There are two major forms of vitamin D: vitamin D2, found in plants and better known as ergocalciferol (or calciferol), and vitamin D3, found in animal tissues and often referred to as cholecalciferol.
This is true not only for cholecalciferol and ergocalciferol obtained from the diet but also for cholecalciferol that is generated from 7-dehydrocholesterol in the skin during exposure to ultraviolet light.
In this reaction the bond between one carbon and one hydrogen atom is eliminated, while simultaneously the same hydrogen atom forms a bond to a new carbon atom, resulting in the molecule cholecalciferol, or vitamin D3.
Though it is not biologically active itself, cholecalciferol is converted by the liver and the kidneys into several forms of vitamin D with various metabolic roles, including regulating calcium (Ca2+) levels in the intestine, kidney, liver, and bone and controlling differentiation of hematopoetic cells in bone marrow to macrophages and osteoclasts for bone formation.
Certain sterols are transformed to calciferols (D vitamins) by ultraviolet light; this process occurs naturally in the skin and is used commercially in the manufacture of vitamin D2 (ergocalciferol) from ergosterol and of vitamin D3 (cholecalciferol) from synthetic 7-dehydrocholesterol.
A specific bloodborne protein, an alpha-1 globulin, alpha-1 globuline liver, where the procarries chemital change to the active hormone begins by hydroxylivern to cholecalciferol.
On the other hand, vitamins A and D3 (cholecalciferol) occur only in animal tissues.
Although about 10 compounds have vitamin D activity, the two most important ones are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3).
One IU of vitamin A activity = 0.3 μg all-trans-retinol = 3.6 μg all-trans-betacarotene. 31 μg cholecalciferol = 40 IU vitamin D. 4As niacin equivalents (NE); 1 mg niacin = 60 mg tryptophan.
Most calcium supplements now also contain vitamin D (usually as cholecalciferol, or D3), supplying about 250 to 300 international units in two tablets.
The vitamin D in milk comes either from the cow's milk itself (the cow also makes cholecalciferol in its skin through the UV activation of 7-dehydrocholesterol) or from the fortification of the milk with cholecalciferol.
