Soy and Breast Cancer—Is there a connection?

One of my most memorable patient interactions related to this topic, was an Asian woman with hormone sensitive breast cancer, who grew up eating soy foods with each meal her entire life. She like others heard that there might be a connection with the consumption of soy and breast cancer. Soy of course is one of the most used ingredients in Asian cuisine in many forms, such as of tofu, soy sauce, miso, edamame, soy milk, soy beans, tempeh, nato and many others. Understandably, it’s no surprise that many of my breast cancer patients ask if soy is safe to consume once you have breast cancer or if they should avoid it all together, as there is a lot of conflicting information out there. My patient asked me if she needed to stop eating soy completely and was extremely depressed by thought of this. She had many well-meaning friends and family members warning her of the dangers of eating soy with her cancer.

Where did the confusion begin? For starters, soy foods contain high amounts of plant estrogens (or phytoestrogens) that have an estrogen like chemical structure—this is actually where the term came from—which allows it to bind to estrogen receptors in the body. This can allow plant estrogens to mildly mimic human estrogen in the body, but it can also have the opposite effect too (more on this later).

Estrogen is of course the primary female sex hormone responsible for sexual and reproductive development. Because estrogen promotes the development, growth, and spread of breast cancer in those diagnosed with hormone sensitive breast cancer (which represents the majority of breast cancer types), many doctors have worried that eating high amounts of soy foods may worsen the prognosis for their patients. Others have feared that soy’s isoflavones, which are one group of plant estrogens, might interfere with a common breast cancer treatment called Tamoxifen. Tamoxifen is a drug that is often used in those with hormone sensitive breast cancer as a treatment or for prevention in those at high risk as it acts like an anti-estrogen. Since isoflavones, Tamoxifen and breast cancer have connections with estrogen there were general concerns with soy. But it would take years for research to test whether this concern was justified.

Soybeans 2

Interestingly, prior to the initial concern of soy and its relationship with breast cancer, soy was originally investigated for its potential role in fighting breast cancer. There had been observations of historically lower cancer incidence rates in Asian cultures compared to the US. One obvious difference between Asian and American cuisine was the significantly higher soy consumption that Asian’s eat. After this realization, earlier research started being conducted in animals and produced conflicting results. This led to a first series of studies in animals and with human cells grown in petri dishes. In the midst of these studies scientists realized something strange—that soy mimicked estrogen compounds and questioned if soy may have negative impacts for breast cancer patients.

Because the news from animal and petri dish studies had already spread, it has been hard to shake the notion that soy increases breast cancer risk, and people were not sure how to react when scientific findings in humans suggested otherwise. Further research revealed that soy intake was not only safe, but could actually potentially help with preventing breast cancer, possibly improve treatment outcomes, and could increase survival rates after diagnosis. For example, a large study of 9,514 women in the US and China, found that soy food consumption after a diagnosis of breast cancer was associated with improved treatment outcomes and lower recurrence rates.

Researchers found that the plant estrogens from soy bind competitively to estrogen receptors and can block estrogen in the body from attaching to breast cancer cells. As a result of these findings, dietary intake of soy foods would be expected to reduce breast cancer risk by having anti-estrogenic effects. Research has since advanced and has included lower and higher qualities of soy sources and has been analyzing the timing of soy intake though out the lifespan.

When you examine all of the data, there is now strong evidence to support that eating a moderate amount of soy foods each day does not increase the risk of breast cancer. What’s more, almost all of the high quality studies and analyses have evidence to support that even high intake of soy foods that we don’t normally see in the United States also doesn’t increase breast cancer risk and can have positive benefits. The central issue in studies currently is the timing of soy intake within lifespan and region of the world however. Their results suggest that if you didn’t eat soy prior to your breast cancer diagnosis, then don’t start now. But if you consumed soy foods prior to your diagnosis then you should continue doing so after diagnosis. We know that we need more research in humans with regards to timing of soy.


So what is considered a moderate serving?

The conservative recommendation that most oncology dietitians, scientists, and the American Institute of Cancer Research are recommending is two servings per day or less of soy foods for breast cancer survivors. For example: a serving would be 1 cup of soy milk, or ½ cup of soy beans/edamame, or 1/3 cup of tofu.

Since we know soy foods are okay, what about highly processed forms of soy?

In a food lab, we are able to separate different compounds from soybeans, such as soy isoflavones, soy protein concentrate, and soy protein isolates.  These can be used in soy supplements, soy protein powders, soy bars, and some meat alternatives such as textured vegetable protein (TVP), soy hotdogs, and some veggie burgers or soy nuggets. Several studies say that high-dose isoflavones from supplements have insufficient data to conclusively say supplements are harmful, but the research suggests they may have detrimental effects—so at this time supplements containing high-dose soy isoflavones are not currently recommended. More extensive future research may shed light on this.

What about other ingredients such as soy oil and soy lecithin?

Soy is often seen as an ingredient in many foods. You’ve likely seen soy oil and soy lecithin on ingredient lists, especially in salad dressings and baked goods. If not take a look to see what you find. Thankfully these ingredients are considered safe. Soy lecithin is a derivative of soy oil, which is used as a binder/emulsifier in foods, and is in foods in minute amounts.

If I am a breast cancer survivor does soy intake improve my survival and lower the chances of a recurrence?

The current evidence thus far suggests that a diet high in soy may improve survival and decrease the risk of recurrence, but these benefits may be more specific to the Asian population. Though research has shown many benefits, high intake of soy is still not currently recommended as a way for breast cancer survivors to reduce the risk of recurrence. The reason for this is that there is speculation about the different forms of soy and their effects, as well as, if women who frequently eat soy might also tend to be healthier in general than those who do not eat soy regularly. Even though soy’s full health benefits are uncertain, we do know that including some soy as part of your diet is healthy. Soybeans are high in fiber, protein, folic acid, potassium, magnesium, can be a great substitute for meat, and can help you to focus on more of a plant based diet.

What if I have triple negative (hormone negative) breast cancer?

For those without hormone sensitive breast cancer, moderate soy intake may still be beneficial. Currently, the conservative recommendation is two servings or less per day. Zhang, et al. showed inverse association between soy intake and all-cause mortality in 6235 women with breast cancers in the US and Canada. Women with the highest isoflavone intake had a 21% decrease in mortality compared with women who had the lowest intake. This reduction was seen only in women with hormone receptor negative tumors, and in women who were not treated with endocrine therapy such as Tamoxifen. 


  • When reviewing human studies that reflect everyday life we see that it’s safe to eat soy each day. If you don’t feel comfortable consuming soy foods, you don’t have to. Just know that you can safely eat soy foods if you do decide to, as plant estrogens do not have the same effect as human estrogen.
  • Studies on humans that looked at soy foods and anti-estrogen medicines like Tamoxifen show that there are no harmful interactions. A small number of studies even suggest that soy foods may be protective for women who take Tamoxifen, but more research is still needed.
  • The American Institute of Cancer Research (AICR) finds no harmful effects from breast cancer survivors eating soy foods.
  • The effects of soy on tumor growth in animals and human cells grown in a petri dish aren’t the same as in a human body.
  • Processed soy is still processed food, and whole food is always a healthier choice. You can still include some processed soy foods like soy nuggets in moderation.
  • No studies have found that phytoestrogens cause cancer to grow in humans.
  • Overall, there is little clinical evidence to suggest that plant estrogens will increase breast cancer risk in healthy women or worsen the prognosis of breast cancer patients.







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