Do Soy Foods Promote Breast Cancer?

Soy beansThere has been alot of debate recently about the safety of soy with respect to the development of breast cancer. One theory that hs been promoted by the anti-soy lobby over the years is that because soy contains estrogen-like compounds, it has the potential to enhance the growth of breast cancer, particularly ER-positive tumours.

The Weston A. Price foundation, which is an organisation funded primarily by meat and dairy farmers, is a classic example of the anti-soy lobby. On this page, the foundation boldly claims, without any evidence, that soy phytoestrogens promote breast cancer. In-fact, what the Weston A. Price foundation won’t tell you is that the majority of studies looking at this issue have found that consumption of soy products both reduces the risk of developing breast cancer and improves survival.

A 2009 study, published in The Journal of the American Medical Association looked at the issue of soy intake and survival after breast cancer diagnosis in a group of 5,042 Chinese breast cancer survivors. Those in the top 25% for soy intake, which corresponded to at least 15.3g of soy protein a day, had 29% reduced mortality and 32% reduced breast cancer recurrence compared to those in the bottom 25% for soy intake (who consumed less than 5.3g per day). Interestingly, soy led to larger decreases in the recurrence of ER-positive breast cancer (-31%) compared to ER-negative tumours (-23%).

Another study, also published in 2009, looked at the intake of various soy isoflavones on the risk of breast cancer recurrence in a group of women recently diagnosed with the disease. That study found that the isoflavones daidzein and glycetin decreased recurrence of breast cancer in post-menopausal women. In the case of daidzein, intake in excess of 1453μg per day resulted in an impressive 52% reduction in recurrence among women also receiving tamoxifen therapy.

Studies of soy intake and breast cancer incidence have yielded similar results with most studies showing between a 10% and 30% reduction in breast cancer risk for regular consumers of soy products. A meta-analysis of 18 studies, published in 2006, found a 14% reduction in the odds of developing breast cancer in high vs low consumers of soy products. The association was strongest for pre-menopausal women who had 30% reduced odds of developing the disease. The researchers also calculated that every 1 gram increase in soy protein resulted in a 3% reduction in the odds of developing breast cancer. The full results of that study can be found here.

Breast cancer incidence and soy intakeFurther evidence of the protective benefits of soy on breast cancer risk can be seen by comparing breast cancer rates in countries with high and low intakes of soy.

If soy was truly as dangerous as the anti-soy lobby groups would have us believe, we would expect death rates from breast cancer to be significantly higher in these countries. Japan, Korea, Thailand, and China have the highest per capita soy intakes in the world, ranging from 5-20g of soy protein per day.

As can be seen on the graph to the right, breast cancer rates in these countries are a mere fraction of the rates seen in Western countries such as the USA, UK, Canada, and Australia where soy intake averages less than 2g per day.