Previtamin D3 Explained

Previtamin D₃ is an intermediate in the production of cholecalciferol (vitamin D₃).

It is formed by the action of UV light, most specifically UVB light of wavelengths between 295 and 300 nm, acting on 7-dehydrocholesterol in the epidermal layers of the skin.^{[1] [2] [3]}

The B ring of the steroid nucleus structure is broken open, making a secosteroid. This then undergoes spontaneous isomerization into cholecalciferol, the prohormone of the active form of vitamin D, calcitriol.

The synthesis of previtamin D₃ is blocked effectively by sunscreens.^[4]

Notes and References

1. MacLaughlin JA, Anderson RR, Holick MF . Spectral character of sunlight modulates photosynthesis of previtamin D3 and its photoisomers in human skin . Science . 216 . 4549 . 1001–3 . May 1982 . 6281884 . 10.1126/science.6281884 . 1982Sci...216.1001M .
2. Webb AR . Who, what, where and when-influences on cutaneous vitamin D synthesis . Progress in Biophysics and Molecular Biology . 92 . 1 . 17–25 . September 2006 . 16766240 . 10.1016/j.pbiomolbio.2006.02.004 . free .
3. Pope SJ, Holick MF, Mackin S, Godar DE . Action spectrum conversion factors that change erythemally weighted to previtamin D3-weighted UV doses . Photochemistry and Photobiology . 84 . 5 . 1277–83 . 2008 . 18513232 . 10.1111/j.1751-1097.2008.00373.x . 23246616 . free .
4. Sayre RM, Dowdy JC . Darkness at noon: sunscreens and vitamin D3 . Photochemistry and Photobiology . 83 . 2 . 459–63 . 2007 . 17115796 . 10.1562/2006-06-29-RC-956 . 23767593 .