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Classical homeostatic physiology sheds light on vitamin D controversies
Simeon I Taylor1, Zhinous S Yazdi2
1Department of Medicine, University of Maryland School of Medicine, Health Sciences Research Facility III, 4th Floor, 670 W. Baltimore Street, Baltimore, MD 21201, USA.
Abstract:
Because vitamin D (VitD) plays a key role in calcium balance, the body has evolved homeostatic mechanisms to maintain 1α,25-dihydroxyvitamin D levels within a physiological range. Historically, measurements of total 25-hydroxyvitamin D [25(OH)D] have provided assessments of VitD status. However, this approach has important limitations because it measures 25(OH)D bound to vitamin D-binding protein (DBP), while it is free 25(OH)D that is biologically relevant. This opinion article discusses precision diagnostic approaches to minimize the confounding effects of interindividual variation in DBP levels. Ratios of VitD metabolites [25(OH)D, 1,25-dihydroxyVitD, and 24,25-dihydroxyVitD] provide DBP-independent indices of VitD status. This precision diagnostic approach could have an especially favorable impact on patients with several clinical conditions, for example, malabsorption, impaired activation of VitD, or estrogen-induced increases in DBP.
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