As discussed in Chapter 4, there are very limited data to suggest that there may be some biological differences in the way in which different ethnic/racial groups respond to calcium and vitamin D, most notably among those of African American ancestry. The extent to which such observations may affect requirements for the nutrients is unknown at this time. Although it is important to take into account biological differences where they may exist among, for example, African Americans, Hispanics, and those of Asian descent, the available data are too limited to permit the committee to assess whether separate, quantitative reference values for such groups are required. The DRIs established in this report are based on the current understanding of the biological needs for calcium and vitamin D across the North American population. Other factors may come into play in terms of ensuring adequate intakes of these nutrients—for example, lactose intolerance or food choices—but as far as is known these factors do not affect the basic biological need for these nutrients. Rather, they are discussed in Chapter 8 as issues relevant to the application of the DRIs by dietary practitioners.
Described in this chapter is the committee’s decision-making regarding the dose–response relationships for calcium and bone health, and for vitamin D and bone health. From these conclusions, DRI values for adequacy are specified. A significant underlying assumption made by the committee is that the DRIs for calcium are predicated on intakes that meet requirements for vitamin D and that the DRIs for vitamin D rest on the assumption of intakes that meet requirements for calcium. In other words, the requirement for one nutrient assumes that the need for the other nutrient is being met. This is an essential assumption, for three reasons:
Given that reference values are intended to act in concert for the purposes of planning diets, health policy makers would be working to meet all nutritional needs; therefore it would be inappropriate to establish requirements for such purposes on the basis that one or more related nutrients would be consumed by the population in inadequate amounts.
An inadequacy in one of the nutrients could cause changes in the efficient handling of or physiological response to the other nutrient that might not otherwise be present. For example, in vitamin D–deficient states with minimal calcium intake, absorption of calcium from the gut cannot be enhanced. The compensatory metabolic response to this scenario is the accelerated conversion of 25-hydroxyvitamin D (25OHD) to its active form (calcitriol) through an increase in parathyroid hormone (PTH) levels. Such perturbations confound the estimation of the true requirement under neutral circumstances.