Phytoestrogens possess a weak antioxidant activity on low density lipoprotein in contrast to the flavonoid quercetin in vitro in postmenopausal women

Background Phytoestrogens are a family of plant-derived compounds with weak estrogenic and antiestrogenic properties. The antioxidant capacity of phytoestrogens has been proposed as one of the important mechanisms that explain their health benefits. Objective To determine the in vitro potency of three phytoestrogens, ubiquitous in food, (blochanin A, daidzein and genistein) as antioxidants of low density lipoprotem (LDL) and to compare them with the well-established antioxidant actions of estradiol and quercetin, an ubiquitous flavonoid which is found in high concentration in onions, tea and berries. Methods LDL was isolated by ultracentrifugation from the plasma of ten healthy postmenopausal women who were not on hormone therapy. Aliquots containing 0.5 mg of protein were incubated for 4 h with CuSO4 15 mumol/l to induce oxidative stress and with one of the five compounds studied: estradiol, quercetin, blochanin A, daidzein, and genistein, in doses of 0, 5, 15, 50, 500, 1000 and 2000 mumol/l. In addition, we studied the combined effect of estradiol 1 mumol/l plus quercetin 1 mumol/l, comparing their antioxidant action with that of each compound separately. Malonaldehyde (MDE nmol/ mg protein) was measured as a marker of LDL oxidation. Results Estradiol and quercetin induced a dose-dependent decrease in MDE concentration (p < 0.01). Comparing the areas under the curve, the antioxidant effect of quercetin was 8 times higher than the one observed with estradiol (P < 0.01). A 50% decrease in MDE was reached by quercetin at a concentration of 3.4 mumol/l, estradiol at 29 mumol/l, genistein at 280 mumol/l, biochanin at 1312 mmol/l and daidzein at 8007 mmol/l. Estradiol 1 mumol/l and quercetin 1 mumol/l did not modify MDE generation separately, but, when incubated combined, there was a significant decrease of MDE (p < 0.02). Conclusion The phytoestrogens studied showed a weak antioxidant activity in vitro. The flavonoid quercetin, in contrast, showed the most potent antioxidant activity in vitro, higher than estradiol. Estradiol and quercetin showed additive antioxidant activity. We speculate that different compounds with variable antioxidant effects could amplify their antioxidant capacity when acting combined.