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are higher for the Canadian than for the U.S. population may be in part due to differences in assay methodologies used.

IMPLICATIONS AND SPECIAL CONCERNS

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.

Uncertainties

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

BOX S-4

Population Segments and Conditions of Interest

Adiposity

Persons living at upper latitudes in North America

Persons who experience reduced vitamin D synthesis from sun exposure

  • Dark skin (including immigrant groups and exclusively breast-fed infants)

  • Use of sunscreen

  • Indoor environments and institutionalized older persons

Alternative diets or changes in dietary patterns

  • Dairy and animal product exclusion

  • Changes in dietary patterns of indigenous Canadian populations

Use of calcium supplements

Oral contraceptive use

Premature infants

Interaction between vitamin D and prescription drugs



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