Figure 1.

Schematic overview of the metabolism of vitamin D. Synthesis of vitamin D3 starts in the skin where 7-dehydrocholesterol is converted into vitamin D3 in response to UVB exposure. Vitamin D3 is hydroxylated by 25-hydroxylases in the liver. The resulting 25-hydroxyvitamin D3 is then hydroxylated in the kidney by 1α hydroxylase, to produce the final activated product, 1,25-dihydroxyvitamin D3.