Advances in Vitamin D nutritional physiology since publication of the DRIs in 1997 are briefly summarized. Available data indicate that (1) Vitamin D's canonical function, optimizing intestinal calcium absorption, is fully expressed at serum 25-hydroxyvitamin D (25OHD) concentration of approximately 80nmol/L; (2) elevated parathyroid activity, typical of aging populations, is minimized at the same 25OHD value and (3) osteoporotic fractures are reduced when serum 25OHD is raised to near 80nmol/L. Depending upon starting value, achieving 25OHD concentrations of 80 or higher may require a daily oral intake of 2200IU (55microg) or more in addition to prevailing cutaneous inputs. The tolerable upper intake level (TUIL), currently set at 2000IU (50microg)/day, is too low to permit optimization of Vitamin D status in the general population. Actual toxicity is not seen below serum 25OHD values of 250nmol/L, a value that would be produced only at continuing oral intakes in excess of 10,000IU (250microg)/day.