Cutaneous synthesis and seasonal variation of vitamin D

THE CONTEXT

Ultraviolet exposure initiates vitamin D synthesis from a cholesterol-synthesis intermediate that accumulates in the skin.

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7-Dehydrocholesterol undergoes a nonenzymic reaction on ultraviolet exposure to form previtamin D.

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Over several hours, previtamin D undergoes a further reaction to form cholecalciferol, which enters the bloodstream.

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In temperate climates, plasma vitamin D concentration is highest at the end of summer and lowest at the end of winter.

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Beyond approximately 40° north or south, winter ultraviolet radiation contains very little of the appropriate wavelength.

NEETx

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