Two compounds share vitamin D activity in humans. Cholecalciferol is designated vitamin D3, whereas ergocalciferol is designated vitamin D2. While cholecalciferol is the form of the vitamin produced in animals, ergocalciferol is produced in plants. Both versions of the vitamin can be produced synthetically for use in supplements and to fortify foods. Because cholecalciferol can be produced in humans through the action of ultraviolet light on 7‑dehydrocholesterol in the skin, it is not technically a vitamin. However, vitamin D retains its vitamin status for historical reasons. The plant form of vitamin D is produced by the action of ultra violet light on the plant compound ergosterol. Because vitamin D interacts with receptors in the nucleus of cells, it is regarded primarily as a hormone. Recent evidence also suggests that vitamin D may also interact with cell membrane bound receptors. Vitamin D is converted in humans to the biologically active hormones 25-hydroxyvitamin D (calcifediol) and 1,25-dihydroxyvitamin D (calcitriol). The traditional role for vitamin D is in the correct metabolism of calcium, however more recent data suggests that vitamin D is also required for correct immune function and regulation of the insulin receptor.
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