Scientist Untangle the Soy-Breast Cancer Paradox

To eat soy or not: That's the question many U.S. women have been asking. Tofu, miso paste and other soybean-based foods are high-quality sources of protein that are low in calories and saturated fat. And studies have shown that they can help prevent cancer.

Yet many doctors recommend that women who have, or are at risk of developing, a common form of breast cancer called estrogen-receptor-positive breast cancer avoid eating soybean-based foods because they contain compounds called isoflavones. Some studies suggest that isoflavones can mimic the hormone estrogen and encourage tumor growth.

Now, in an animal study, researchers at the Georgetown Lombardi Comprehensive Cancer Center in Washington, D.C., have uncovered a possible reason for the apparent Jekyll-and-Hyde nature of soy - how it can both prevent cancer and fuel its spread.

The researchers found that rats that were given soybean isoflavones to eat throughout their lives - in particular, one type of soybean isoflavone called genistein - had improved immunity against cancer. But rats that weren't given the isoflavone until after developing breast cancer didn't have that same immune response to kill cancer cells. Instead, these rats had higher rates of cancer growth and higher rates of recurrence after their tumors were removed.