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DRI DIETARY REFERENCE INTAKES FOR Vitamin C, Vitamin E, Selenium, and Carotenoids
ASSESSING NUTRIENT INTAKES OF GROUPS
Using the Estimated Average Requirement for Groups
The prevalence of nutrient inadequacy for a group of individuals may be estimated by comparing the distribution of usual intakes with the distribution of requirements. The Estimated Average Requirement (EAR) is the appropriate Dietary Reference Intake (DRI) to use for this purpose. In most situations, a cutpoint approach may be used to estimate the prevalence of inadequate intakes within the population group under study; this approach is a simplification of the full probability method of calculating the prevalence of inadequacy described by the National Research Council (NRC, 1986). The cutpoint approach allows the prevalence of inadequate nutrient intakes in a population to be approximated by determining the percentage of the individuals in the group whose usual intakes are less than the EAR for the nutrient of interest. This approach assumes that the intake and requirement distributions are independent, that the variability of intakes among individuals within the group under study is greater than the variability of their requirements, and that the requirement distributions are symmetrical.1
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 (Nusser et al., 1996). This can be accomplished by collecting dietary data for each individual over many days or by statistical adjustments to the distribution that are based on information or assumptions about the day-to-day variation (Nusser et al., 1996). When this adjustment is performed (and intakes are thus more representative of the usual diet), the intake distribution narrows and gives a more precise estimate of the proportion of the group with usual intakes below the EAR. An explanation of an adjustment procedure was presented by the National Research Council (NRC, 1986) and is also described in the upcoming report on using DRIs for dietary assessment.
Figure 9-2 and Figure 9-3 are a graphical representation of this approach for vitamin C. They compare the adjusted distribution of intakes of vitamin C for men and women 19 years of age and older from the Third National Health and Nutrition Examination Survey
For free-living populations, it is reasonable to assume that the variability in requirements is smaller than the variability in intakes. For vitamin C, vitamin E, and selenium, requirement distributions are assumed to be symmetrical, and the intake and requirement distributions are thought to be independent.