The Myth About Soy Causing Breast Cancer

For over two thousand years, soy has been regarded as one of the healthiest natural foods on earth. Nowadays, soybeans are being cultivated and widely consumed all over the world.

Well, it’s just a myth.

High in protein, soybeans contain all eight essential amino acids making it one of the very few plants that offer complete non-animal proteins. Whole soy foods are an excellent source of fiber, omega-3 fatty acids, B vitamins, and calcium. It has been widely accepted that consumption of moderate amounts of soy could decrease the cholesterol level and the risk for cardiovascular diseases.

But, how come such a healthy natural food was feared by some people who think it may cause breast cancer?

Research Can Be Misleading

Ironically, it was all started when researchers became intrigued by the fact that East Asian women, among whom soy-based foods have been the staple of their diet for hundreds of years, have much lower breast cancer incidences comparing with Western countries. Scientists suspected that the isoflavones in soybeans, a protein that is structurally similar to human estrogen, may have protective effects against the development of breast cancer.

In the 1990’s, a few research groups designed experiments in animals, mainly in mice and rats, to study the effect of high dose, purified isoflavones from soy on the growth of breast cancer cells. What they found was that some breast cancer cells in mice and rats showed increased growth pattern after being injected or fed with high levels of isoflavones. Those data were later extrapolated and served as the main evidence to caution people about the potential risk of soy consumption in developing breast cancer as well as breast cancer recurrence.

However, experts quickly pointed out the flaws of those studies. They argued that the metabolism of isoflavones in mice and rats are significantly different from it in human (surprise?) and the level of isolated isoflavones used in those studies were at such high levels that were almost never consumed by the East Asian population, let alone the women in Western countries.

Subsequently, numerous epidemiological and clinical studies have repeatedly shown that consumption of whole soy products pose no increased risk for women to develop breast cancer. On the contrary, there is significant evidence that points to the decreased breast cancer risk among women who consistently consume soy products since their early child life.

Studies of soy consumption among breast cancer survivors (including North American women) have also shown no increased risk of breast cancer recurrence. As matter of fact, there is a trend of decreased breast cancer recurrence among women whose diet contains high amounts of soy products.

The Soybean Mindset Today

Our current understanding of soy-based isoflavones is that they belong to a family of compounds called “Selective Estrogen Receptor Modulators” or “SERMs”. Despite structural similarities, these compounds work on different types of estrogen receptors in different human tissues and exert different biological activities.

For example, human estrogen primarily works on estrogen receptor alpha which is mainly present in human breast and uterus tissues. On the other hand, isoflavones from soy primarily activate estrogen receptor beta which is not present in breast or uterus tissue but can be found in bone, brain, kidney, heart, and prostate. Therefore, they function differently from human estrogen and, in many ways, could serve as a counteracting force that potentially prevents breast cancer cells from developing.

What we don’t know for certain is at what level and how long of consumption of soy would be needed to provide the benefit of preventing breast cancer. We also don’t know whether a high dose of purified or isolated isoflavone supplement is safe to consume.

After decades of intense research effort on soy and breast cancer, the overwhelming evidence from human studies has largely discredited the false claim that soy consumption could potentially increase the risk of breast cancer or breast cancer recurrence.

American Cancer Society has recently reiterated position regarding the health benefits of soy products:

“Soy and soy-derived foods are an excellent source of protein and, for this reason, a good alternative to meat. Soy contains several phytochemicals, some of which have weak estrogenic activity and seem to protect against hormone-dependent cancers in animal studies. Other compounds in soy foods have antioxidant properties and may have anticancer activities. There is considerable public and scientific interest in the role of soy foods in the prevention of cancer in general and breast cancer in particular, although scientific support for such a role is inconsistent. For the breast cancer survivor, current evidence suggests no adverse effects on recurrence or survival from consuming soy and soy foods, and there is the potential for these foods to exert a positive synergistic effect with tamoxifen.”

It’s Just a Myth!

We, along with most experts in this field, believe that moderate intake of whole soy-based products, not the purified soy supplements, is not only safe but also potentially beneficial in preventing breast cancer, lowering cholesterol, helping weight reduction, and decreasing the risk of cardiovascular diseases.

For more detailed scientific discussion about soy, we encourage you to listen to this presentation by Dr. Anna Wu, Professor of Preventative Medicine at University of Southern California, on “Soy Isoflavones and Breast Cancer” at the 2013 annual meeting of America Society of Clinical Oncology (the most authoritative cancer research and treatment organization in the world).


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