Consumption of flavonoids in onions and black tea: lack of effect on F2-isoprostanes and autoantibodies to oxidized LDL in healthy humans

Background: Oxidative damage to lipids in vivo may be involved in the development of atherosclerosis and cancer. Onions and black tea are foods rich in flavonoids, predominantly the flavonoid quercetin, which is a potent in vitro inhibitor of membrane lipid peroxidation and LDL oxidation. Objective: Our objective was to investigate the effects of consuming a high-flavonoid (HF) diet enriched with onions and black tea on indexes of oxidative damage in vivo compared with a low-flavonoid (LF) diet. Design: Thirty-two healthy humans were studied in a randomized crossover design. Indexes of oxidative damage used were plasma F2-isoprostanes (a biomarker of lipid peroxidation in vivo) and the titer of antibodies to malondialdehyde (MDA)-modified LDL. Results: There were no significant differences in the intake of macronutrients or assessed micronutrients, plasma F2-isoprostane concentrations, and MDA-LDL autoantibody titer between the HF and LF dietary treatments. In the men, however, plasma concentrations of the F2-isoprostane 8- epi -prostaglandin F2 α were slightly higher after the HF treatment phase than after the LF treatment [0.31 ± 0.029 nmol/L (111 ± 10.4 ng/L) compared with 0.26 ± 0.022 nmol/L (92 ± 7.8 ng/L); P = 0.041]. In all subjects, plasma quercetin concentrations were significantly higher after the HF treatment phase than after the LF treatment: 221.6 ± 37.4 nmol/L compared with less than the limit of detection of 66.2 nmol/L. Conclusion: Flavonoid consumption in onions and tea had no significant effect on plasma F2-isoprostane concentrations and MDA-LDL autoantibody titer in this study and thus does not seem to inhibit lipid peroxidation in humans.