are higher for the Canadian than for the U.S. population may be in part due to differences in assay methodologies used.
The final risk assessment step is risk characterization, which highlights implications of the DRI outcomes and special concerns including the population segments shown in Box S-4. The nature and extent of the risk associated with these population segments vary.
On balance, the uncertainties surrounding the DRI values for calcium are less than those for vitamin D because the evidence base is considerably larger for calcium, and the physiology and metabolism of calcium are better understood. The following key issues were identified as introducing uncertainty into DRI values for calcium and vitamin D, as based on bone health outcomes:
The tendency for study protocols to administer a combination of calcium and vitamin D, reducing the opportunity to ascertain effects of each nutrient independently;
The lack of data examining the responses and health outcomes
Population Segments and Conditions of Interest
Persons living at upper latitudes in North America
Persons who experience reduced vitamin D synthesis from sun exposure
Alternative diets or changes in dietary patterns
Use of calcium supplements
Oral contraceptive use
Interaction between vitamin D and prescription drugs