Antioxidant Role. At a vitamin C intake of 90 mg/day, the plasma ascorbate concentration reaches 50 µmol/L which has been shown to inhibit LDL oxidation in vitro in both cellular and cell free systems (Jialal et al., 1990). Although it is not known whether vitamin C prevents LDL oxidation in vivo, if it does this might be relevant in the prevention of heart disease (Jialal et al., 1990). Also, as discussed earlier, since neutrophils are at 80 percent saturation at an EAR of 75 mg/day, this should potentially protect intracellular proteins from oxidative injury when these cells are activated during infectious and inflammatory processes (Anderson and Lukey, 1987; Halliwell et al., 1987).
Plasma Vitamin C Concentrations. Based on data from the Third National Health and Nutrition Examination Survey (NHANES III), although more than 75 percent of adult men have dietary vitamin C intakes higher than the EAR of 75 mg/day (Appendix Table C-1), only 50 percent have plasma vitamin C concentrations greater than 38 µmol/L (0.67 mg/dL) (Appendix Table F-1). This plasma concentration is estimated from the data of Levine et al. (1996a) to correspond to an intake of 75 mg/day of vitamin C (Figure 5-3). This finding is not surprising since the NHANES III vitamin C plasma concentrations are for both smokers and nonsmokers, and it is known that plasma vitamin C concentrations are reduced by about 40 percent in male smokers (Pelletier, 1977; Weber et al., 1996). In addition, as discussed in the earlier section “Environmental Tobacco Smoke,” exposure of nonsmokers to environmental tobacco smoke can result in a decline in plasma ascorbate concentrations (Tribble et al., 1993; Valkonen and Kuusi, 1998). Findings from the first three years (1988 to 1991) of NHANES III indicate that 38 percent of the participants were smokers and an additional 23 percent were nonsmokers exposed to environmental tobacco smoke at home or work (Pirkle et al., 1996).
Based on vitamin C intakes sufficient to maintain near-maximal neutrophil concentrations with minimal urinary loss, the data of Levine et al. (1996a) support an EAR of 75 mg/day of vitamin C for men. Since the data were based on men and no similar data are available for women at the present time, it is assumed that women will have a lower requirement due to their smaller lean body mass, total body water, and body size. This assumption is supported by the findings previously discussed that women maintain higher plasma