the possibility that total intakes (diet plus supplements) above 2,000 mg/day may increase the risk for kidney stones, and demonstrate no increase in benefits relative to bone health. There is also some limited evidence that the long-term use of calcium supplements may increase the risk for cardiovascular disease. Although no attempt was made to compare systematically the data used for the North American population that is the subject of this report with data from other countries focused on persons who are genetically and environmentally different from those in the United States and Canada, it should be recognized that calcium requirements may be subject to a variety of factors that have not yet been fully elucidated and so therefore cannot yet be integrated into DRI reviews.
For vitamin D, the challenges introduced by issues of sun exposure cannot be ignored. This nutrient is unique in that it functions as a prohormone, and the body has the capacity to synthesize the nutrient if sun exposure is adequate. However, concerns about skin cancer risk preclude making recommendations about sun exposure; in any case, there are a number of unknowns surrounding the effects of sun exposure on vitamin D synthesis. At this time, the only solution when DRIs are to be set for vitamin D is to proceed on the basis of an assumption of minimal sun exposure and set a reference value assuming that all of the vitamin D must come from the diet. Moreover, the possibility of risk for persons typically of concern because of reduced synthesis of vitamin D, such as persons with dark skin or older persons in institutions, is minimized given the assumption of minimal sun exposure for the DRIs.
One unknown in the process of DRI development for vitamin D is the degree to which waning kidney function with aging may be relevant. It appears that increasing serum 25OHD levels do not typically increase calcitriol levels in aging persons with mild renal insufficiency, and a dietary strategy to address the concern is not evident.
Although ensuring adequacy is important, there is now an emerging issue of excess vitamin D intakes. A congruence of diverse data on health outcomes ranging from all-cause mortality to cardiovascular risk suggests that adverse health outcomes may be associated with vitamin D intakes that are much lower than those classically associated with hypervitaminosis D and that appear to occur at serum 25OHD levels achievable through current levels of supplement use.
The extensive review of the data required to conduct this study and to determine DRIs for calcium and vitamin D that are consistent with existing scientific understandings has answered many questions. But, the process has also identified or left unanswered other questions due to the limita-