KANSAS CITY — Consumption of soy protein or soy isoflavone may help reduce the risk of death and recurrence in women who have had breast cancer, according to new research published Dec. 9 in the Journal of the American Medical Association.
The findings run counter to a line of thinking that has suggested soy intake may increase the risk of breast cancer or worsen the condition of those already diagnosed with the disease because soy is known as a phytoestrogen, which may act like a weak form of estrogen in the body.
The study’s authors, led by Xiao Ou Shu, a professor of medicine and a cancer epidemiologist at Vanderbilt University Medical Center in Nashville, Tenn., found soy intake reduced the amount of naturally occurring estrogen and acted in a similar manner to tamoxifen, a drug that helps treat cancer by blocking the action of estrogen in the body.
"In our study, we found that soy food intake was associated with improved survival regardless of tamoxifen use, while tamoxifen use was related to improved survival only among women who had low or moderate levels of soy food intake," the researchers said. "Tamoxifen was not related to further improvement of survival rates among women who had the highest level of soy food intake. More importantly, women who had the highest level of soy food intake and who did not take tamoxifen had a lower risk of mortality and a lower recurrence rate than women who had the lowest level of soy food intake and used tamoxifen, suggesting that soy food intake and tamoxifen use may have a comparable effect on breast cancer outcomes."
As part of the study, the researchers examined 5,042 participants of the Shanghai Breast Cancer Survival Study. The participants included women aged 20 to 75 who were diagnosed as having primary breast cancer between March 2002 and April 2006 and were permanent residents of Shanghai, China.
The women provided information to researchers on a variety of factors, including cancer diagnosis and treatment, lifestyle factors and disease progression, at 6 months, 18 months, 36 months and 60 months after diagnosis. Soy food consumption did not vary by education level, number of pregnancies, tumor stage, hormone receptor status, ginseng use, income, or family history of breast or ovarian cancer.
The hazard ratio associated with the highest quartile of soy protein intake was 0.71 for total mortality and 0.68 for recurrence compared with the lowest quartile of intake. In other words, women with the highest intake of soy had a 29% reduced risk of death and a 32% reduced risk of cancer recurrence compared with those participants who ate less than 5.3 grams of soy per day.
"The association of soy food intake with mortality and recurrence appears to follow a linear dose-response pattern until soy food intake reached 11 grams per day of soy protein; no additional benefits on mortality and recurrence were observed with higher intakes of soy food," the researchers noted. "This study suggests that moderate soy food intake is safe and potentially beneficial for women with breast cancer."
The researchers cautioned that more studies need to be done to replicate the findings and to "disentangle whether the benefit of soy food consumption on breast cancer prognosis is the result of soy isoflavones or other soy constituents or is the result of a combination of the effects of multiple soy constituents."