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from graded doses of calcium or vitamin D intake so as to elucidate dose–response relationships;

  • The interaction between calcium and vitamin D to the extent that it would appear that adequate calcium intake greatly diminishes the need for vitamin D relative to bone health outcomes;

  • The unique situation in which a nutrient (vitamin D) is physiologically managed by the body as a hormone, introducing a myriad of variables and feedback loops related to its health effects;

  • The paucity of data and resulting uncertainty concerning sun exposure, which confounds the interpretation of dose–response data for intakes of vitamin D. This, coupled with the apparent contribution of sun exposure to overall vitamin D nutriture in North American populations, leads to an inability to characterize and integrate sun exposure with dietary intake recommendations as much as may be appropriate, given the concern for skin cancer risk reduction. Thus, for individuals who experience sun exposure, the uncertainty of the DRI is greater than for those who do not;

  • The lack of clarity concerning the validity of the serum 25OHD measure as a biomarker of effect;

  • The variability surrounding measures of serum 25OHD concentrations owing to different methodologies used;

  • The evidence of the non-linear nature of the relationship between serum 25OHD concentrations and total intake of vitamin D, suggesting that lower levels of intake have more impact on serum 25OHD concentrations than previously believed and that higher intakes may have less impact;

  • The limited number of long-term clinical trials related to calcium and vitamin D intake and health outcomes; and

  • The need to set ULs based on limited data in order to ensure public health protection.

For vitamin D, the challenges introduced by issues of sun exposure are notable. This nutrient is unique in that it functions as a hormone and the body has the capacity to synthesize it. However, concerns about skin cancer risk preclude incorporating the effects of sun exposure in the DRI process. At this time, the only solution is to proceed on the basis of the assumption of minimal sun exposure and set reference values assuming that all of the vitamin D comes from the diet. This is a markedly cautious approach given that the vast majority of North Americans obtain at least some vitamin D from inadvertent or intentional sun exposure. Therefore, the estimated intake data for vitamin D cannot stand alone as a basis for broad public health action. Rather, national policy should consider intake data in the context of measures of serum 25OHD, a well-established biomarker of



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