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Soy Doesn’t Cause Breast Cancer—It Prevents It

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For years, American women at high risk for breast cancer have worried that eating soy could increase their risk of developing the disease. After all, soy contains phytoestrogens, which mimic certain qualities of the female hormone estrogen—and high levels of estrogen can spur tumor growth. The worry has been particularly intense for women being treated for breast cancer and for those who have survived it.

Now they can relax—and enjoy foods made from soy along with everyone else. A major study done on US women who have had breast cancer has put the fears to rest. It confirms that research done on Asian women, who regularly eat much more soy than most Americans do, applies to American women, too.

Major finding: A diet rich in such soy foods as tofu, edamame and tempeh doesn’t increase the risk for cancer recurrence in women with a history of breast cancer—and in fact, may help prevent it. What’s more, women with breast cancer who are currently undergoing treatment may get better results if they eat some soy every day.

HOW SOY SAVES THE DAY

Soy has both estrogen-like properties and estrogen-blocking properties. That combination prompted researchers at Tufts University to explore whether women with breast cancer should eat more soy-based foods—or steer clear of them. To do this, they examined the diets of 1,664 women with histories of breast cancer—some were undergoing treatment, while others were breast cancer survivors. The women were of diverse ethnicities and were followed for nearly 10 years.

The subjects were participants in the Breast Cancer Family Registry, a large, ongoing study of families with histories of breast cancer or genetic mutations that make them more susceptible to developing breast and ovarian cancer. The study was published in the journal Cancer. Results

  • Women who ate the most foods rich in isoflavones—the major phytoestrogens in soy—responded better to cancer treatment.
  • Women who ate the most soy lived longest. Details: Those who consumed the highest levels of soy isoflavones (more than 1.5 mg/day) had a 21% lower mortality rate over 10 years compared with women who consumed the lowest levels (less than 0.3 mg/day, which is less than about one serving a day).

SOY IS MOST PROTECTIVE FOR ONE KIND OF BREAST CANCER

The apparent protective power of soy depended on what kind of breast cancer a woman had. For women with “hormone receptor positive” breast cancer, including those taking hormonal adjuvant therapies, eating more soy was neutral. There was no harm but no benefit in terms of mortality.

Women whose breast cancer was “hormone receptor negative,” however, really benefited from soy. These are women who are not candidates for hormonal adjuvant therapies such as tamoxifen. For these women, who don’t benefit from these medications that help prevent recurrence, eating more soy is one step they can take to improve their odds. And since the study also found that women who started eating more soy after diagnosis benefited, it’s clear that while a lifelong soy habit is healthy, it’s never too late to start.

The findings echoed the results of earlier research published in The American Journal of Clinical Nutrition that women who ate more soy-based foods after diagnosis were 25% less likely to have recurring tumors than women who ate little or no soy after diagnosis. So this is the second major study in the US demonstrating that women with breast cancer who eat soy tend to do better—not worse.

What about isoflavone supplements? Unfortunately, this study didn’t cover them, so no conclusions can be drawn about whether they are beneficial or safe.

HOW MUCH SOY IS ENOUGH?

The data from this and other research—which shows that soy consumption is strongly linked to breast cancer prevention overall—is now crystal clear. Everyone should feel good about eating soy, including women with breast cancer, women whose family members had breast cancer and girls going through puberty. Men, too! Not only does soy reduce the risk for breast cancer in women, it also protects against prostate cancer in men and heart disease and diabetes in both men and women. And based on research, the earlier in life that people start eating soy-based foods, the better.

It doesn’t take very much soy to get the benefits, either. For example, two glasses of soy milk and one half-cup serving of edamame a day would do it, according to Dr. Kucuk. Nor do you have to eat soy every single day—just shoot for this kind of daily average consumption over the week.

Any type of soy-based food is beneficial. While it’s always a healthy practice to eat minimally processed foods, any food that contains soy protein and soy isoflavones and doesn’t contain harmful ingredients is good. So if tofu and miso aren’t your thing, try a soy burger or another meat-mimicking product. An anticancer veggie burger? Sure!

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Source: Omer Kucuk, MD, chief of genitourinary medical oncology at Winship Cancer Institute of Emory University and a professor at Emory University School of Medicine, both in Atlanta. He wrote an editorial commenting on the study titled, “Dietary isoflavone intake and all-cause mortality in breast cancer survivors: The Breast Cancer Family Registry,” by researchers at The Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy, Tufts University, Boston, et al., published in Cancer. Date: August 21, 2017
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