FIG. 8.

Synthesis and metabolism of vitamin D3. Vitamin D3 is synthesized from 7-DHC in the skin via UV light through an intermediate previtamin D3, which is ultimately converted by simple body heat. Classically, it was thought that vitamin D3 was metabolized in the kidneys and liver through the steps outlined, resulting in the bioactive form of calcitriol. Recent studies suggest that another form of metabolism using the enzymes p450scc and CYP27B1 can also break down this molecule, and creates far more numerous bioactive forms of vitamin D3, and it can do this in the skin (as well as certain other sites in the body). 7-DHC, 7-dehydrocholesterol. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars