There has been much speculation about phytoestrogens - particularly soy products, and breast cancer risk. This is mainly based on the idea that because these foods contain oestrogen, they can stimulate the breast tissue like our own body’s oestrogen or prescribed oestrogen might. The plant oestrogens that are found in these foods are much, much weaker than the oestrogen we have in our body.

In 2006 a study was published which looked at all the research on phytoestrogens and breast cancer between 1978 and 2004. This type of research is called a meta-analysis. The results showed that there was a small reduction in risk of breast cancer. The risk of breast cancer was reduced more in premenopausal women who consumed soy compared to postmenopausal women.

There is conflicting in vivo and in vitro data on the individual isoflavones found in soy, genistin, daidzin and their metabolites genistein and daidzein 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.

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.

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 of this study concluded that it is not recommended for women at high risk for 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. There is no evidence to suggest that eating a diet of soy foods in amounts consistent with an Asian diet is detrimental to breast health.

Therefore, it seems to be fine to eat a diet with “real” soy foods such as tofu, soy beans or breads made from soy flour; but to avoid the highly processed soy-based supplements, as they work differently to the way soy exists in nature.

Reference

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
http://jnci.oxfordjournals.org/cgi/content/abstract/98/7/459

Content updated October 24, 2007