Evidence from in vitro and animal studies has shown that vitamin C can regenerate or spare α-tocopherol (Halpner et al., 1998), but studies in guinea pigs and humans have not confirmed that this interaction occurs to a significant extent in vivo (Jacob et al., 1996). Calculation of redox potentials indicates that ascorbate can recycle the flavonoid radical (Bors et al., 1995), and Skaper et al. (1997) showed that ascorbic acid acts synergistically with the flavonoid quercetin, to protect cutaneous tissue cells in culture against oxidative damage induced by glutathione deficiency.
A variety of interactions of ascorbate with the redox-active trace metals iron and copper have been reported (the potential pro-oxidant effects are discussed later in the section “Pro-oxidant Effects”). Ascorbic acid is involved in the regulation of iron metabolism at a number of points. Ascorbate-related reduction of iron to the ferrous state is involved in iron transfer and storage pathways. Ascorbic acid added to meals facilitates intestinal absorption of nonheme iron, possibly due to lowering of gastrointestinal iron to the more absorbable ferrous state or amelioration of the effect of dietary iron absorption inhibitors (Hallberg, 1985). However, studies in which the vitamin is added to meals over long periods have not shown significant improvement of body iron status, indicating that ascorbic acid has less effect on iron bioavailability than has been predicted from tests with single meals (Hunt et al., 1994).
Some evidence indicates that excess ascorbic acid intake may affect copper metabolism in a variety of ways, including inhibition of intestinal absorption and ceruloplasmin oxidase activity and labilization of ceruloplasmin-bound copper for cellular transport (Harris and Percival, 1991). High concentrations of plasma ascorbate in premature infants has been suggested to decrease ceruloplasmin ferroxidase activity and thereby compromise antioxidant protection (Powers et al., 1995). However, the significance of these effects in humans is questionable, because high ascorbate intakes among men on a metabolic unit did not inhibit copper absorption (Jacob et al., 1987b). In addition, the findings of decreased ceruloplasmin ferroxidase activity due to high physiologic ascorbate concentrations have been attributed to an artifact of nonphysiological assay pH (Løvstad, 1997).