Vitamin D Status: Current Opinion on Critical Levels for Plasma Calcium and Bone Mineral Homeostasis

Currently there is an unprecedented level of interest regarding the purported wide-ranging beneficial effects of an adequate vitamin D status translating into marked increases in test requests for clinical laboratories. The well characterised endocrine pathway of vitamin D metabolism and action is solely responsible for vitamin D regulating plasma calcium and phosphate homeostasis. A large body of data confirm that vitamin D exerts activities within each of the major bone cells and that these same cells are capable of synthesising the active metabolite, 1,25-dihydroxyvitamin D from 25-hydroxyvitamin D. Such data arising from in vitro studies, animal models and clinical sources are consistent with a paradigm that local metabolism of vitamin D by bone cells to form 1,25-dihydroxyvitamin D and its consequent local actions within bone cells exerts an anabolic effect to increase bone mineral status. The data reviewed here provide plausible mechanisms for both catabolic and anabolic actions of vitamin D on bone depending on dietary calcium intake.