<unk>  D (the inactive version) is mainly from two forms:<unk>  D3 and<unk>  D2.<unk>  D3, or cholecalciferol, is formed in the skin after exposure to sunlight or ultra violet radiation or from D3 supplements or fortified food sources.<unk>  D2, or ergocalciferol, is obtained from D2 supplements or fortified food sources. These two forms of<unk>  D are metabolized in the liver and stored as 25-hydroxyvitamin D. Before biological use, the storage form must be converted into an active form. One common active form is 1,25 dihydroxyvitamin D. The term<unk>  D in this article, refers to group of molecules including cholecalciferol, ergocalciferol, 25-hydroxyvitamin D, and the active forms. The role of<unk>  D is best characterized as enabling calcium absorption and regulating calcium homeostasis.<unk>  D also play a role in phosphate absorption.
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