Figure 1 shows the schematic overview of vitamin D metabolism that starts in the liver, where vitamin D is hydroxylated to 25(OH)D, the main circulating
vitamin D metabolite used for vitamin D deficiency diagnosis [5]. Further hydroxylation of
25(OH)D to 1, 25(OH)D is catalyzed by 1α-hydroxylase which is expressed in multiple tissues
and binds to vitamin D receptors that in turn regulates various genes [5].