Eating Soy Foods May Prolong Survival in Breast Cancer Patients

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Some women with breast cancer may survive longer if they consume soy-based foods, according to a new study.

A decrease in all-cause mortality was seen in women with hormone receptor–negative tumors, and in those women who did not receive hormone therapy, such as tamoxifen, as part of their cancer treatment.

The researchers published their results in Cancer.

The estrogen-like properties of soy foods have raised concerns about a potential increased risk of breast cancer, and questions remain about advising women to avoid or increase their intake of soy foods to reduce breast cancer risk or progression.   

“Isoflavones-the component of soy that has estrogen-like properties-have been shown to slow the growth of breast cancer cells in laboratory studies, and epidemiologic analyses in East Asian women with breast cancer found links between higher isoflavone intake and reduced mortality; however, other research has suggested that the estrogen-like effects of isoflavones may reduce the effectiveness of hormone therapies used to treat breast cancer,” said lead author Fang Fang Zhang, MD, PhD, of the Friedman School of Nutrition Science and Policy at Tufts University in Boston.

Zhang and colleagues examined the association of dietary intake of isoflavone with all-cause mortality in an ethnically diverse cohort of 6,235 women with breast cancer enrolled in the Breast Cancer Family Registry. The women completed a food frequency questionnaire, including 5,178 women who reported pre-diagnosis diet and 1,664 women who reported post-diagnosis diet.

Over a median follow-up of 9.4 years, the researchers documented 1,224 deaths and observed a 21% decrease in all-cause mortality for women in the highest quartile vs lowest quartile of dietary isoflavone intake (hazard ratio [HR], 0.79; 95% CI, 0.64–0.97; P for trend = .01).

Lower mortality associated with higher intake was limited to women who had tumors that were hormone-receptor negative (HR, 0.49; 95% CI, 0.29–0.83; P for trend = .005) and those who did not receive hormone therapy for their breast cancer (HR, 0.68; 95% CI, 0.51–0.91; P for trend = .02).

“Based on our results, we do not see a detrimental effect of soy food intake among women who were treated with endocrine therapy,” said Zhang. “For women with hormone receptor–negative breast cancer, soy food products may potentially have a protective effect. Women who did not receive endocrine therapy as a treatment for their breast cancer had a weaker, but still statistically significant, association.”

The findings suggest that survival may be better in patients with a higher consumption of isoflavones. The researchers noted that they examined only naturally occurring dietary isoflavones, not isoflavones from supplements.

Also, the study categorized women in the highest quartile as those who consumed 1.5 milligrams or more of isoflavone per day-equivalent to a few dried soybeans. The researchers noted that individuals tend to underestimate their food intake when filling out questionnaires.

In an accompanying editorial, Omer Kucuk, MD, of the Winship Cancer Institute of Emory University in Atlanta, stated: “We now have evidence that soy foods not only prevent breast cancer but also benefit women who have breast cancer. Therefore, we can recommend women to consume soy foods because of soy’s many health benefits.”