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Vitamin D synthesis in the skin
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In the skin, vitamin D starts with a molecule that is already there: 7-dehydrocholesterol. When ultraviolet B radiation from the sun, a specific band of light, reaches it, that molecule turns into previtamin D.
That gives a simple rule: anything that changes how much UVB radiation reaches the skin changes how much vitamin D is produced.
Melanin, the pigment that gives skin its color, absorbs that radiation and keeps it from driving the conversion. That is why someone with more pigment produces less vitamin D from the same exposure and needs more time in the sun to produce the same amount.
Think of a window blind: the less light it lets through, the less happens on the other side. The same goes for geography: above 37 degrees of latitude, less UVB reaches the skin and production falls.
In brief
Ultraviolet B radiation converts 7-dehydrocholesterol in the skin into previtamin D.
Anything that changes how much UVB radiation reaches the skin changes how much vitamin D the skin produces.
A window blind: the less light it lets through, the less happens on the other side.
Melanin absorbs UVB and blocks that conversion: more pigment means less production and more exposure needed.
Above 37 degrees of latitude, less UVB arrives and production falls.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingVitamin D, StatPearlsPublished
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