Biochemistry of vitamin D

Vitamin D, a sunshine vitamin, has its major source in human skin where 7-dehydrocholesterol upon exposure to UV rays gets converted to previtamin D3. Other major sources include fatty fish and cod liver oil. Vitamin D3’s activation occurs in liver and kidney forming 25 (OH) D3 (calcidiol) and 1, 25(OH)2 D3 (calcitriol), respectively. The best marker for determining human status of vitamin D is measuring serum levels of 25 (OH) D3, which is the most stable form of this vitamin. Vitamin D deficiency (25 (OH) D3 level <20ng/mL) was quite rampant in South Asia where it ranged from 48% to 58.4% in different countries of the region. In addition to the classical genomic effects of calcitriol on its target cells, there are direct nongenomic effects by binding to the cell membrane of certain target tissues such as small intestine and bone and facilitating the absorption of calcium and phosphate.