Soyfoods have become controversial in recent years,…even among health professionals,…exacerbated by misinformation found on the Internet.” Chief among the misconceptions is that soy foods promote breast cancer, because they contain a class of phytoestrogen compounds called isoflavones. Since estrogens can promote breast cancer growth, it’s natural to assume phytoestrogens might too.
But, people don’t realize there are two types of estrogen receptors in the body—alpha and beta. And, unlike actual estrogen, soy phytoestrogens “preferentially bind to and activate [estrogen receptor beta]. This distinction is important, because the 2 [types of receptors] have different tissue distributions…and often function differently, and sometimes in opposite ways.” And, this appears to be the case in the breast, where beta activation has an anti-estrogenic effect, inhibiting the growth-promoting effects of actual estrogen—something we’ve known for more than ten years. There’s no excuse anymore.
The effects of estradiol, the primary human estrogen, on breast cells are completely opposite to those of soy phytoestrogens, which have antiproliferative effects on breast cancer cells, even at the low concentrations one gets in one’s bloodstream eating just a few servings of soy—which makes sense, given that after eating a cup of soybeans, the levels in our blood cause significant beta receptor activation.
So, where did this outdated notion that soy could increase breast cancer risk come from? The concern was “based largely on research that showed that [the main soy phytoestrogen] genistein stimulates the growth of mammary tumors in [a type of] mouse.” But, it turns out, we’re not actually mice. We metabolize soy isoflavones very differently from rodents. The same soy leads to 20 to 150 times higher levels in the bloodstream of rodents. The breast cancer mouse in question was 58 times higher. So, if you ate 58 cups of soybeans a day, you could get some significant alpha activation, too. But, thankfully, we’re not hairless athymic ovariectomized mice, and we don’t tend to eat 58 cups of soybeans a day.
At just a few servings of soy a day, with the excess beta activation, we would assume soy would actively help prevent breast cancer. And, indeed, “[s]oy intake during childhood, adolescence, and adult life were each associated with a decreased risk of breast cancer.” Those women who ate the most soy in their youth appear to grow up to have less than half the risk.
This may help explain why breast cancer rates are so much higher here than in Asia—yet, when Asians come over to the U.S. to start eating and living like Americans, their risk shoots right up. For example, women in Connecticut—way at the top of the breast cancer risk heap—in their fifties have, like, ten times more breast cancer than women in their fifties living in Japan. But, it’s not just genetic, since when they move here, their breast cancer rates go up generation after generation, as they assimilate into our culture.
Are the anti-estrogenic effects of soy foods enough to actually change the course of the disease? We didn’t know, until the first human study on soy food intake and breast cancer survival was published in 2009 in the Journal of the American Medical Association, suggesting that “[a]mong women with breast cancer, soy food consumption was significantly associated with decreased risk of death and [breast cancer] recurrence.” Followed by another study, and then another, all with similar findings.
That was enough for the American Cancer Society, who brought together a wide range of cancer experts to offer nutrition guidelines for cancer survivors, to conclude that, if anything, soy foods should be beneficial. Since then, two additional studies have been published, for a total of five, and they all point in the same direction. Five out of five, tracking more than 10,000 breast cancer patients.
Pooling all the results, soy food intake after breast cancer diagnosis was associated with reduced mortality (meaning a longer lifespan) and reduced recurrence—so, less likely the cancer comes back. Anyone who says otherwise hasn’t cracked a journal open in seven years.
And, this improved survival was for both women with estrogen receptor negative tumors and estrogen receptor positive tumors, and for both younger women, and for older women. Pass the edamame.
This is probably the same reason flax seeds are so protective. See Flax Seeds & Breast Cancer Survival: Epidemiological Evidence and Flax Seeds & Breast Cancer Survival: Clinical Evidence.
What about women who carry breast cancer genes? I touched on that in BRCA Breast Cancer Genes & Soy, and it’s the topic of my next video, Should Women at High Risk for Breast Cancer Avoid Soy?
What about genetically modified soy? I made a video abut that too; see GMO Soy & Breast Cancer.
Who Shouldn’t Eat Soy? Glad you asked. Watch that video too! 🙂
Not all phytoestrogens may be protective, though. See The Most Potent Phytoestrogen is in Beer and What are the Effects of the Hops Phytoestrogen in Beer?
Michael Greger, M.D.
Michael Greger, M.D., is a physician, New York Times bestselling author, and internationally recognized professional speaker on a number of important public health issues. Dr. Greger has lectured at the Conference on World Affairs, the National Institutes of Health, and the International Bird Flu Summit, testified before Congress, appeared on The Dr. Oz Show and The Colbert Report, and was invited as an expert witness in defense of Oprah Winfrey at the infamous “meat defamation” trial. Currently Dr. Greger proudly serves as the Director of Public Health and Animal Agriculture at the Humane Society of the United