Vitamin D is, at this point, probably one of the trendiest vitamins around. Everyone suddenly seems to be getting their vitamin D level tested (specifically vitamin D3 or 25(OH)D, also called calcidiol) and, when levels are found to be deficient, taking supplements. In November 2010, the Institute of Medicine tripled its daily recommendations for vitamin D from 200 International Units to 600 [1]. Severe vitamin D deficiency causes rickets, which leads to a softening and weakening of the bones, so milk has been fortified with vitamin D to prevent rickets. Less dramatic vitamin D deficiency has been implicated in ailments ranging from cancer to heart disease to schizophrenia to autoimmune diseases to colds and the flu. But how does vitamin D act in the body — how can it contribute to so many different physiological processes?