Phytoestrogens and breast cancer

There is a great deal of confusion about the safety of soy with regards to breast cancer and breast cancer risk, based on the mis-belief that because soy contain oestrogen-like compounds, it can stimulate breast tissue like endogenous (our own body’s) oestrogen or pharmaceutical oestrogen might. Soy beans and soy products contain the class of phytoestrogens called isoflavones. The research has overall shown that consumption of soy in the diet, in amounts equivalent to that in an Asian diet is safe, actually reduces the risk of breast cancer and does not increase the risk of breast cancer recurrence in breast cancer survivors. Furthermore, the discovery of the Oestrogen receptor- Beta (ER-β) and how isoflavones preferentially bind to this receptor, has contributed to the understanding of the safety of soy (see Oestrogen Receptors below).

Examination of the all of the research on phytoestrogens and breast cancer has demonstrated the following important findings:

The research also discusses some cautions with soy supplements and highlights some conflicting information regarding soy constituents:

Additionally, phytoestrogen foods also have anti-cancer effects unrelated to their oestrogenic actions, which appear to be important. Soy isoflavones can also act as antioxidants, which may contribute to their anti-cancer properties.

Oestrogen Receptors (ER)

Phytoestrogens  bind to oestrogen receptors (ERs) and their activity is influenced by the oestrogen environment, how they bind to the oestrogen receptor and particularly to which oestrogen receptor they bind to.

There are different types (and subtypes) of ERs. Our own body’s oestrogen can bind to oestrogen-receptor alpha (ER-α) and oestrogen-receptor beta (ER-β), and binds with both receptors with equal ability. Phytoestrogens have a similar chemical structure to our own body’s oestrogen and can also bind to these ERs. When oestrogen binds to the ER-α, it stimulates growth of hormone-sensitive tissue. For example, the processes involved in the normal menstrual cycle are activated by oestrogen interaction with ER-α. These receptors are also related to the growth of hormone-sensitive tumours such as breast and endometrial cancer (or prostate cancer in men). ER-β has functions distinct from ER-α and seems to counteract processes brought about by ER-α. Isoflavones, found in soy and red clover, as well as in other legumes, can bind to both ER-α and ER-β, but they preferentially bind to and activate ER-β. For this reason, they are sometimes classified as selective (o)estrogen receptor modulators (SERMs). Because of the preferential binding of isoflavones to ER-β, which is a natural counterplayer to the ER-α-mediated hormonal effects, activation of ER-β by isoflavones protects tissues from excessive oestrogenic effects and thus helps to protect breast tissue from the excessive oestrogenic effects.


1 Trock, BJ et al 2006. ‘Meta-Analysis of Soy Intake and Breast Cancer Risk’ J of the National Cancer Institute 98 (7), pp 459-471).