Vitamin D: biology, deficiency, and treatment

Vitamin D functions in calcium and phosphate absorption

Vitamin D is a group of fat-soluble compounds that primarily function to increase intestinal absorption of calcium and phosphate, which is essential for maintaining normal blood mineral levels and supporting bone health. In humans, the most important forms are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Although the name “vitamin D” is a misnomer, these compounds act like a hormone: they are produced in the skin with adequate UVB sunlight exposure and can also be obtained from diet, fortified foods, and supplements. After intake or skin synthesis, vitamin D is converted in the liver to calcifediol (25-hydroxyvitamin D), which is commonly measured in serum to assess vitamin D status. Calcifediol is then further hydroxylated by the kidneys (and certain immune cells) to form calcitriol (1,25-dihydroxyvitamin D), the biologically active form. Calcitriol binds to the vitamin D receptor (VDR), a nuclear receptor that regulates gene expression in target tissues, including the intestine, by modulating transport proteins involved in calcium absorption (such as TRPV6 and calbindin). This VDR-mediated regulation helps maintain calcium and phosphate balance and promotes healthy bone mineralization.

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Vitamin D2 and vitamin D3 as main human forms

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