Cholecalciferol is produced by the action of sunlight on a cholesterol precursor in the skin. This compound is then converted to calcidiol (25(OH) D3) in the liver, whereupon calcidiol is converted in the kidney to calcitriol (1,25(OH)2D3), the active form of vitamin D. However, recently it has been shown that deficiency of either calcidiol or calcitriol is associated with inflammation, insulin resistance and increased mortality in the general population. Furthermore, when both calcidiol and calcitriol were deficient, the mortality risk was much higher than the deficiency of either alone. A possible explanation is that some of the non-renal tissues might critically depend on the endogenous conversion of calcidiol to calcitriol and not on circulating levels of calcitriol. Thus, low circulating levels of calcidiol might be associated with tissue level functional calcitriol deficiency despite adequate circulating levels of calcitriol.