attributed to the effect of high serum ascorbate levels inhibiting ceruloplasmin ferroxidase activity, thereby creating an excess of reactive ferrous ions (Powers et al., 1995). This result and other reports of ascorbate inhibition of ceruloplasmin ferroxidase activity (Gutteridge, 1991) have subsequently been attributed to an artifact of using a nonphysiological pH buffer in the ceruloplasmin ferroxidase assay (Løvstad, 1997).
Results of studies testing the effects of supplemental vitamin C intake on markers of oxidant damage to deoxyribonucleic acid (DNA) and chromosomes are discussed in an earlier section and are summarized in Table 5-4, Table 5-5, and Table 5-6. The results are mixed, with studies showing a decrease, increase, or no change in oxidant damage measures. A study of 30 apparently healthy adults supplemented with 500 mg/day of vitamin C for 6 weeks reported an increase in 8-oxoadenine, but a decrease in the more mutagenic DNA lesion, 8-oxoguanine (Podmore et al., 1998). Supplementation of apparently healthy volunteers with vitamin C and iron resulted in increases in some DNA damage markers, decreases in others, and a rise in total DNA base damage at 6 weeks, which disappeared at 12 weeks (Rehman et al., 1998). Other evidence from in vitro and in vivo data as well as epidemiological studies have not shown increased oxidative DNA damage or increased cancer risk associated with high intakes of vitamin C (Block, 1991; Fontham, 1994; Fraga et al., 1991; Rifici and Khachadurian, 1993).
Other Adverse Effects. Other adverse effects observed following high vitamin C intakes include diminished high-altitude resistance (Schrauzer et al., 1975), delayed-type allergic response (Metz et al., 1980), and erosion of dental enamel (Giunta, 1983). Additional studies confirming these findings were not found.
Identification of Distinct and Highly Sensitive Subpopulations. Data show that individuals with hemochromatosis, glucose-6-phosphate dehydrogenase deficiency, and renal disorders may be susceptible to adverse effects from excess vitamin C intake. Vitamin C may enhance iron absorption and exacerbate iron-induced tissue damage in individuals with hemochromatosis (McLaran et al., 1982). Individuals with renal disorders may have increased risk of oxalate kidney stone formation from excess vitamin C intake (Auer et al., 1998; Ono, 1986; Urivetzky et al., 1992). Hemolysis has been associated with ascorbic acid administration in newborns with glucose-6-phosphate dehydrogenase deficiency and in normal premature infants (Ballin et al., 1988; Mentzer and Collier, 1975). There is also anecdotal evidence of hemolysis following ascorbic acid intake in adults