A diet high in certain soy compounds, called soy isoflavones, may reduce the risk of breast cancer recurrence among survivors of some types of breast cancer, according to new research.

Researchers found post-menopausal breast cancer survivors in the study who consumed the most soy isoflavones, around 42.3 milligrams a day, had a considerably decreased risk of recurrence of their breast cancer compared with those who consumed the least, around 15.2 mg a day.

The link between soy compounds  and breast cancer risk is not completely understood. Research has shown the plant compounds may affect estrogen levels in the body, but other work has yielded conflicting results – some research seems to show a protective effect against breast cancer, while other work finds the compounds may increase some women's risk of the disease.

Reduction in cancer's recurrence

The new study began in 2002 with 524 patients, ages 29 to 72, who had been diagnosed with early or locally advanced breast cancer and who were receiving hormone therapy after surgery.

All of the women had breast cancer that was positive for estrogen receptors (meaning their tumors that grew in response to estrogen), progesterone receptors (the cancer cells were fed by progesterone) or both, according to researchers from Harbin Medical University in China.

The patients were being treated with either tamoxifen or anastrozole, two drugs designed to stall tumor growth by affecting the body's estrogen levels. They were also asked to fill out a questionnaire that included questions about their consumption of soy products, including tofu, soymilk, soybean sprouts and soy flour.

The researchers found the recurrence rate was 12.9 percent lower among patients whose diets were highest in soy isoflavones than those whose diets were lowest. Among post-menopausal women taking anastrozole, the effect was most pronounced — the recurrence rate was 18.7 percent lower in women consuming the most soy, compared with those consuming the least.

What the results mean for American women

Compared to the Asian population, for which soy protein is a diet staple, only 37 percent of Americans eat soy foods and beverages at least once a month or more, according to the 2010 Consumer Attitudes about Nutrition report by the United Soybean Board.

More than 12 percent of women born today will be diagnosed with breast cancer at some point in their lives, according to the National Cancer Institute.

Previous studies have examined the influence of soy isoflavones intake on breast cancer patients, however, little is known about their effects on those taking hormone-based therapies, researchers said.

Though several findings have found soy consumption beneficial to humans, some research indicates one soy isoflavone, called genistein, can block the effects of tamoxifen and promote the growth of estrogen receptor positive cells in mice.

While the study is consistent with other findings regarding the effects of high intakes of plant compounds on post-menopausal, hormone-sensitive breast cancer patients, further studies are needed to confirm results, and the effect should be studied in other populations, the study said.

The study was published online today (Oct. 18) in the Canadian Medical Association Journal.