Vitamin D has a significant role in calcium homeostasis and metabolism. As vitamin D is synthesized in adequate amounts by most mammals exposed to sunlight, it is not strictly a vitamin, and may be considered a hormone as its synthesis and activity occur in different locations. Accordingly, the Dietary Reference Intake for vitamin D assumes no synthesis occurs and all of a person's vitamin D is from food intake. Evidence indicates the synthesis of vitamin D from sun exposure is regulated by a negative feedback loop that prevents toxicity, but because of uncertainty about the cancer risk from sunlight, no recommendations are issued by the Institute of Medicine (US) for the amount of sun exposure required to reach vitamin D requirements. Vitamin D from the diet or dermal synthesis from sunlight is biologically inactive activation requires enzymatic conversion (hydroxylation) in the liver and kidney. Dermal synthesis of vitamin D from cholesterol is dependent on sun exposure (specifically UVB radiation). Very few foods contain vitamin D synthesis of vitamin D (specifically cholecalciferol) in the skin is the major natural source of the vitamin.

Cholecalciferol and ergocalciferol can be ingested from the diet and from supplements.

In humans, the most important compounds in this group are vitamin D 3 (also known as cholecalciferol) and vitamin D 2 (ergocalciferol).

Vitamin D refers to a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, iron, magnesium, phosphate, and zinc.

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