Description
The two most important D vitamins are vitamin D3 and vitamin D2.
D vitamins and calciferols arise from provitamins by the splitting of the B-ring of the sterane backbone, catalysed by UV radiation of sunlight. Contrary to vitamin D2, which has to be sourced from food, vitamin D3 can be produced in the liver. Vitamin D3, formed in the skin or ingested together with vitamin D2 in food, is bound to a vitamin D binding protein in the plasma, transported into the liver and hydroxylated in position 25 to form 25-OH-D.