Soy has long been a concern for women who have breast-cancer risk factors or have had the disease. The concern was due to a natural compound found in soybeans called soy isoflavones, which have a weak estrogen-like effect, says Dr. Xiao-Ou Shu, a professor and cancer epidemiologist at Vanderbilt-Ingram Cancer Center in Nashville, Tenn. Estrogen has been thought to promote the growth of breast-cancer cells. While it is true that soy isoflavones do have ‘some estrogen-like effect,’ says Shu, ‘it is much weaker than the actual hormone in the body.’
That’s a good thing, but what’s really exciting is that the latest and largest study to date in this area has actually linked eating soy to a reduced risk of breast-cancer recurrence. The study, published in The American Journal of Clinical Nutrition, followed nearly 10,000 breast-cancer survivors from the U.S. and China for almost eight years. After monitoring the diets of these women, the researchers found that those who consumed at least 10 mg of soy isoflavones a day had a 25 percent lower risk of breast-cancer recurrence than those who ate less than four mg per day.
Experts have also previously worried that eating soy foods might interfere with the effectiveness of tamoxifen, a drug used to reduce the recurrence of breast cancer in women. The results of this study, however, showed that tamoxifen users who regularly consumed higher amounts of soy isoflavones had a lower risk of recurrence than women who did not take the drug and consumed the lowest amount of soy isoflavones.
‘What needs to be made clear is that this study was done based on dietary soy,’ says Shu. ‘There have been no studies done on whether the effect is the same from soy isoflavone supplements.’