carotene and other carotenoids. However, adult men and women participating in NHANES III underreported energy intake by about 23 percent, as well as fat intake (which serves as a carrier for vitamin E) when expressed as a percentage of total energy intake (Briefel et al., 1997).

Adjusting for Day-to-Day Variation

Because of day-to-day variation in dietary intakes, the distribution of 1-day (or 2-day) intakes for a group is wider than the distribution of usual intakes even though the mean intake may be the same (for further elaboration, see Chapter 9). To reduce this problem, statistical adjustments have been developed (NRC, 1986; Nusser et al., 1996) that require at least 2 days of dietary data from a representative subsample of the population of interest. However, no accepted method is available to adjust for the underreporting of intake, which may average as much as 20 percent for energy (Mertz et al., 1991).

DIETARY INTAKES IN THE UNITED STATES AND CANADA

Sources of Dietary Intake Data

The major sources of current dietary intake data for the U.S. population are the Third National Health and Nutrition Examination Survey (NHANES III), which was conducted from 1988 to 1994 by the U.S. Department of Health and Human Services, and the Continuing Survey of Food Intakes by Individuals (CSFII), which was conducted from 1994 to 1996 by the U.S. Department of Agriculture (USDA). NHANES III examined 30,000 subjects aged 2 months and older. A single 24-hour diet recall was collected for all subjects. A second recall was collected for a 5 percent nonrandom subsample to allow adjustment of intake estimates for day-to-day variation. The 1994 to 1996 CSFII collected two nonconsecutive 24-hour recalls from approximately 16,000 subjects of all ages. Both surveys used the food composition database developed by USDA to calculate nutrient intakes (Perloff et al., 1990). National survey data for Canada are not currently available, but data for vitamin C have been collected in Québec and Nova Scotia. The extent to which these data are applicable nationwide is not known.

Appendix D gives the mean and the first through ninety-ninth percentiles of dietary intakes of vitamin C and vitamin E by age from the CSFII, adjusted for day-to-day variation by the method of Nusser et al. (1996). Appendix C provides comparable information



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