Does Eating Soy Cause Breast Cancer? Here’s What Experts and Studies Say

Soy is a great source of high-quality protein for vegetarians, vegans, and plant-based eaters, filled with vitamins, minerals, fiber, and omega-3 fatty acids. But how much soy is too much soy? In addition to all those nutrients, soy contains phytoestrogens called isoflavones — chemical compounds that are similar to the human hormone estrogen, though not the exact same and also weaker in strength.

There is research about the protective nature of phytoestrogens, but some believe that too much of these isoflavones can act as an endocrine disruptor, messing with the natural role of hormones and possibly the increasing risk of breast cancer, perhaps even fueling tumor growth in people who have estrogen-related breast cancer. So, what is the real link between soy and cancer? And how much soy is too much soy?

Actually the opposite is true. In large studies of Asian populations in which the women eat a lot of soy, the evidence is strong that the more soy you eat, the lower your overall risk of breast cancer, and soy may even have a protective effect in keeping estrogen production in check.

A PLOS-One analysis of more than 30 studies showed that soy intake reduced the risk of breast cancer for both pre- and postmenopausal women in Asian countries (though the studies have not found as strong a protective effect in American women, possibly because Asian women tend to start eating soy at a much younger age than their Western counterparts). Other studies have found that soy may help lower the risk of cardiovascular disease, diabetes, and hypertension.

The theory about why phytoestrogens may help ward off breast cancer goes something like this: Isoflavones are both weaker than the estrogen produced in your body and they. latch on to the Beta receptors for estrogen, thereby blocking actual estrogen from doing so. So if those plant substances replace the more potent hormone in your cells acting as a brake on actual estrogen, according to experts who explained this mechanism to The Beet. By doing so, they could actually help to ward off cancers that need animal-based estrogen to thrive.

"When it comes to estrogens, especially those that our own bodies make, there are two types of receptors," explains Lee Crosby RD. "Alpha receptors and beta receptors for estrogen. Alpha receptors act like accelerators and tell cells to grow, while beta receptors act as braes and tell cells to stop growing. Estrogen binds to the alpha receptors and sends the signal to grow, while plant-estrogen binds to the beta receptors and does the opposite: Tells the cells to stop growing."

“The women in many of these studies are economic vegetarians,” explains Marisa C. Weiss, MD, the founder and chief medical officer of BreastCancer.org. “They eat soy throughout most of their life because it is an inexpensive protein — they’re not eating a lot of meat or dairy.” So the question is, she adds, “Is the soy lowering their risk of breast cancer, or is it because they are eating less meat and less dairy? Is it because they tend to be thinner? We don’t really know if it’s the soy itself, but we do know that people who eat soy throughout their life have a lower risk of breast cancer.”

The science gets trickier when you look at soy intake in women who are breast cancer survivors. This is because when studies are done either in test tubes or in lab rats, it appears that genistein, a major form of isoflavone, can encourage breast cancer tumors to grow. However, this effect has not been found in studies of actual human women (and those rats were not being fed whole soy foods, but isoflavone extracts).

In fact, studies have found that for women who are survivors of breast cancer, eating soy may actually be protective against a recurrence: A recent study in the journal Cancer found that for North American women with breast cancer, those who ate the most soy had the lowest risk of dying over the next decade. Another large study in the Journal of the American Medical Association that followed more than 5,000 breast cancer survivors found that women who ate the most soy had the lowest rate of recurrence and death at a four-year follow-up.

Because isoflavone and the breast-cancer drug tamoxifen both bind to estrogen receptors, there has been concern that soy could interfere with the treatment; but the JAMA study found that for women who had low-to-moderate soy intake while on tamoxifen, there was an increased rate of survival, and for women on tamoxifen who had high soy intake, the soy did not seem to affect their prognosis one way or the other.

Dr. Weiss pointed out that in the Asian population studies, the women were not stopping into Trader Joe’s to load up on soy corn dogs. They are eating whole-food soy, such as tofu, edamame beans, tempeh, and soy milk. “Tofu is made from squeezing the soybean, then separating out the curd,” she explains. It is a completely natural food, low in cholesterol, fat-free, and filled with both protein and fiber, and as Dr. Weiss said, if you can buy organic, without pesticides, even better.

And of course, one of the key issues of eating soy is that it replaces much less healthy proteins in your diet, most notably, red meat. A study from the Harvard School of Public Health study found that women who ate red meat when they were growing up had a higher risk for breast cancer as adults. For each serving of red meat they ate per day as an adolescent, they had a 22 percent higher risk of premenopausal breast cancer; for each serving per day they ate as young adults, they had a 13 percent higher risk of breast cancer overall.

The not-so-great news for me is that the soy products that are in many vegetarian and vegan products — kids’ beloved nuggets and corn dogs, not to mention all those protein shakes, health bars, and supplements on supermarket shelves—are not made with all-natural soybeans, but with soy protein isolates. “This is a concentrated pharmaceutical extract,” Dr. Weiss explains. “We don’t know what the health impact of these isoflavones may be, and I would avoid anything that could have a hormonal effect in concentrated doses.”

And indeed, in a paper from the American Society of Clinical Oncology that discusses the benefits of soy, the authors caution that supplements with soy isoflavones should be avoided since they provided very high doses of isoflavones and have not been studied closely enough. “Whenever there is concern about something, we want to operate from caution, it’s better safe than sorry,” says Dr. Weiss. “Always choose real food over processed food. If there is a veggie burger made of ground edamame, choose that over one made of soy isolates.”

Start eating these soybean-based foods as young as possible to get the most protection against breast cancer. Soy products, however, are like any kind of processed food whether plant-based or meat-based: a hodgepodge of ingredients made in a lab that may or may not be doing harmful things to our bodies.

What you should do is take Dr. Weiss’s advice to heart: “Just eat real food.”

For more expert advice, visit The Beet's Health & Nutrition articles.