Is Soy Really Bad for Breast Cancer?

“O Soy, Soy, wherefore art thou Soy?”

I think Romeo had fewer impediments to Juliet than Soy to thousands of breast cancer sufferers. Controversy about the ingestion of soy increasing the risk of breast cancer is not new and is persistent. Weekly, our breast cancer patients ask us about the safety of soy.  

The problem started when we discovered that soybeans and their related products are among the richest foods in total phytoestrogens. These phytoestrogens are present primarily in the form of isoflavones, daidzein, and genistein.  

In the early days, our understanding of estrogens is that any kind of estrogen that binds with hormone receptors in the breasts can cause the proliferation of the tissues potentially causing more breast cancer cells if there is the presence of breast cancer. This old understanding unfortunately has gotten stuck in our minds after all these years so that even now, patients still avoid the ingestion of soy.

The reality is that most naturally occurring phytoestrogens act as selective estrogen receptor modulators or SERMs. In a normal consumption of soybeans, and when these phytoestrogens bind with estrogen receptors in the breast and in the body, they do not elicit a physiological response in humans. In other words, they DO NOT cause the proliferation of breast tissue or breast cancer. 

Below, I shared one of the major clinical studies in breast cancer which has given us new thoughts on the possible benefits of ingestion of soy and breast cancer survival.  

The Shanghai Breast Cancer Survival Study, is a large, population-based cohort study of 5,042 female breast cancer survivors in China. Women aged 20 to 75 years with diagnoses between March 2002 and April 2006 were recruited and followed up through June 2009. Information on cancer diagnosis and treatment, lifestyle exposures after the cancer diagnosis, and disease progression was collected at approximately 6 months after cancer diagnosis and was reassessed at 3 follow-up interviews conducted at 18, 36, and 60 months after diagnosis.  

This study specifically looked at the question of how soy may be good or bad for breast cancer recurrence. The result was surprising in that soy food consumption after the cancer diagnosis, measured as soy protein intake, was inversely associated with mortality and recurrence. In other words, the group of patients that had the highest ingestion of soy had the lowest mortality and recurrence rate.

Among the many components of soy food, soy isoflavones are the most studied phytochemicals for their health-related effects. It has been shown that soy isoflavones compete with endogenous (self-secreted) estrogens in the binding of estrogen receptors, increase the synthesis of sex hormone–binding globulin (thus lowering the biological availability of sex hormones), inhibit 17β-hydroxysteroid dehydrogenases (thus reducing estrogen synthesis), and increase the clearance of steroids from the circulation. These anti-estrogenic properties may be one of the underlying mechanisms through which soy food consumption is associated with better breast cancer outcomes.

Besides the benefits of these phytoestrogens, soy foods are rich in nutrients including B vitamins, fiber, potassium, magnesium, and a sizable amount of high-quality protein. Unlike some plant proteins, soy protein is considered a more complete protein, containing all nine essential amino acids that the body cannot make which must be obtained from the diet.  Fermented soy such as miso, tempeh, or natto further breaks down the soybeans and allows an even better digestible option that promotes absorption.     

In fact, the American Cancer Society agrees with the positive finding on soy with its quote:

There is growing evidence that eating traditional soy foods such as tofu may lower the risk of cancers of the breast, prostate, or endometrium, and there is some evidence it may lower the risk of certain other cancers.

Because of the aforementioned benefits, we suggest including soy in the diet for our patients with breast, prostate, and endometrial cancers. In addition, for patients suffering from estrogen-receptor-positive (ER+) breast cancer, we also recommend taking Estro-DIM which promotes healthy levels of estrogen in the body. DIM or diindolylmethane is a compound converted from indole-3-carbinol made from cruciferous vegetables. Naturally, when Kale, broccoli, cauliflower, Brussels sprouts, and cabbage are exposed to stomach acid, indole-3-carbinol is created, which is then converted into DIM.

In summary, based on a majority of research studies, ingestion of soy is safe for breast cancer patients and may provide additional preventive benefits that will require continuous investigations. And at last, Juliet rejoiced, “My bounty is as boundless as the soy, my love as deep…”