Forms
| Name | Chemical composition | Structure |
|---|---|---|
| Vitamin D1 | molecular compound of ergocalciferol with lumisterol, 1:1 | |
| Vitamin D2 | ergocalciferol (made from ergosterol) | |
| Vitamin D3 | cholecalciferol (made from 7-dehydrocholesterol in the skin). | |
| Vitamin D4 | 22-dihydroergocalciferol | |
| Vitamin D5 | sitocalciferol (made from 7-dehydrositosterol) |
Several forms (vitamers) of vitamin D exist (see table). The two major forms are vitamin D2 or ergocalciferol, and vitamin D3 or cholecalciferol; vitamin D without a subscript refers to either D2 or D3 or both. These are known collectively as calciferol. Vitamin D2 was chemically characterized in 1932. In 1936, the chemical structure of vitamin D3 was established and proven to result from the ultraviolet irradiation of 7-dehydrocholesterol.
Chemically, the various forms of vitamin D are secosteroids; i.e., steroids in which one of the bonds in the steroid rings is broken. The structural difference between vitamin D2 and vitamin D3 is in their side chains. The side chain of D2 contains a double bond between carbons 22 and 23, and a methyl group on carbon 24.
Vitamin D3 (cholecalciferol) is produced by ultraviolet irradiation (UV) of its precursor 7-dehydrocholesterol. This molecule occurs naturally in the skin of animals and in milk. Vitamin D3 can be made by exposure of the skin to UV, or by exposing milk directly to UV (one commercial method).
Vitamin D2 is a derivative of ergosterol, a membrane sterol named for the ergot fungus, which is produced by some kinds of phytoplankton, invertebrates, yeasts, and higher fungi such as mushrooms. The vitamin ergocalciferol (D2) is produced in all of these organisms from ergosterol, in response to UV irradiation. Like all forms of vitamin D, it cannot be produced without UV irradiation. D2 is not produced by green land plants or vertebrates, because they lack the precursor ergosterol. The biological fate for producing 25(OH)D from vitamin D2 is expected to be the same as for 25(OH)D3, although some controversy exists over whether or not D2 can fully substitute for vitamin D3 in the human diet.
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