Because soy isoflavones have a structure similar to estrogen, they can bind to estrogen receptors and have similar effects. A meta-analysis suggested that adding soy foods to the diet reduced the frequency of hot flushes in postmenopausal women.^[17] Soy isoflavones may also help mitigate the increased risk of cardiovascular disease that is associated with menopause.^[6][5] Soy isoflavones had similar effects on hot flushes and vaginal dryness as hormone-replacement therapy (HRT) but did not improve the overall menopause rating scale (MRS).^[18][19] Another study that used self-reporting of the MRS found some improvement in sleep disturbances as well.^[20] This suggests that soy isoflavones did not improve many of the other symptoms of menopause that HRT can improve, such as anxiety, irritability, depression, exhaustion, and joint pain.^[18][19]

Although several studies support the use of soy isoflavones as a treatment for the symptoms of menopause, there is high heterogeneity between these studies, likely due to a variety of study designs, dosages studied, and reported outcomes. Because of this, they are not yet recommended as a stand-alone treatment option for menopause symptoms.

Although soy protein appears to reduce LDL cholesterol by 4.3% on average,^[3] the contribution to this effect from the soy isoflavones is unknown.^[21][22] A meta-analysis did find that soy isoflavones reduced LDL cholesterol and total cholesterol in people with high cholesterol. However, soy protein had a similar effect.^[23] A 2008 meta-analysis of randomized controlled trials found that soy isoflavones did not improve LDL cholesterol levels in women with normal cholesterol.^[24] The evidence remains uncertain, and more research is needed before soy isoflavones can be recommended as a cholesterol-lowering agent.

Because soy and soy isoflavones have estrogen-like properties, they could affect breast cancer risk and recurrence. The evidence to date seems to support this. A 2020 meta-analysis found that increased soy intake was associated with a significantly lower risk of breast cancer, but they did not measure soy isoflavone intake specifically, and so this benefit could be a result of any of the compounds found in soy products.^[15] Increased dietary soy was also found to reduce the risk of breast cancer recurrence in women who were using cancer treatments such as tamoxifen, which work by blocking the effects of estrogen.^[25][26] However, there are some studies that found no association between breast cancer risk in relation to dietary soy intake.^[27][28][29]

Because soy isoflavones work by preferentially binding to estrogen receptor beta (ERβ), they could slow down the growth of certain types of cancer. However, there are some types of breast cancer that are more sensitive to the effects of estrogen receptor alpha (ERα), and in these cases, soy isoflavones could speed up the growth of cancer cells.^[13] The risks and benefits of soy isoflavone use for breast cancer prevention and treatment remain unclear, but soy products in dietary amounts are unlikely to result in any significant change in breast cancer risk. Caution is warranted when using higher concentrations of soy isoflavones in supplements.^{[30][31][32][29]}

Most of these studies use food diaries to measure soy intake, so the exact soy isoflavone intake is sometimes not measured, and the results are heterogeneous. The current evidence is promising, but more research is needed before soy isoflavones can be recommended to treat or prevent breast cancer.

There are multiple potential ways that soy isoflavones could improve gut health. Studies done in mice have found that soy isoflavones can improve gut motility, the secretory function of the intestine, and the integrity of the endothelial barrier.^[33] In in vitro studies, they have been shown to promote the growth of certain types of probiotic microbes.^[34] Other studies found that soy isoflavones did not influence the composition of the gut microbiota.^[35] The potential benefits of soy isoflavones for gut health in humans have not been clearly shown in clinical trials.

Soy isoflavones could have an effect on bone density, particularly in postmenopausal women. Meta-analyses have found that soy isoflavones increased markers of bone formation in postmenopausal women,^[2][36] and a study that followed up with the participants after 3 years of treatment with genistein found that it could improve bone mineral density in osteopenic postmenopausal women.^[37] These effects are most pronounced if soy isoflavones are taken earlier in menopause, and the effects are diminished if started later in menopause. There is no substantial evidence that soy isoflavones play a role in bone health in premenopausal women or in men.

While more clinical trials are needed to assess the appropriate timing and dosage and the clinical relevance of the effect soy isoflavones have on bone density, incorporating soy foods into the diet may be worthwhile for perimenopausal and postmenopausal women.^[38][39]

Studies done on rats found that supplementing with soy isoflavones (specifically genistein) increased insulin levels and lowered HbA1c in diabetic rats.^[40][41] A study in monkeys found that consuming soy protein high in soy isoflavones increased the insulin response to glucose.^[42]

In human studies, supplementing with soy and soy isoflavones also seemed to improve insulin sensitivity, lowered HbA1c, and improved fasting insulin.^[43][44] Although these results are promising, the changes in insulin and HbA1c levels were small, and the studies involved people with type 2 diabetes who were already taking antidiabetic medications. Further research into the appropriate dosage and the clinical relevance of the effect on insulin sensitivity is needed.

Equol is another type of soy isoflavone and is produced through the metabolism of daidzein by gut bacteria. It is thought to have strong antioxidant, estrogenic, and antiestrogenic effects and might be more beneficial than daidzein and genistein. However, some people are not able to produce equol, possibly due to the composition of their gut microbiome. Because of this, people who are “equol-producers” are likely to experience more benefit from soy isoflavone intake than people who are “nonequol-producers”.^[45][46]