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.