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Vitamin D

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Vitamin D is a fat soluble vitamin that maintains normal levels of calcium and phosphorus in the bloodstream.

Often known as calciferol.

Forms of Vitamin D:

Vitamin D₁: lamisterol[?]
Vitamin D₂: ergocalciferol[?]
Vitamin D₃: cholecalciferol[?] (made from 7-dehydrocholesterol)

Note: this needs review by experts for accuracy

Vitamin D, sunlight and skin color

Vitamin D is synthetized by the human body, but in quantities that are not always sufficient. The level of synthesis depends on exposure to sunlight, so in winter and in polar areas it acts more like a vitamin, and in summer and in equatorial areas it acts less like a vitamin. So it's usually treated as a vitamin, but one that isn't required in some areas and seasons.

Light-colored skin lets more sunlight through, which leads to increased production of vitamin D₃, which is necessary for calcium absorption and bone growth. In sun-poor regions, this advantage outweights the increased cancer risk, and therefore humans in northern latitudes have evolved lighter skin. (See human skin color for a more detailed discussion of other effects of skin colour, and the disadvantages of light-colored skin).

Dark-skinned people who live in sun-poor regions often lack vitamin D₃ (one reason for the fortification of milk with vitamin D in some countries). Light-skinned people in sun-rich regions typically have an overabundance of vitamin D₃.

The Inuit are a special case: even though they live in an extremely sun-poor environment, they have retained their relatively dark skin. This can be explained by the fact that their traditional diet provides plenty of vitamin D.