This complex is transported to the liver, where vitamin D undergoes hydroxylation in the 25 position to form 25-hydroxyvitamin D (25D), which then circulates to the kidney and is hydroxylated at the 1 position by the 1-α-hydroxylase to form the hormonal form of vitamin D, 1,25-dihydroxyvitamin D (1,25 2D).
Once vitamin D is made in the skin (D 3) or obtained in the diet (D 2 or D 3), it enters the circulation bound to vitamin D–binding protein. The amount of vitamin D 3 made in the skin can be affected by an individual's skin color, age, and use of sunscreen products, as well as the time of day, season, and latitude. When the skin is exposed to ultraviolet B (UVB) radiation between the wavelengths of 290 and 315 nm, 7-dehydrocholesterol, a compound present in the skin, is converted to previtamin D 3, which isomerizes to form vitamin D 3. Both forms of vitamin D can be used to fortify food however, only cholecalciferol can be made endogenously in skin. Cholecalciferol is produced in skin or found naturally in fatty fish such as salmon or mackerel. Ergocalciferol is produced from irradiated fungi or yeast. Vitamin D refers to vitamin D 2 (ergocalciferol) or vitamin D 3 (cholecalciferol).
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