Vitamin D: biology, deficiency, and health

Vitamin D overview and misconception as vitamin

Vitamin D is a group of structurally related, fat-soluble compounds that help increase intestinal absorption of calcium and phosphate and support many other biological functions. The two major forms in humans are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Although commonly called a vitamin, the name is a misnomer: vitamin D acts more like a secosteroid hormone. It is produced in the skin when exposed to UVB sunlight, and it can also be obtained from diet, fortified foods, and supplements. In the body, vitamin D is converted in the liver to calcifediol (25-hydroxyvitamin D) and then in the kidneys (and some immune cells) to 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 many tissues, including those involved in calcium/phosphate balance and immune function. A major misconception is that vitamin D is “just a vitamin” like the others; in reality, it is only conditionally essential because skin synthesis can supply it when sunlight exposure is adequate.

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Fat-soluble secosteroid hormone and key forms

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