Can isoflavones mimic the hormone oestrogen?

Isoflavones, because of their chemical similarity to oestradiol, have been mistakenly considered to act exactly like oestrogen eventhough they have weak oestrogen-like properties at best and sometime even oppose the action of oestrogen or display neutral activity. 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 they are contrary to population findings 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', akin to a docking station. There are 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 they have a preference to bind to ERβ rather than ERα ¹ . Binding to ERβ appears to 'put the breaks on' the more oestrogenic effects of ERα. In addition, isoflavones bind differently to oestrogen receptors compared to oestrogen, sending different signals. For example, genistein binds to oestrogen receptors in a different way to oestradiol, and more similarly to the way the anti-oestrogen breast cancer drug Raloxifene ³³ ^, ³⁴ binds. A large study of over 7,700 postmenopausal women found that Raloxifene was highly protective against breast cancer ³⁵ .

Two studies using rodent models that are more relevant to the human body, showed that miso (a fermented soy food) and genistein or a soy phytochemical concentrate (SPC) significantly enhanced the effectiveness of Tamoxifen to treat chemically induced mammary cancer ³⁶ ^, ³⁷ . While these findings have only been observed in these animal models for breast cancer, it is possible that soy foods could have a similar beneficial effect in humans.