Similarly, in an April 2009 American Journal of Clinical Nutrition study of nearly 74,000 Chinese women, age 40 to 70, those who consumed a daily serving or more of soy had a significantly lower risk of developing breast cancer in their premenopausal years than women who ate soy less frequently. Also in this study, the women who started eating soy consistently in adolescence had an even lower risk than those who started later. “This study suggests that high soyfood intake may reduce the risk of breast cancer,” says Wei Zheng, M.D., Ph.D., professor and chief, Division of Epidemiology at Vanderbilt University and an author of the study. However, in this study eating soyfoods did not protect women from developing breast cancer after menopause.
Studies are conflicting about the benefits of soyfood consumption later in life. Researchers hypothesize that in younger women, when the body’s estrogen levels are high, isoflavones in soy may compete with the body’s natural estrogen and reduce risk of breast cancer. After menopause, however, natural estrogen levels are much lower and so it’s thought that the isoflavones act like estrogen. Higher estrogen levels are linked with higher risk for breast cancer. That doesn’t mean that eating soyfoods like tofu and edamame—in moderation—after menopause is unsafe, says Zheng. “No data show that eating soyfoods increases breast-cancer risk in postmenopausal women.”