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by trevorkoob 2210 days ago
7-dehydrocholesterol is photolyzed by UV to previtamin D. There is then spontaneous isomerization between this previtamin D, vitamin D and 2 inactive isomers. As vitamin D is used by the body, and the inactive isomers remain in the skin, there is an equilibrium - as vitamin D is used by the body it is drawn from the skin, leading to more conversion of inactive isomers to active vitamin D.