Figure 2.

Schematic representation of the synthesis and metabolism of vitamin D. The main sources of vitamin D, i.e., supplementation, UV exposure and dietary intake are highlighted. A detailed description of the production of the active form, 1,25(OH)₂D, and the regulation of its amount is also illustrated. UVB: ultraviolet B; 7-DHC: 7-dehydrocholesterol; DBP: vitamin D binding protein; PTH: parathyroid hormone; FGF23: fibroblast growth factor. Figure was redrawn from Szymczak-Pajor et al. [80].