[Vitamin D : More than just a bone hormone]

Vitamin D plays an essential role in the primary therapy and prevention of osteoporosis. Established vitamin D effects are renal and intestinal resorption of calcium and phosphate for optimal bone mineral density; however, the widespread distribution of the vitamin D receptor (VDR), a member of the nuclear steroid hormone receptor family, provides extensive evidence for additional pleiotropic effects of the vitamin D ligand. Vitamin D deficiency and insufficiency are common and were reported as risk factors for a variety of diseases in observational studies. In addition, extensive research from experimental studies also illustrated extraskeletal effects of vitamin D. More randomized controlled trials are ongoing to test the effects of high dose vitamin D supplementation in numerous chronic diseases. This article summarizes the current state of knowledge regarding effects on muscle, on glucose metabolism in type 2 diabetes mellitus and on cardiovascular risk factors and diseases. Furthermore, we discuss the influence of vitamin D on the immune system, whereby vitamin D might provide beneficial effects not only for infectious but also for autoimmune diseases, such as type 1 diabetes mellitus. Current evidence leads to the conclusion that vitamin D deficiency should be avoided in these diseases. Recommendations for optimal dosage are discussed.