Figure 1.

Metabolic pathways regarding the production, activation, and effects of vitamin D. The highest amount of vitamin D is produced in the skin, via the conversion of 7-DHC to vitamin D3. Vitamin D2 is mostly assumed through foods. Vitamin D2 and Vitamin D3 are further hydroxylated in carbonium 25 and 1. 1,25(OH)D exerts its genomic effects by binding to VDR and RXR, and translocating to the nucleus, where it interacts with VDRE. Non-genomic effects are mainly involved in calcium–phosphate homeostasis. 7-DHC: 7dehydrocolesterol, vitamin D3: vitamin D3, vitamin D2: vitamin D2, 25(OH)D: 25-hydroxylated vitamin D, 1,25(OH)D: 1,25-hydroxylated vitamin D, VDR: vitamin D receptor, RXR: retinoic acid receptor, VDRE: vitamin D response element.