Vitamin D plays an essential role in the regulation of calcium and phosphorus absorption and
metabolism for bone health. Nevertheless, the influence of vitamin D is more than just mineral
and skeletal homeostasis. The existence of vitamin D receptors (VDR) in several tissues and
organs implies that vitamin D physiology encompasses beyond bone maintenance [1].
Furthermore, the enzyme responsible for the conversion of 25[OH]D to its biologically active
form [Vitamin D (1, 25[OH]2D)] has been recognized in several other tissues aside from
kidneys with evidence growing that extra renal synthesis of 1, 23[OH]2D may be just as
important in regulating the cell growth of cellular differentiation via paracrine or autocrine
regulatory mechanisms [2–4]. 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].