2 Vitamin D

Authored by: Janos Zempleni , John W. Suttie , Jesse F. Gregory III , Patrick J. Stover , Wei Zheng and Dorothy Teegarden

Handbook of Vitamins

Print publication date:  July  2013
Online publication date:  July  2013

Print ISBN: 9781466515567
eBook ISBN: 9781466515574
Adobe ISBN:


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Vitamin D designates a group of fat-soluble compounds that were first recognized for their anti-rachitic function. Vitamins D2 (ergocalciferol) and D3 (cholecalciferol) are the major forms relevant to human health. Vitamin D2 is derived from the plant steroid, ergosterol. Vitamin D3 is made from its precursor, 7-dehydrocholesterol, in human skin when exposed to the ultraviolet (UV) irradiation in the sunlight. Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylations in the body for activation [14]. Because vitamin D can be produced endogenously, it does not meet the classical criterion of a vitamin. It is classified as a hormone [5] with an endocrine system that produces the active form of the hormone 1,25-dihydroxyvitamin D [1,25(OH)2D] primarily in the kidney but which acts in distant sites. However, since it was first identified as an essential nutrient and dietary intake is required in certain circumstances, it is classified as a vitamin.

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