Rather than protecting against all breast cancers, high levels of soy food consumption appears to specifically reduce the risk of estrogen receptor (ER)-positive tumors and human epidermal growth factor receptor 2 (HER2)-negative tumors, Japanese researchers report in the International Journal of Cancer.

Dr. Takeshi Suzuki, at Aichi Cancer Center Research Institute in Nagoya, and associates conducted a study of 678 women with breast cancer and 3,390 controls matched by age and menopausal status with no history of cancer.

The researchers “observed a significantly reduced risk” of breast cancer among the women who ate the most soy and were ER-positive, HER2-negative or both.

For women who ate the most soy compared with those who ate the least amount the odds of having ER-positive breast cancer were reduced by 26 percent and for women with HER2-negative breast cancer, the reduced risk was 22 percent.

Soy intake was not significantly associated with HER2-positive or ER-negative tumors, or with the presence or absence of the progesterone receptor (PR).

However, Suzuki’s team found that “when the three receptors were jointly examined, a reduced risk was observed only for patients with ER-positive/PR-positive/HER2-negative tumors,” with a 27-percent reduction seen only in the women who ate the most soy.

“These findings are biologically plausible, and suggest a potential benefit of soybean products in the prevention of breast cancer,” the investigators conclude.

SOURCE: International Journal of Cancer, October 2008.