The first hydroxylation, which occurs in the liver, converts vitamin D to 25-hydroxyvitamin D, also known as 'calcidiol.' The second hydroxylation occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxyvitamin D, also known as 'calcitriol'. Vitamin D obtained from sun exposure, foods, and supplements is biologically inert and must undergo two hydroxylations in the body for activation. It is also produced endogenously when ultraviolet (UV) rays from sunlight strike the skin and trigger vitamin D synthesis. Vitamin D (also referred to as 'calciferol') is a fat-soluble vitamin that is naturally present in a few foods, added to others, and available as a dietary supplement.

For a general overview of Vitamin D, see our consumer fact sheet on Vitamin D.įor information on vitamin D and COVID-19, see Dietary Supplements in the Time of COVID-19. This is a fact sheet intended for health professionals.