Fig. 2

Vitamin D is obtained through two main sources: cutaneous synthesis and oral intake. UVB radiation (290–315 nm) photolyzes 7-dehydrocholesterol to pre-vitamin D3 which in turn is converted to vitamin D3 by isomerization through a thermo-sensitive reaction in the epidermis. Upon binding to vitamin D binding protein (DBP), the synthesized vitamin D3 traverses the systemic circulation to the liver. In the liver, vitamin D3 is hydroxylated to produce 25-hydroxyvitamin D3 [25(OH)D3], the major circulating vitamin D metabolite. DBP transports 25(OH)D3 to the kidney where it is converted to calcitriol (1,25-dihydroxyvitamin D3 [1,25(OH)₂D₃]), a potent steroid hormone and the active metabolite of the vitamin D. Created with BioRender.com