Vitamin D biology, deficiency, and treatment

Vitamin D functions in calcium and phosphate absorption

Vitamin D is a group of fat-soluble compounds (mainly vitamin D3/cholecalciferol and vitamin D2/ergocalciferol) that increase intestinal absorption of calcium and phosphate, which is essential for maintaining normal blood mineral levels and supporting bone health. In the body, vitamin D is converted in the liver to calcifediol (25-hydroxyvitamin D, measured as 25(OH)D) and then further hydroxylated in the kidneys (and some immune cells) to calcitriol (1,25-dihydroxyvitamin D), the biologically active form. Calcitriol exerts its effects by binding to the vitamin D receptor (VDR), a nuclear receptor that regulates gene expression. When calcitriol binds the VDR, it modulates the transcription of transport proteins involved in mineral absorption in the intestine (such as TRPV6 and calbindin), thereby enhancing calcium and phosphate uptake. This VDR-mediated regulation in the intestine, along with coordinated roles in bone, kidney, and parathyroid function, helps maintain calcium and phosphorus balance in the blood and contributes to healthy bone mineralization.

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Vitamin D2 and vitamin D3 as main human forms

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