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.[1] 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.[2][3] However, there are some studies that found no association between breast cancer risk in relation to dietary soy intake.[4][5][6]
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.[7] 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.[8][9][10][6]
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.