Myth-buster
A meta-analysis of 40 trials found soy isoflavones didn't significantly thicken the endometrium — but the result fell just short of statistical significance (p=0.059).
This contradicts the widespread belief that soy isoflavones act like estrogen in the body, but because the finding is on the edge of significance and this is among the first large-scale analyses of its kind, it doesn't settle the debate — it shifts the burden of proof.
Researchers pooled data from 40 randomized trials in postmenopausal women and found that taking soy isoflavones (about 75 mg per day) for roughly six months did not cause a meaningful thickening of the uterine lining, a marker of estrogenic activity. The effect was nearly nonexistent — the average change was just -0.22 mm, with the confidence interval barely missing zero — and the same null result held for other estrogen-sensitive measures like vaginal cell maturation and hormone levels. So the idea that soy is estrogen-like in a way that could affect reproductive tissues is not supported by this analysis.
Where this fits in the evidence
This is among the first studies we've indexed on Soy for Reduced Endometrial Thickness — treat it as an early signal until more research accumulates.
The study
- Meta-Analysis
- n = 3,285
- 2025-01
- Advances in nutrition (Bethesda, Md.)
This is a plain-language summary of a research finding, not medical advice. Pillser surfaces research signals to help you decide what's worth investigating — always consult a qualified professional before changing what you take.