Vitamin D Metabolism & Sources of Vitamin D
There are two naturally occurring forms of vitamin D – vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol) – which have slightly different chemical structures. Cholecalciferol is made by the conversion of 7-dehydrocholesterol in the epidermis and dermis in humans upon exposure to UVB radiation.[1] Ergocalciferol is made in mushrooms and yeast and the synthetic form is often used to fortify foods and in dietary supplements. Few foods, mostly fatty fish, are naturally rich in vitamin D. Milk and other dairy products in the USA are fortified with vitamin D. After ingestion, vitamin D undergoes two separate hydroxylations. One in the liver to 25-hydroxyvitamin D (25[OH]D), the precursor hormone which is tested to evaluate for vitamin D status owing to its half-life of approximately 3 weeks.[2,3] The second hydroxylation is catalyzed by the 1-α hydroxylase enzyme, to the biologically active 1,25 dihydroxyvitamin D (1,25[OH]2D) mainly in the kidney but also in other target tissues.[4] Other target tissues include the skin and vasculature and it is thought that 1-α hydroxylation at these sites facilitates autocrine and paracrine signaling and is responsible for the non-calcemic actions of vitamin D, which are detailed later in this review.[5] 1,25[OH]2D interacts with the vitamin D receptor (VDR) to affect physiological changes. The VDR has been found in tissues that are not directly involved in calcium metabolism, suggesting that vitamin D may play other important roles in the body.[6–9] Vitamin D becomes inactivated by the 25(OH)D-24-hydroxylase, which creates the biologically inactive 24,25-dihydroxyvitamin D.
Pediatr Health. 2010;4(1):89-97. © 2010 Future Medicine Ltd.
Cite this: Low Levels of 25-hydroxyvitamin D in the Pediatric Populations: Prevalence and Clinical Outcomes - Medscape - Feb 01, 2010.
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