Towards a trial-based definition of vitamin D deficiency

Abstract

Vitamin D is not a vitamin (ie, an essential micronutrient), but rather the parent compound of an endocrine system primarily involved in the regulation of intestinal calcium absorption. It is synthesised in the skin as a result of the action of ultraviolet (UV) light on 7-dehydrocholesterol, and oral intake (other than supplement use) makes a negligible contribution to vitamin D concentration in most people. It is not biologically active, but two metabolites, 25-hydroxyvitamin D (25[OH]D) and 1,25-dihydroxyvitamin D, bind to the vitamin D receptor (the latter with a 1000 times the affinity of the former) and stimulate the active absorption of calcium in the upper small bowel.