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Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc
approach assumes that the intake and requirement distributions are independent, an assumption made for the nutrients addressed in this report. It further assumes that the variability of intakes among individuals within the group under study is at least as large as the variability of their requirements. This assumption is warranted in free-living populations. Finally, it assumes that the requirement distribution is symmetrical. This is thought to be true for all nutrients discussed in this report except iron, for which requirement distributions are skewed. Additional information on assessing the adequacy of group intakes of iron is provided in the section on “Nutrient-Specific Considerations”.
Before determining the percentage of the group whose intake is below the EAR, the intake distribution should be adjusted to remove the effect of day-to-day variation in intake. This can be accomplished either by collecting dietary data for each individual over a large number of days or by statistical adjustments to the intake distribution that are based on assumptions about the day-to-day variation (derived from repeat measurements of a representative subset of the group under study) (Nusser et al., 1996). When this adjustment is performed and observed intakes are thus more representative of the usual diet, the intake distribution narrows, giving a more precise estimate of the proportion of the group with usual intakes below the EAR (Figure 14-2). An explanation of this adjustment procedure has been presented in two previous reports (IOM, 2000; NRC, 1986).
An example of using the EAR cut-point approach to assess the dietary zinc adequacy of women aged 51 to 70 years follows. Dietary intake data are available from the Third National Health and Nutrition Examination Survey (NHANES III), which includes intakes from both food and supplements. Although the NHANES food-intake data were based on a single 24-hour recall for all individuals, replicate 24-hour recalls were conducted on a subset of the participants, and these estimates of day-to-day variation have been used to adjust the intake distributions (see Appendix Tables C-25 and C-26). The EAR for zinc for women is 6.8 mg/day. In the U.S. population, about 25 percent of adult women aged 51 to 70 years did not consume adequate amounts of zinc from food sources alone (Appendix Table C-25), as this proportion had estimated intakes below the EAR. When dietary supplements were included, there was little difference in the proportion below the EAR, suggesting that few individuals with low zinc intakes use zinc supplements.
The assessment of nutrient adequacy for groups of people requires unbiased, quantitative information on the intake of the nu-