Soy is safe at normal dietary intake and has no measurable hormone-disrupting effect. It offers a modest benefit for hot flushes, but high supplemental doses are not recommended for people with hormone-dependent cancer.
Soy contains phytoestrogens, also known as isoflavones (such as genistein and daidzein). These compounds resemble the female hormone oestrogen but behave differently in the body. A large meta-analysis of 40 randomised trials involving more than 3,200 postmenopausal women shows that soy isoflavones at a typical dose of 75 mg per day have no measurable oestrogenic effect on the uterus, hormone levels (FSH, oestradiol) or vaginal tissue. The concern that soy 'acts like a hormone' is therefore not supported by this evidence at normal consumption amounts.
Where soy isoflavones do appear to offer some benefit is in hot flushes and night sweats around the menopause. The effect is modest but probably real. Notably, the effect is strongest in women whose gut bacteria convert daidzein into equol. Not everyone produces equol, which explains why results can vary considerably from person to person. For bone health, findings are contradictory: some studies observe less bone loss in the spine, others see no difference. There is currently insufficient evidence for a firm conclusion here.
In the area of cardiovascular disease, several population studies show an association between higher soy intake and a lower risk of coronary heart disease. This is, however, primarily associative evidence from observational research, not evidence of direct cause and effect. The same applies to cancer: epidemiological studies suggest that people who eat soy foods regularly throughout their lives have a slightly lower risk of breast and prostate cancer, but the mechanisms have not been fully clarified and the evidence is not definitive.
There are also possible risks worth noting. In children who already have reduced thyroid function (hypothyroidism), there are indications that isoflavones may further suppress the thyroid. In healthy adults this effect is small or absent. More importantly, high supplemental doses of soy isoflavones are not recommended for people with hormone-dependent cancers (such as certain forms of breast or prostate cancer). Ordinary food intake -- such as tofu, edamame or soy milk -- is considered safe on the basis of available evidence. For cognition and urogenital complaints in women, there is insufficient evidence that soy isoflavones are beneficial.
Evidence comes from one large meta-analysis of RCTs (PMID 39433088, n=3285), multiple systematic reviews and meta-analyses (PMID 33809928, 40725220, 27886135), and observational/associative research. The thyroid risk is based on limited evidence (PMID 32824177). For cancer risk and bone health, the evidence is limited and partly contradictory.