Wednesday, April 07, 2010
vitamine D
Vitamin D: The Basics
Vitamin D refers to two biologically inactive precursors: vitamin D3 (cholecalciferol), produced mainly in the skin post exposure to ultraviolet (UV) radiation, and vitamin D2 (ergocalciferol), produced exogenously and entering the circulation solely after gastrointestinal absorption.[3] Both vitamin D2 and D3 require hydroxylation reactions in the liver and kidney, closely regulated by the parathyroid hormone (PTH), to form the biologically active metabolite of vitamin D, 1,25(OH)2D (calcitriol).[4] Vitamin D status is best measured by 25(OH)D concentrations or levels and not calcitriol levels for several reasons, including longer half-life (~3 weeks compared to ~8 hours for calcitriol), higher circulating serum concentrations, and tight regulation of calcitriol by PTH, resulting in falsely elevated calcitriol levels despite vitamin D deficiency.[5] Currently, there is no universally accepted "normal" measure of 25(OH)D levels; however, it has been suggested that concentrations above 30 ng/mL are associated with decreased fracture rates and maximal parathyroid suppression.[6]
Although the classic function of vitamin D has been to increase the intestinal absorption of calcium for proper bone health, its role in health maintenance is beginning to expand with the finding of vitamin D receptors (VDRs) in many cells throughout the body, including cardiomyocytes, vascular smooth muscle, and endothelium.[5,7] Additionally, recent studies have found that individuals with vitamin D deficiency have increased incidence of CVD.[7]
