Soy isoflavone and ascorbic acid supplementation alone or in combination minimally affect plasma lipid peroxides in healthy postmenopausal women.

The objective of this study was to assess synergistic antioxidant properties of vitamin C and isoflavones. The design was a placebo-controlled crossover trial: 500 mg vitamin C, 5 mg/kg body weight isoflavones, 500 mg vitamin C plus 5 mg/kg body weight isoflavones, or placebo. Total lipid peroxides, plasma vitamin C, and blood pressure were measured. Eight of 10 healthy postmenopausal women completed the study. A multiple analysis of variance was performed and least-squares difference post-hoc test utilized to determine where differences occurred. Significance was defined as P <.05. There was a significant reduction in total lipid peroxides between baseline and isoflavone treatments (3.22+/-0.72 vs 2.47+/-0.82 nmol/mL, P <.05). Mean systolic blood pressure was higher during isoflavone intervention than placebo (117+/-14 vs 125+/-15 mm Hg, P= .042). Supplementation with vitamin C and isoflavones did not produce a synergistic antioxidant effect. A slight but significant increase in systolic blood pressure occurred with isoflavone supplementation. A larger study should be conducted to fully explore the potential interactions between these antioxidants.