But don't isoflavones mimic the hormone oestrogen?

Isoflavones, because of their chemical similarity to oestradiol, have been mistakenly considered to act exactly like oestrogen. This has resulted in confusion and some practitioners recommending that women with breast cancer or those at increased risk of breast cancer should avoid soy foods and beverages. However, there is no clinical evidence to support these recommendations and it is contrary to population evidence where women who have the highest intakes of soy also have the lowest rates of breast cancer.

For oestrogen to carry out its effects in the body, it must bind to a 'receptor'. There are currently two main types of oestrogen receptors in the body, the classical oestrogen receptor alpha (ERα) and the more recently discovered oestrogen receptor beta (ERβ). These receptors are differently distributed in the body. It is thought that the reason isoflavones do not act the same way as oestrogen is because isoflavones have a preference to bind to ERβ rather than ERα ¹ . In addition, the isoflavone genistein binds to oestrogen receptors in a different way to oestradiol. It is thought that genistein may bind to oestrogen receptors in a similar way to the anti-breast cancer drug Raloxifene ²⁶ , ²⁷ . A large study of over 7,700 postmenopausal women found that Raloxifene was highly protective against breast cancer ²⁸ .

One study showed that miso (a fermented soy food) significantly enhanced the effectiveness of Tamoxifen treatment in rats that had chemically induced mammary cancer ²⁹ . While this effect has only been observed in this animal model for breast cancer, it is possible that soy foods could have a similar beneficial effect in humans.