In contrast to some previous research, high levels of isoflavone intake were not associated with greater mortality among women receiving hormonal therapy.

"Based on our results, we do not see a detrimental effect of soy intake among women who were treated with endocrine therapy, which has been hypothesized to be a concern," said Zhang.

"For women with hormone-receptor-negative breast cancer, soy food products may potentially have a beneficial effect and increase survival. Women who did not receive endocrine therapy as a treatment for their breast cancer had a weaker, but still statistically significant, association."

The investigators noted that they examined only naturally occurring dietary isoflavones, not isoflavones from supplements.

How isoflavones from foods interact with breast cancer cells is unclear, but research has shown that they have antioxidant, anti-inflammatory, anti-angiogenic, and other effects that could influence tumor survival and growth.

In an accompanying editorial, Omer Kucuk of the Winship Cancer Institute of Emory University, said that they now have evidence 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," he wrote.