The question

Women with breast cancer sometimes shy away from soy, which contains substances that mimic the effects of estrogen, a hormone known to fuel the growth of many cancers. They fear that soy consumption could make a fatal cancer recurrence more likely. Is that the case?

This study

The researchers analyzed data on 6,235 women (average age, 52) who had been diagnosed with breast cancer for the first time. On average, the women consumed about 2 milligrams of soy isoflavones daily.

Over about 9½ years, 1,224 of the women died. Those whose soy intake was the highest were 21 percent less likely to have died than were women whose diet included the least soy.

The benefit was most pronounced in women whose breast cancer was hormone-receptor negative (meaning that the cancer cells did not have receptors that estrogen and progesterone attach to fueling their growth) and among those who had not been treated with anti-estrogen therapy. High soy intake did not increase mortality risk among women who had gotten hormonal therapy. Mortality risks overall were essentially the same regardless of race or ethnicity.

Who may be affected?

Women with breast cancer. Soy contains isoflavones, which are phytoestrogens (plant estrogens) that have qualities similar to estrogen. Common dietary sources of soy include tofu, miso, edamame, soy milk and soy sauce.


Information on the women’s diets and breast cancer treatments came from their responses on questionnaires.

Dietary data included only food sources of soy, not supplements. Mortality data covered deaths from any cause, not just breast cancer. Some older studies had suggested an increased risk for cancer growth spurred by soy consumption.

Find this study

Online in Cancer ­(canceronline.­wiley.com; search for “dietary isoflavone”).

Learn more about

Information on breast cancer is available at cancer.gov. Learn more about soy and breast cancer at ww5.komen.org (search for “soy and breast cancer”).

The research described in Quick Study comes from credible, peer-reviewed journals.