Figure 1.

Basic physiology of vitamin D action. Humans obtain vitamin D mainly through endogenous production of previtamin D₃ and vitamin D₃ in the skin, followed by subsequent conversions in the liver and kidney. When exposed to the sun, 7-dehydrocholesterol converts to previtamin D₃ and vitamin D₃ by ultraviolet B (UVB) radiation and heat, respectively, in the skin. A small amount of vitamin D can be present in natural food as vitamin D₂ and D₃. Vitamin D₂/D₃ go through hydroxylation two times in the liver (vitamin D₂/D₃ → 25 (OH) D₃) and the kidney (25 (OH) D3 → 1α,25 (OH)₂D₃) to become a biologically active form of vitamin D₃, 1α,25 (OH)₂D₃;1α,25 (OH)₂D₃ exerts its biological actions by binding to the nuclear vitamin d receptor (VDR), which associates with a retinoid x receptor (RXR) in the nucleus. The VDR/RXR heterodimers bind to the vitamin D response element (VDRE) in the promoter region of vitamin D-regulated genes and initiate expression of various genes. VDR is also found in the plasma membrane, and the liganded plasma membrane VDR activates an intracellular signaling transduction involved in many physiological actions.