Is Soy Safe to Eat After Breast Cancer?

April 5, 2011 -- For years, breast cancer survivors were often counseled to avoid soy foods and supplements because of estrogen-like effects that might theoretically cause breast tumors to grow.

Now, a new study of more than 18,312 women shows that eating soy foods did not increase risk of breast cancer recurrence.

The new findings are being presented at the American Association for Cancer Research (AACR) 102nd Annual Meeting in Orlando, Fla.

“If you regularly eat soy, you don’t need to worry or avoid it, and women who want to lead a healthy life, can safely include some soy in their diets,” says study researcher Xiao Oh Shu, MD, PhD, a professor of medicine at Vanderbilt Epidemiology Center of Vanderbilt University Medical Center in Nashville, Tenn.

In addition to the isoflavones which may act like estrogens in the body, “soy has many anticancer properties, antioxidants, nutrients, micronutrients, or vitamins that may contribute to its beneficial effect on health,” Shu says.

Shu and colleagues analyzed data from four large studies of women with a history of breast cancer diagnosed between ages 20 and 83. Soy intake was assessed using questionnaires in all of these studies. The study only looked at soy foods, not supplements.

They found that the odds of breast cancer recurrence were not increased among women who consumed the highest amount of soy in their diet, compared with women who ate less soy foods. Women who were from Shanghai, China, where one of the studies took place, consumed more soy than American women.

Checking for Risk of Breast Cancer Recurrence

After an average of nine years after their breast cancer diagnosis, women who consumed the highest amount of soy, or more than 23 milligrams of soy per day, had a 9% lower risk of dying from any cause and a 15% reduced risk for breast cancer recurrence, compared to women who consumed 0.48 milligrams of soy per day or less.

These results were not considered statistically significant and therefore may be due to chance.

According to Shu, 23 milligrams per day of soy consumed is the equivalent of one glass of soy milk or a half cup of tofu.

Leif Ellisen, MD, PhD, of the Gillette Center for Breast Cancer at the Massachusetts General Hospital Cancer Center in Boston, says women often ask him whether soy is safe after breast cancer.

“There are a lot of health benefits in soy, but there has been some theoretical concern that soy has molecules that resemble estrogen which may increase risk of breast cancer recurrence,” he says.

But “rather than any negative effect, this study suggested a benefit for these patients in terms of overall health and breast cancer recurrence,” he says. “This is quite reassuring for women who were concerned that they might have to eliminate healthy soy foods from their diets.”

Ellisen says the news will affect how he counsels patients. “I used to say ‘the potential negative effects are only theoretical,’ but now I am much more likely to say ‘the good evidence suggests that if anything soy may be beneficial,’” he says. “This is a strong study and it gives us a lot of support to say you should not be eliminating soy and if anything, you could increase soy in your diet.”