The Soy & Breast Cancer Conundrum

By Victoria Shanta Retelny, R.D., "The Soy Conundrum,"November/December 2009

Does eating soyfood prevent or promote breast cancer? Or both?

Soy is touted as a food that can prevent breast cancer—and also implicated as one that might promote it. It all comes down to compounds in soy, called isoflavones, that act as weak estrogens in the body. Researchers still don’t know whether isoflavones spur the growth of tumors by acting like estrogen or prevent breast cancer by competing with the breast’s natural estrogen. Scientists who looked at the effect of individual isoflavones from soy on breast cancer cells in test tubes have found both results. Two recent studies, however, which looked at dietary habits, are helping scientists to better understand who might reap the greatest protection from soy.

A 2008 review in the British Journal of Cancer found that Asian women who ate about one serving of soy (e.g., 1/2 cup tofu or 1 cup soymilk) a day had a lower risk of breast cancer compared to those who ate less soy. Those who started eating soy regularly in their adolescent years reaped the greatest protection.

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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.”


Bottom Line: Adding soy to your diet in midlife might not offer much protection against breast cancer. But it probably won’t hurt either: soyfoods are a healthy, protein-rich, low-saturated-fat alternative to foods like red meat, says Sally Scroggs, M.S., R.D., at MD Anderson Cancer Center in Houston. Because theoretically soy isoflavones can act like estrogen, it’s best to eat soyfoods in moderation at any age—up to two servings daily, which is equivalent to 1⁄2 cup tofu or edamame and 1 cup soymilk.