Vitamin D维生素 D

Place · Level 3

从阳光抵达皮肤 · 到肝脏改名 · 到肾脏点火 · 最后服务全身

Sun → Skin

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阳光 · 皮肤

The plain version first: sunlight builds vitamin D because the UVB in it strikes your skin and remodels a cholesterol 'cousin' molecule there into the raw material for D. It's also why winter, overcast days, or staying indoors leave your body making less of its own D. Here's what actually happens at the molecular level.

The basal layer of skin holds a molecule called 7-dehydrocholesterol (7-DHC) — originally an intermediate on the cholesterol synthesis pathway that the body conveniently repurposes as the precursor for vitamin D. When UVB photons (280–315 nm) strike the B-ring of 7-DHC, a photochemical reaction breaks the C9-C10 σ bond, opens the ring, and yields previtamin D3.

This step needs no enzyme — pure photon energy. But there are three hidden prerequisites:

Wavelength must be right — UVA (315–400 nm) doesn't work, visible light doesn't work, glass blocks UVBSkin pigment intercepts — melanin in darker skin absorbs UVB, cutting synthesis efficiency by ~50%Angle must be sufficient — above 35°N in winter, the low sun angle means UVB is almost entirely absorbed by the atmosphere, which is why 'winter depression + northern populations low on D' is a geographic fact
Minutes later, previtamin D3 spontaneously isomerizes at skin temperature into D3 (cholecalciferol) — the starting point for the body's own D production.

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