Isoflavonoids do not inhibit in vivo lipid peroxidation in subjects with high-normal blood pressure

The isoflavonoids genistein and daidzein have been shown to have antioxidant activity in vitro, but their effects on in vivo oxidation have not been assessed. The newly described F-2-isoprostanes are believed to currently represent the best available marker of in vivo lipid peroxidation. Therefore we have assessed the effects of a 55 mg daily isoflavonoid supplement on urinary F-2-isoprostane concentrations in subjects with high-normal blood pressure (BP). A total of 59 subjects completed an 8-week parallel design, randomized, double blind, and placebo-controlled study. F-2-isoprostanes, isoflavonoids and creatinine were measured in 24-h urine samples taken at baseline and at the end of the intervention. There were significant increases in urinary excretion of genistein (5.22 +/- 0.75 mg/day, P < 0.0001) and daidzein (2.53 +/- 0.43 mg/day, P < 0.0001) in the group taking the isoflavonoid supplement. Creatinine excretion was significantly correlated with F-2-isoprostanes at baseline (r = 0.45, P < 0.01). After adjustment for baseline values, there was no significant difference between groups in creatinine adjusted post-intervention F-2-isoprostane concentrations (P = 0.74). In addition, changes in genistein and daidzein excretion were not significantly correlated with changes in F-2-isoprostanes in the isoflavonoid treatment group. These results are not consistent with the suggestion that the two soy derived isoflavonoids have in vivo antioxidant activity at a level of intake achievable: by dietary means and in subjects with high-normal BP. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.