In the past, physicians have often warned breast cancer patients not to eat soy. The new research represents "a complete turnaround" from the previous understanding about the link between soy consumption and breast cancer, says Sally Scroggs, a registered dietician and senior health education specialist at M.D. Anderson's Cancer Prevention Center in Houston.
"We have gone from saying, 'No soy for breast cancer survivors' to, 'It's not going to hurt,'" Scroggs says. "Now it looks like we can say, 'It may help.'"
The study followed more than 5,000 women in China who had undergone a mastectomy for about four years. The women who consumed the most soy protein (about 15 grams or more a day) had a 29% lower risk of dying and a 32% decreased risk of breast cancer recurrence compared to the women who consumed less than about 5 grams of soy protein a day, according to the study, which appears in the December 9 issue of the Journal of the American Medical Association. The National Cancer Institute and the U.S. Department of Defense's Breast Cancer Research Program funded the study.
Women who ate between 9.5 and 15 grams of soy protein saw nearly the same decrease in risk as the women who ate more than 15 grams. In fact, the researchers found no additional benefits to eating more than 11 grams of soy protein a day. (An 8-ounce glass of soy milk and a cup of shelled edamame contain about 7 and 14 grams of soy protein, respectively.)
In all, 534 women had a breast cancer recurrence or died from breast cancer during the study period.
Soy foods—such as milk, tofu, and edamame—are rich in naturally occurring estrogens (especially isoflavones) that can mimic the effects of estrogen in the female body. Because the most common types of breast cancer depend on estrogen to grow, experts once feared that soy isoflavones could stimulate the estrogen receptors in breast-cancer cells, even though the estrogens in soy are much weaker than those produced by the body.
The current study suggests the exact opposite: Soy may actually reduce the amount of estrogen that's available to the body.
"Soy isoflavones may compete with estrogens produced by the body. Soy isoflavones may also reduce the body's production of estrogen, and increase clearance of these hormones from the circulation—all of which together reduce the overall amount of estrogen in the body," says the lead author of the study, Xiao Ou Shu, MD, PhD, a cancer epidemiologist at the Vanderbilt-Ingram Cancer Center of Vanderbilt University Medical Center in Nashville, Tenn.
Dr. Shu says, however, that factors beyond estrogen may be at work. Other components of soy foods, such as folate, protein, calcium, or fiber (or some combination thereof) may also be responsible for the health benefits reported in the study, she says.