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Vitamin D consists of a group of fat-soluble prohormones, the most important of which are vitamin D2 and D3, with measurement of 25-hydroxyvitamin D (25-OHD) closely representing a person's vitamin D2 and D3 status. D2 (ergocalciferol) is plant and fungal derived, while vitamin D3 (cholecalciferol) is made from 7-dehydrocholesterol in the skin. This conversion of 7-dehydrocholesterol to previtamin D3 is governed by both the intensity and appropriate wavelength of the ultraviolet (UV) B irradiation reaching 7-dehydrocholesterol. Adequate amounts of vitamin D3 can be made in the skin after only 10–15 min of sun exposure at least twice a week without sunscreen. However, with longer exposure to UVB rays, equilibrium is achieved in the skin and the vitamin degrades as fast as it is generated. Serum concentrations of vitamin D have been found to vary with age, race, sex, season and geographic location, and subclinical deficiency is common, particularly in temperate climates.1
Once in its physiologically active form vitamin D is released into the circulation, binds to a carrier protein in the plasma (vitamin D-binding protein (DBP)) and is transported to various target organs. The hormonally active form of vitamin D mediates its biological effects by binding to the vitamin D receptor (VDR), which is principally located in the nuclei of target cells. This VDR is constitutively expressed in monocytes, activated macrophages, dendritic cells, natural killer cells, and T and B cells. Activation has potent antiproliferative, prodifferentiative and immunomodulatory functions; both immune enhancing and immunosuppressive.2 It is these immunomodulatory properties of vitamin D that have particularly attracted interest in recent years with regards …
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