(NHANES III) with the EARs for vitamin C. These data include intake from both food and supplements. Although the NHANES food intake data are 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 derived from this subset have been used to adjust the intake distributions (see Appendix Table C-1 and Table C-2).

The EARs for vitamin C are 60 mg/day for women and 75 mg/day for men. Based on the U.S. population that doesn't smoke, approximately 11 percent of women and 21 percent of men did not consume adequate amounts of vitamin C (from food sources and supplements) (Figure 9-2).

Those who smoke require an additional 35 mg/day of vitamin C, which would result in an adjusted average requirement of 95 mg/day for females and 110 mg/day for males. This is shown in Figure 9-3, in which a higher prevalence of inadequacy is estimated for smokers compared to nonsmokers. Even when vitamin C supplements are included, 53 percent of men and 30 percent of women who smoke were below the requirement. These prevalences indicate that a substantial percentage of Americans who smoke may have inadequate intakes of vitamin C and thus would be expected to have less than optimal ascorbate saturation of neutrophils.

The assessment of nutrient adequacy for groups of people requires unbiased, quantitative information on the intake of the nutrient of interest by individuals in the group. Care must be taken to ensure the quality of the information on which assessments are based, so they are not underestimates or overestimates of total nutrient intake. Estimates of total nutrient intake, including amounts obtained from supplements, should be acquired. It is also important to use appropriate food composition data with valid nutrient values for the foods consumed. In the example for vitamin C intakes, a database of vitamin C values for all foods that contribute substantially to the intakes of this nutrient, as well as a database with the vitamin C composition of the supplements consumed by the population under study, are required.

Overestimates of the prevalence of inadequate intakes could result if the data used are based on intakes that are systematically underreported or if foods rich in vitamin C are underreported. Conversely, underestimates of the prevalence of inadequacy would result if vitamin C-rich foods were overreported. A more extensive discussion of potential sources of error in self-reported dietary data can be found in the upcoming report on using DRIs in dietary assessment.