Clinical impact of Vitamin D hydroxylation efficiency

Vitamin D is a major regulator of bone and mineral metabolism. However, on the basis of epidemiologic and experimental data numerous extra-skeletal effects are claimed for this vitamin. Indeed, in recent years this vitamin that can be synthesized in the skin, has turned into a true hormone. Moreover, the complexity of synthesis, metabolism and mechanism of action, allow us to identify a sophisticated “vitamin D endocrine system” which includes, besides the active molecule (calcitriol), several specific enzymes of synthesis and degradation, a dedicated plasma transport protein (D binding protein, DBP), a specific receptor (vitamin D receptor or VDR) and some regulating hormones.1 Further to regulation of calcium homeostasis and bone metabolism this system is implicated in extra-skeletal effects through activation of vitamin D receptors (VDR ) in almost every tissue.