New research indicates that dietary soy products are safe and even beneficial for women diagnosed with breast cancer. Published early online in CANCER, a peer-reviewed journal of the American Cancer Society, the findings may help resolve controversies over soy’s potential link to breast cancer outcomes.
Soy foods are considered among the healthiest for human consumption, but their estrogen-like properties have raised concerns of a potential increased risk of breast cancer. This is because in hormone receptor-positive cancer, the most common form of the disease, there are some concerns that high estrogen levels help cancer cells grow and spread, though this remains controversial.
“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; however, other research has suggested that the estrogen-like effects of isoflavones may reduce the effectiveness of hormone therapies used to treat breast cancer,” explained Fang Fang Zhang, MD, PhD, of the Friedman School of Nutrition Science and Policy at Tufts University. “Because of this disparity, it remains unknown whether isoflavone consumption should be encouraged or avoided for breast cancer patients.”
To provide some clarity, Dr. Zhang and her colleagues looked at the relationship between dietary intake of isoflavones and death from any cause in 6235 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 largely confined to women with hormone receptor-negative tumors and women who were not treated with anti-estrogen 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.