin D is a fat-soluble steroid prohormone that plays an important role in regulating body levels of calcium and phosphorus, and in the mineralization of bone. (A prohormone displays no significant hormone activity itself but is a chemical precursor of a hormone.) The term "vitamin D" is imprecise because it can refer to one or more members of a group of steroid molecules. Also, vitamin D is not a true vitamin because in the important form known as D3 (see below), it can (in the presence of adequate sunlight) be manufactured in the body without the need for dietary supplementation. It is more accurate to describe vitamin D as a conditional vitamin.
The two major forms of vitamin D are vitamin D2, also called calciferol or ergocalciferol, and vitamin D3, or cholecalciferol. Vitamin D2 occurs in certain plants and in yeast. Vitamin D3 is generated in the skin of animals when sunlight is absorbed by a precursor molecule – a derivative of cholesterol known as 7-dehydrocholesterol.
Dietary sources of vitamin D, include egg yolk, fish oil, and plants that contain vitamin D2. However, natural diets typically don't have enough vitamin D in them for human need, and exposure to sunlight or consumption of foodstuffs purposefully supplemented with vitamin D are necessary to prevent deficiencies.
Vitamin D, either as D2 or D3, isn't significantly biologically activity. Rather, it must be metabolized within the body to the hormonally-active form known as 1,25-dihydroxycholecalciferol. This transformation occurs in two steps, as shown in the diagram:
Vitamin D deficiency causes rickets in growing animals and osteomalacia in mature animals, both conditions being characterized by weak, deformed bones. The discovery in 1919–24 of vitamin D and its production in skin and foods by ultraviolet irradiation led to the elimination of rickets as a major medical problem in children.