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:

• There is a small reduction in the risk of breast cancer associated with phytoestrogen consumption. The risk of breast cancer was reduced more in premenopausal women who consumed soy compared to postmenopausal women
• It has also been shown that a diet high in phytoestrogen in early life (before puberty) may be important for the anti-cancer effects of phytoestrogens in later life.
• Recent reviews have shown that women whom consume a moderate amount of soy throughout their life have a lower breast cancer risk compared to women who do not consume soy. Again, it is proposed that this protective effect is due to soy intake earlier in life.
• Recent reviews of human studies looking at breast cancer survivors has shown that a moderate consumption of a diet containing soy isoflavones does not increase the risk of breast cancer recurrence in Western women, and Asian breast cancer survivors have a better prognosis if they continue eating a soy diet.

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

• There is conflicting in vivo (real life) and in vitro (test tube) data on the individual isoflavones found in soy, depending on the dose. Sometimes there was a stimulation of human breast cells in the research, and other times there was a prevention of stimulation. (It is important to consider that drawing conclusions from test tube studies of isolated compounds found in foods in no way represents what occurs in the body when a food is eaten. Whole foods contain differing amounts of complex compounds that can behave differently when they are consumed as a food, versus what happens to an isolated compound in a test tube. It is generally agreed that while test tube evaluation of compounds provides important information, there are more suitable methods of assessing safety when evaluating food products).
• Importantly, it is suggested that highly processed soy supplements such as soy protein isolate, isoflavone-rich soy extracts or isoflavone capsules act differently to foods made from soybeans or soy flour. The authors a review which examined all the research on phytoestrogens and breast cancer between 1878-2004, concluded that it is not recommended for women at high risk of breast cancer or breast cancer survivors to take these highly processed soy supplements, such as soy protein isolate, isoflavone-rich soy extracts or isoflavone capsules.¹ (Soy germ may be an exception as it very similar to whole soy – discuss with your naturopath whether soy germ may be appropriate)
• There is no evidence to suggest that eating a diet of soy foods in amounts consistent with Asian diet is detrimental to breast health. This finding was again confirmed in a review published in 2008 in Nutrition Journal, stating that there is no evidence that isoflavone intake increases breast cell proliferation (growth) in pre- or postmenopausal with or without a history of breast cancer and that eating phytoestrogens at levels consistent with historical Asian soy food does not result in adverse stimulatory effects on breast tissue.

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).