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Last updated on April 18, 2014 at 1:21 EDT

Soy May Benefit Breast Cancer Survivors

December 9, 2009

Experts have found that women in China who had breast cancer and a higher intake of soy food had an associated lower risk of death and breast cancer recurrence.

Xiao Ou Shu of Vanderbilt University Medical Center and colleagues examined the association between soy isoflavone intake with breast cancer recurrence and survival.

They found that soy foods are rich in isoflavones, a major group of phytoestrogens that have been hypothesized to reduce the risk of breast cancer.

However, the estrogen-like effect of isoflavones and the potential interaction between isoflavones and tamoxifen have led to concern about soy food consumption among breast cancer patients, the authors wrote.

But since so few American women eat soy-based products, Shu decided to study soy in more than 5,000 breast cancer survivors in China, where soy is a staple.

The researchers divided the women into four groups based on how much soy they ate. Shu said those women with low soy levels consumed an average of about half a cup of soy milk a day, while the high-soy group had about three cups a day.

The study, published today in The Journal of the American Medical Association, showed that after four years, 7.4 percent of those who ate the most soy had died, compared with 10.3 percent of those who ate the least soy.

Walter Willett, chairman of the nutrition department at the Harvard School of Public Health, said those results suggest that it’s probably safe for breast cancer survivors and other women to have one or two servings of soy foods a day, but he cautions against women taking large doses of genistein pills or other soy supplements, which are unproven.

While studies in the past have shown that genistein, an estrogen-like compound in soy, promotes the growth of breast tumor cells in lab dishes and animals, Shu says the latest study suggests that women benefited from eating whole soy, possibly because other nutrients from the plant act together in healthy ways.

Scientists are still trying to understand all of soy’s hormonal effects, as it is possible that soy acts like the breast cancer drug tamoxifen, which blocks the effects of estrogen, according to Willett.

Claudine Isaacs of Georgetown’s Lombardi Comprehensive Cancer Center believes American patients may respond to soy very differently from those in China.

Shu says research suggests that prenatal and childhood exposure to hormones and other chemicals can program how the body responds to them.

Studies have shown soy is most effective at preventing cancer in women who are exposed to it during and after adolescence.

Therefore, Isaac says it is possible that a middle-aged American woman who adds soy to her diet for the first time may not reap the same benefits as Chinese women who grow up with it.

Willett believes it makes sense not to go to extremes.

“Having some soy in the diet is probably a good thing, especially if it replaces red meat, but there’s still good reason not to go overboard,” he said.

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