Ascorbate scavenging of myeloperoxidase-derived oxidants from phagocytic white cells may also be protective against in vivo LDL oxidation because HOCl-oxidized proteins have been identified in human atherosclerotic lesions (Hazell et al., 1996). In an in vitro system, ascorbate at a physiologically relevant concentration of 50 µmol/L (0.9 mg/dL) was the most effective antioxidant for preventing LDL oxidation due to myeloperoxidase-derived RNS (Byun et al., 1999).
Table 5-4 summarizes the results of five experimental human studies in which cellular markers of DNA damage were measured after various vitamin C intakes. Three of the studies varied vitamin C alone, while the other two studies varied vitamin C and other micronutrients.
Of the three studies that varied only vitamin C intake, one showed that 60 or 250 mg/day decreased sperm 8-hydroxy-7, 8-dihydro-2′-deoxyguanosine (8-oxodG), a measure of oxidative stress, but did not affect lymphocyte or urine 8-oxodG or DNA strand breaks (Fraga et al., 1991). In contrast, the second study showed no effect of either 60 or 6,000 mg/day vitamin C on lymphocyte DNA or chromosome damage as measured by comet assay (Anderson et al., 1997). The third study showed both decreases and increases in measures of lymphocyte DNA oxidative damage after vitamin C supplementation of 500 mg/day (Podmore et al., 1998). In a subsequent report of results from the study of Podmore et al. (1998), the investigators hypothesized that increases in serum and urine 8-oxodG following the decreases of lymphocyte 8-oxoguanine and 8-oxodG suggest a role for vitamin C in the repair of oxidant-damaged DNA (Cooke et al., 1998).
The two studies that co-supplemented with vitamin E and β-carotene (Duthie et al., 1996) or iron (Rehman et al., 1998) demonstrated mixed results in that both decreases and increases in lymphocyte DNA oxidant damage measures. Since the contribution of vitamin C alone to the results of these studies cannot be determined, these studies cannot be used to estimate a vitamin C requirement. Results of the latter study involving supplementation of apparently healthy individuals with both vitamin C and iron are discussed in the section “Tolerable Upper Intake Levels.”
Inverse correlations of lymphocyte ascorbate and glutathione con-