The first step, in the liver, requires P450 enzymes such as CYP2R1 and CYP27A1 (25-hydroxylases) to form the major circulating form 25(OH)D. Vitamin D (D 2, D 3) has no biological activity without a two-step hydroxylation process. Sun-dried mushrooms, UV-B irradiation of the yeast sterol ergosterol, as well as vitamin supplements are considered as main sources of vitamin D 2. Vitamin D 3 can also be obtained exogenously from animal sources such as fish oils or fortified dairy products and vitamin supplements. Vitamin D 3 constitutes around 80-90% of the circulating metabolites and is synthesized mainly in the skin by the action of ultraviolet (UV) light (280-315 nm). Vitamin D refers to two biological precursors - ergocalciferol (D 2) and cholecalciferol (D 3).
However, intensive research over the last two decades has indicated that vitamin D may also be a critical modulator of several non-skeletal systems and related diseases.
In humans, the active vitamin D metabolite, 1,25-dihydroxyvitamin D (1,25(OH) 2D), has been well recognized for its role in calcium and phosphate homeostasis. Most plants and animals that are exposed to sunlight have the ability to produce vitamin D.
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