Effects of phytoestrogen supplementation on hormones in postmenopausal women: a systematic review and meta-analysis

Background The decline in ovarian estradiol production after menopause is associated with endothelial dysfunction, systemic inflammation, and increased cardiovascular risk. Because conventional hormone therapy may carry potential risks, phytoestrogens—plant-derived compounds structurally similar to 17β-estradiol—have been proposed as alternative hormonal modulators. However, whether phytoestrogens meaningfully influence circulating sex hormone levels in postmenopausal women remains uncertain. Purpose To evaluate the effects of phytoestrogen supplementation on circulating sex hormone levels in postmenopausal women. Methods A systematic search of PubMed, Embase, and Cochrane was conducted to identify randomized controlled trials comparing phytoestrogens with control (placebo or no intervention) on circulating hormone levels in postmenopausal women. Prespecified subgroup analyses were performed according to phytoestrogen class, including isoflavones, lignans, flavonoids, and other potential modifiers. Treatment effects were pooled using standardized mean differences (SMD) with 95% confidence intervals (CI). Results Thirty-nine randomized controlled trials, including 3,360 postmenopausal women, were analyzed; 1,673 (49.8%) received phytoestrogens. No meaningful differences were observed between phytoestrogen and control groups in circulating estradiol (SMD 0.18; 95% CI −0.01 to 0.36; p=0.04), estrone (SMD −0.00; 95% CI −0.15 to 0.15; p=0.77), or sex hormone–binding globulin (SHBG) (SMD −0.04; 95% CI −0.21 to 0.12; p=0.59). Subgroup analyses according to phytoestrogen class—including flavonols, isoflavones, and lignans—showed no statistically significant effects. Conclusions Phytoestrogen supplementation does not significantly modify circulating estradiol, estrone, or SHBG levels in postmenopausal women. These findings do not support a clinically relevant systemic estrogenic effect and suggest that potential cardiovascular benefits, if present, are unlikely to be mediated by increases in circulating estrogen levels.