| Definition: | process whereby 7‑dehydrocholesterol, precursor of vitamin D3, produces previtamin D3 by absorbing 240 nm to 315 nm ultraviolet radiation and moreover changes it to vitamin D3 through a thermal isomerization process Note 1 to entry: Vitamin D is a generic term for vitamin D2 and/or vitamin D3. Vitamin D synthesis in mammalian skin produces vitamin D3. Vitamin D2 is formed in plants in an analogous process when ultraviolet radiation activates ergosterol.
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