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What's The Deal With Soy And Breast Cancer?

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I avoid soy like the plague because of a family history of breast cancer. What is the real story on soy and breast cancer? -- Adrienne

About 75 percent of breast cancer tumors have estrogen receptors, meaning they grow in response to estrogen. And so, if breast cancer runs in your family, it stands to reason that you'd want to closely regulate exposure to a hormone that promotes tumor growth. For many people, doing so includes avoiding soy -- but luckily for those who love their edamame or veggie burgers, this isn't necessary at all. Yes, soy mimics estrogen and estrogen is linked to some hormonally active cancers, but that does not mean that soy itself is linked to cancer.

"There has been concern over the years regarding soy consumption and development or recurrence of breast cancer, and the basis for this concern is isoflavones, a compound in soy with a similar chemical structure to estrogen," Dr. Erica Mayer, M.D., M.P.H., a medical oncologist at the Susan F. Smith Center for Women's Cancers at Dana-Farber, explains to HuffPost. "The idea is that this could potentially fuel growth of breast cancer, but that doesn't bear out in the data."

Breast cancer and soy studies typically fall into two categories -- lab studies in rodents, or large-scale, observational studies of human behavior over time. Some rodent studies have shown that if you give rats or mice a very high dose of pure isoflavones, it can promote tumor growth. But, as Mayer points out, this result cannot be extrapolated to how humans process soy, both because of physiological differences between humans and rodents and because of the major differences between isoflavone supplements and dietary soy.

And then there's the other side of the research spectrum: the observational, population-level studies. These studies have generally followed large Asian populations, where soy is most commonly consumed. And, not only does this high soy diet not contribute to increased rates of cancer, it actually seems to lower them in many cases.

"Even though animal studies have shown mixed effects on breast cancer with soy supplements, studies in humans have not shown harm from eating soy foods," echoes Marji McCullough, strategic director of nutritional epidemiology for the American Cancer Society in a blog post for the organization. "Moderate consumption of soy foods appears safe for both breast cancer survivors and the general population, and may even lower breast cancer risk. Avoid soy supplements until more research is done."

While there is no definitive explanation for why soy may be able to lower breast cancer risk, there are a few factors that are likely involved. On a micro-level, though isoflavones mimic estrogen, there is also some evidence that they have anti-estrogen properties, including a role in blocking estrogen from binding to receptors (and helping them to bind in the blood instead) and inhibiting the formation of estrogen in fat tissue, according to the American Cancer Society. More globally, for overweight or obese women, a plant-based diet can lead to weight loss and weight loss is associated with a decreased risk of breast cancer.

"The message that [the] oncology community is trying to get out is that instead of following concerns about soy in the diet, women should be focusing on exercise and weight loss as preventive measures for breast cancer," explains Mayer. "We really do have very good evidence and a growing accumulation of data on the link between obesity and breast cancer risk and the beneficial effect of exercise. That's not the case for soy."