Eating soy foods has a protective effect in both women with hormone receptor-negative breast cancer and in women with the cancer who are not receiving hormone therapy, according to a study.

The research, which appears to resolve a long-running concern that soy consumption may not be good for women with breast cancer, also showed that soy does not increase the risk of death among breast cancer patients receiving anti-estrogen therapy.

The study, “Dietary isoflavone intake and all-cause mortality in breast cancer survivors: The Breast Cancer Family Registry,” was published in the journal Cancer.

While soy foods are healthy, many scientists have had concerns about women with hormone-receptor breast cancer consuming them.

“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 epidemiological analyses in East Asian women with breast cancer found links between higher isoflavone intake and reduced mortality,” Fang Fang Zhang, MD, PhD, of the Friedman School of Nutrition Science and Policy at Tufts University, said in a press release.

But “other research has suggested that the estrogen-like effects of isoflavones may reduce the effectiveness of hormone therapies used to treat breast cancer,” he said. “Because of this disparity, it remains unknown whether isoflavone consumption should be encouraged or avoided for breast cancer patients.”

To understand whether eating isoflavones increases breast cancer patients’ risk of death, Zhang and colleagues examined 6,235 women listed in the Breast Cancer Family Registry. The researchers asked them to fill out a Food Frequency Questionnaire, then followed them for a median of 9.4 years to see how many would die of all causes.

Women who consumed high amounts of isoflavones had a 21 percent lower risk of dying of all causes than women who consumed low amounts.

The risk of death was much lower in women with hormone receptor-negative tumors — 51 percent — and women who were not treated with an anti-estrogen therapy like tamoxifen — 32 percent.

Importantly, the study revealed that women receiving anti-estrogen therapy did not benefit from eating isoflavones, but they also were not at increased risk of death.

“Based on our results, we do not see a detrimental effect of soy food intake among women who were treated with endocrine therapy,” Zhang said. “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.”

Although scientists have yet to understand how isoflavones interact with breast cancer cells, researchers believe soy products’ antioxidant, anti-inflammatory, and anti-angiogenic effects may be what protect breast cancer patients.

Omer Kucuk, MD, of Emory University’s Winship Cancer Institute, wrote in an accompanying editorial that because of the “evidence that soy foods not only prevent breast cancer but also benefit women who have breast cancer,” women should “consume soy foods because of soy’s many health benefits.”