There is a belief that a diet rich in soy is beneficial for reducing breast cancer risk or treating the disease. However, the evidence for soy is patchy, confusing and often misinterpreted. Dr Matthew Lam looks at the facts.

New research from the Memorial Sloan-Kettering Cancer Center in the US has added to this body of evidence in an attempt to clarify the role of soy in breast cancer. 

What do we know so far?

In epidemiological studies conducted in Asian populations, soy intake has been shown to be associated with a reduced risk of breast cancer. However, these findings have not been replicated in studies carried out on Western populations so it is unclear exactly how and why soy may help prevent breast cancer in some people but not others. Cellular and animal studies have shown soy to be capable of both promoting and reducing cancer cell growth. Although there are mixed conclusions to be drawn, it’s important to note that soy intake has not been associated with an increase in breast cancer risk in any population studies.

Isoflavones – the real issue

The reason there is so much interest in soy and breast cancer is because soy contains high levels of compounds known as isoflavones. These compounds are chemically similar to estrogen so it’s not surprising that isoflavones can promote the growth of breast cancer cells over-expressing estrogen receptors (ER-positive breast cancer). In other studies carried out in breast cancer cells, isoflavones have shown to decrease cell growth and reduce the activity of genes associated with disease progression.

What does the new research show?

The aim of the study was to find out what the effects of soy intake in breast cancer patients were on molecular features associated with breast cancer – including those involved in the growth and progression of the disease. Women were randomised to receive soy protein or milk protein for between seven and 30 days and tissue samples were collected before and after for genetic and molecular analysis.

Their analysis revealed that the activity of genes associated with breast cancer cell growth were elevated in women taking soy protein compared to the group taking milk protein. Interestingly, soy increased the activity of some genes which may have an impact on breast cancer, including the gene FGFR2, which is known to increase cancer cell growth. However, the impact of this on patients’ tumour growth, how well they respond to treatment, or their overall outcome was not looked at in this study and remains unclear.

What does it all mean?

There are limitations to this study which make it difficult to come to any firm conclusions. Yes, soy protein increased the activity of genes associated with cancer cell growth but with no information on how this impacts the patient in the long term, it’s hard to say whether it would be problematic. The authors state that their findings raise some concern that soy may exert a stimulating growth effect on breast cancer in certain women. This isn’t particularly useful in a day-to-day context and doesn’t really answer any of the questions concerned with how long-term intake of dietary soy may influence breast cancer risk.

The current evidence still shows that eating soy in moderation is not harmful to humans. There is not enough clear evidence to suggest that eating soy will reduce or increase your risk of breast cancer.

About the author


Dr Matthew Lam is a Senior Research Communications Officer at Breast Cancer Now. He has a PhD in breast cancer research and becomes enraged in the presence of pseudoscience.