The resurgence of the importance of vitamin D in bone health.

Dietary reference intakes (DRIs) for Vitamin D (VitD) have been traditionally established based on plasma 25-OHD concentration sufficient to prevent rickets and osteomalacia. While nutritional rickets is still prevalent in developing countries, hypovitaminosis D is becoming widespread around the world regardless the latitude. Emerging evidence has unravelled the physiological roles of VitD beyond calcium homeostasis. Hypovitaminosis D has been linked to cancers, diabetes, CVD, periodontal diseases and influenza. Hypovitaminosis D is multifactorial and is related to VitD scarcity in foods, latitude, solar-irradiation, atmospheric-pollution, skin-pigmentation, clothing, sunscreen-use and indoor activities, etc. Plasma 25-OHD concentration range from 25-138 nmol/L. A higher plasma 25-OHD concentration is linked to higher bone-mass in adolescents, pre- and post-menopausal women. Plasma 25-OHD > or =75 nmol/L has been shown to enhance calcium absorption, suppress PTH elevation, reduce the risks of bone loss and fractures, and certain extra-skeletal diseases. VitD supplementation with 10 microg/d is insufficient to lower fracture risks. Combined VitD and calcium supplementation in higher doses has been found superior to VitD alone to increase bone-mass in adolescents and to reduce non-vertebral fractures in postmenopausal women. In future, DRIs for VitD are likely to be established beyond its skeletal roles to include multiple health outcomes. However, the desirable level of VitD has yet to be defined. Furthermore, redefining the upper-tolerable-level of VitD intake is necessary to prevent hypercalcemia and toxicity. There is also a urgent need to harmonize laboratory methods in VitD assay in different laboratories.