efits with regard to cardiovascular disease and cancer. This plasma vitamin C concentration is achieved at a dietary intake of approximately 90 mg/day vitamin C (Levine et al., 1996a). Thus, in the United States or Canada, it may be difficult to do a large-scale trial that demonstrates a health benefit for vitamin C unless the subjects are prescreened to have dietary intakes less than 90 mg/day and plasma levels less than than 50 µmol/L (1.0 mg/dL) of vitamin C.
Some 70 to 90 percent of usual dietary intakes of ascorbic acid (30 to 180 mg/day) are absorbed, although absorption decreases to about 50 percent and less with single doses above 1 g (Kallner et al., 1979; Levine et al., 1996b). The type of food consumed has not been shown to have a significant effect on absorption of either intrinsic or supplemental vitamin C. The bioavailability of the vitamin naturally found in foods or in the form of a supplement has not been shown to be significantly different from that of pure synthetic ascorbic acid (Johnston and Luo, 1994; Mangels et al., 1993).
Vitamin C participates in redox reactions with many other dietary and physiological compounds, including glutathione, tocopherol, flavonoids, and the trace metals iron and copper (Jacob, 1995).
Interactions of ascorbate with the endogenous antioxidant glutathione have been shown in both rodents and humans. In apparently healthy men fed a low-ascorbate diet of 5 to 20 mg/day, plasma total glutathione (reduced [GSH] and oxidized [GSSG] forms) and the ratio of GSH/GSSG, both indicators of oxidative stress, were significantly decreased (Henning et al., 1991). In apparently healthy adults supplemented with 500 mg/day of ascorbic acid, erythrocyte glutathione rose significantly (Johnston et al., 1993). The results indicate that ascorbate may contribute to antioxidant protection by maintaining reduced glutathione.