Figure 1. Transforming pathways of vitamin D in human body.

Vitamin D can be obtained from foods and synthesized through convertion of 7-dehydrocholesterol by UVB in the skin. The absorbed vitamin D transports into liver by binding to vitamin D binding protein (DBP), then vitamin D is hydrolysed into form 25-hydroxyvitamin D₃ (25(OH)D₃) by 25-hydroxylase in the liver. Again, 25(OH)D₃ is hydrolysed into form 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃) in the kidney.