The rap: “Researchers have linked soy to an early form of breast cancer.”
The real story: “When I started out in soy research 20 years ago, most researchers were convinced that soy was a great food for preventing cancer,” says Gertraud Maskarinec, a professor at the University of Hawaii Cancer Center.
That’s because women in Japan, China, and Singapore, where soy is a diet staple, had (and still have) lower rates of breast cancer than U.S. women.
That belief was shaken in 1996, when a pilot study suggested that soy protein and soy isoflavones—compounds in soybeans that are similar to, but much weaker than, estrogen—stimulated the breast to produce more abnormal cells, which could boost cancer risk.
“The study did an incredible amount of harm to women, because it was interpreted out of proportion to what it was capable of showing,” says Maskarinec.
The researchers looked at the breast fluid of 24 women who for six months ate no soy and for six months ate 37 grams a day of soy protein. (That’s what you’d get in about a pound of tofu or 1½ cups of shelled and cooked edamame.)
But studies that try to collect fluid from the breast are tricky. “There isn’t always enough, and some days there are cells in the fluid and some days there are none,” explains Maskarinec, whose 2013 study contradicted the earlier results.
“In our study, 82 women consumed a diet containing either two servings of soy foods each day or less than three servings a week, and they ate each diet for six months,” she notes.
More soy made no difference. “Less than half the women were able to provide enough fluid. In what we were able to collect, we found no indication that they had more aberrant cells when they consumed lots of soy foods.”
A better way to test whether soy promotes cancer, suggests Maskarinec, is to look at breast density. Women who have dense breasts—their breasts have an abundance of fibrous or glandular tissue and not much fat—have a higher risk of breast cancer.
“If you give hormone therapy—estrogen plus progestin—to postmenopausal women, within three months the density of their breast tissue goes up,” says Maskarinec. Not so with soy.
“We have done quite a few studies, and we have seen absolutely no change in breast density from consuming soy or isoflavones.”
Another alarm over soy and the breast: in 1998, researchers reported that genistein, a major soy isoflavone, stimulated the growth of estrogen receptor-positive (ER+) breast tumors in a special strain of mice.
But “a recent study showed that these mice metabolize isoflavones very differently than humans,” points out Mark Messina, of Loma Linda University in California. “So they may not be a suitable model for what happens in people.” (Messina, a leading authority on soy, consults for companies that make soy foods and supplements.)
Maskarinec’s bottom line: “The evidence shows that soy foods don’t seem to increase or decrease the risk of breast cancer in Western women.”
In 10 studies that tracked nearly 250,000 U.S. and European women for two to 13 years, for example, those who got the most soy isoflavones from food were no more —or less—likely to be diagnosed with breast cancer than those who got the least.
“On the other hand, soy foods do seem to help protect Asian women,” says Maskarinec. In four studies that tracked a total of more than 130,000 women in Japan, China, and Singapore for seven to 11 years, those who got the most isoflavones from their food were 24 percent less likely to be diagnosed with breast cancer than those who got the least.
Why the difference? For starters, the “most” isoflavones typically means only about 2 milligrams a day in the United States and Europe, but 25 to 50 mg a day in Asia. The “least” is also far higher there.
What’s more, “In Asian societies, girls grow up eating soy foods, and isoflavones may affect breast tissue early in life in a way that provides some protection later on,” says Maskarinec.
What about women with breast cancer? Could soy foods cause tumors to recur? That’s looking less and less likely.
In two studies that tracked 4,658 U.S. women with breast cancer for an average of seven years, those who got the most isoflavones (at least 10 mg a day) from soy foods had a 24 percent lower risk of breast cancer recurrence than those who got the least (less than 4 mg a day).
But the soy eaters in both studies were more health conscious, so something else about them could explain their lower risk.
Sources: Cancer Epidemiol. Biomarkers Prev. 5: 785, 1996; Nutr. Cancer 65: 1116, 2013; J. Nutr. 139: 981, 2009; Cancer Res. 58: 3833, 1998; Am. J. Clin. Nutr. 94: 1284, 2011; Breast Cancer Res. Treat. 125: 315, 2011; Am. J. Clin. Nutr. 96: 123, 2012.
Other relevant links:
- Can consuming soy help with these potential health problems? See: How to Diet: What are the Benefits of Soy?
- Should you avoid soy to protect your thyroid? See: How to Diet: Does Soy Affect Thyroid Function?
- Do estrogen-like chemicals, such as BPA, cause breast cancer? See: Food Safety: Can Environmental Estrogens Cause Breast Cancer?