Soy foods are not only safe for women diagnosed with breast cancer, but may even help some patients live longer, a new study said Monday.
It has long been controversial as to whether women diagnosed with breast cancer should be advised to eat more or less soy foods, especially for those who receive hormone therapies as part of cancer treatment.
That's because there are some concerns that their estrogen-like properties may help cancer cells grow and spread and make hormone therapies less effective.
But in laboratory studies, isoflavones, the component of soy that has estrogen-like properties, have been shown to slow the growth of breast cancer cells, and epidemiological analyses in East Asian women with breast cancer found links between higher isoflavone intake and reduced mortality.
To provide some clarity, Fang Fang Zhang of the Friedman School of Nutrition Science and Policy at Tufts University and her colleagues looked at the relationship between dietary intake of isoflavones and death from any cause in 6,235 American and Canadian women diagnosed with breast cancer.
Over a median follow-up of nine years, women with breast cancer who consumed high amounts of isoflavones had a 21 percent lower risk of dying than women who consumed low amounts.
This decrease was seen only in women with hormone-receptor-negative tumors, and in women who were not treated with endocrine therapy such as tamoxifen, which blocks the effects of estrogen.
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.
