Often known as calciferol.
Forms of Vitamin D:
There are two forms of the vitamin. Vitamin D2 is derived from ergosterol in the diet, whereas vitamin D3 is derived from cholesterol via 7-dehydrocholesterol. Ultraviolet light (from sunlight) is responsible for the production of both forms of the vitamin. However, in certain parts of the world with limited sunlight (e.g. the British Isles) there is the possibility that the quantity of vitamin D is not always sufficient. To prevent this possibility milk is now fortified with vitamin D2. A deficiency of vitamin D leads to rickets which is a softening of the bones owing to faulty mineralization.
The active form of the vitamin is calcitriol which is synthesized from either D2 or D3 in the kidneys. Calcitriol binds to a protein transcription factor which can regulated gene expression. The outcome is the maintenance of calcium and phosphorous levels in the bone and blood with the assistance of parathyroid hormone and calcitonin.
Because the level of calcitriol synthesis ultimately depends on exposure to sunlight, dark-skinned people who live in sun-poor regions historically would often lack vitamin D. Protection from vitamin D deficiency, and thus rickets, might be one reason light-skinned humans evolved in cloudier regions.
See also:
Vitamin D, sunlight and skin color
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