Soy foods and breast cancer
For years women with breast cancer were told to avoid soy because it contains plant oestrogens. Large studies show moderate soy food intake is safe and may slightly reduce recurrence, including on tamoxifen.
Soy isoflavones (genistein, daidzein) bind oestrogen receptors weakly and preferentially activate ER-beta, which raised concern about stimulating hormone-receptor-positive breast cancer or antagonising tamoxifen. Pooled analyses of prospective cohorts in the United States and China (After Breast Cancer Pooling Project, 9,514 survivors) found that higher soy food intake (at least 10 mg isoflavones/day) was associated with about 25% lower recurrence, with no adverse interaction with tamoxifen or aromatase inhibitors, and Asian cohorts show lower incidence with lifelong soy intake, particularly when started in adolescence. Randomised trials of isoflavone supplements show no change in mammographic density or breast epithelial proliferation. The American Cancer Society and WCRF regard moderate soy food consumption as safe for survivors. The caveat that remains is concentrated isoflavone supplements, which reach doses far above food intake and are not recommended. Soy is a case study in how a plausible mechanism, amplified by supplement marketing on both sides, misled clinical advice for two decades.
How it works
Isoflavones are selective oestrogen receptor modulators with weak, ER-beta-biased activity and additional anti-proliferative and anti-angiogenic actions; at dietary doses they do not stimulate ER-positive breast tumour growth in humans.
- Large pooled cohorts with consistent direction
- Trial evidence of safety on intermediate endpoints
- Corrects a harmful piece of common advice
- Observational for recurrence
- Supplement doses untested for safety
- Effect may depend on lifelong exposure and gut metabolism (equol producers)
Latest papers
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