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Overview of vitamin D

Sassan Pazirandeh, MD
David L Burns, MD
Section Editors
Kathleen J Motil, MD, PhD
Marc K Drezner, MD
Deputy Editor
Jean E Mulder, MD


Vitamin D is a fat-soluble vitamin. Very few foods naturally contain vitamin D (fatty fish livers are the exception), so dermal synthesis is the major natural source of the vitamin. Vitamin D from the diet or dermal synthesis is biologically inactive and requires enzymatic conversion to active metabolites (figure 1). Vitamin D is converted enzymatically in the liver to 25-hydroxyvitamin D (25[OH]D), the major circulating form of vitamin D, and then in the kidney to 1,25-dihydroxyvitamin D, the active form of vitamin D.

Vitamin D and its metabolites have a significant clinical role because of their interrelationship with calcium homeostasis and bone metabolism. Rickets (children) and osteomalacia (children and adults) due to severe vitamin D deficiency are now uncommon except in populations with unusually low sun exposure, lack of vitamin D in fortified foods, and malabsorptive syndromes. Subclinical vitamin D deficiency, as measured by low serum 25(OH)D, is very common. In the National Health and Nutrition Examination Survey (NHANES) 2005 to 2006, 41.6 percent of adult participants (≥20 years) had 25(OH)D levels below 20 ng/mL (50 nmol/L) [1]. This degree of vitamin D deficiency may contribute to the development of osteoporosis and an increased risk of fractures and falls in older adults. Vitamin D may also regulate many other cellular functions.

This topic review provides an overview of vitamin D. Other reviews discuss specific issues related to vitamin D:

(See "Causes of vitamin D deficiency and resistance".)

(See "Overview of rickets in children" and "Etiology and treatment of calcipenic rickets in children".)

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Literature review current through: Nov 2017. | This topic last updated: Nov 28, 2017.
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