Long-term combined administration of quercetin and daidzein inhibits quercetin-induced suppression of glutathione antioxidant defenses.

In this study, we investigated the effects of long-term administration of quercetin with or without daidzein on glutathione and the enzymes involved in its metabolism in rat liver in vivo. Male Sprague-Dawley rats were divided randomly into four groups and given oral quercetin (20 mg/day) and daidzein (20 mg/day) alone or in combination, or vehicle alone for six weeks. The serum and liver alpha-tocopherol concentrations were significantly increased following administration of quercetin and daidzein alone or in combination. Glutathione concentration and glutathione reductase activity was significantly (p < 0.05) decreased with quercetin treatment, while no such effect was observed with daidzein treatment. Interestingly, decrease in glutathione concentration and glutathione reductase activity by quercetin treatment was inhibited by combined administration of daidzein and quercetin. The malondialdehyde concentration was significantly decreased following administration of quercetin and daidzein alone or in combination. These results suggest that quercetin, but not daidzein, acts as a pro-oxidant agent by decreasing glutathione concentration and glutathione reductase activity. Interestingly, this pro-oxidant effect of quercetin was inhibited by the combined administration of quercetin and daidzein.