Previous studies have suggested that the oestrogen-like properties of soy isoflavones may reduce the effectiveness of hormone therapies used to treat breast cancer, while others, including epidemiological analysis in East Asian women, found links between higher isoflavone intake and reduced mortality.
“Because of this disparity, it remains unknown whether isoflavone consumption should be encouraged or avoided for breast cancer patients,” said Fang Fang Zhang, of the Friedman School of Nutrition Science and Policy at Tufts University.
Therefore, 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 tumours and women who were not treated with anti-estrogen therapy such as tamoxifen (which blocks the effects of oestrogen).
In contrast to some previous research, high levels of isoflavone intake were not associated with greater mortality among women receiving hormonal therapy.
The investigators noted that they examined only naturally occurring dietary isoflavones, not isoflavones from supplements.
“Based on our results, we do not see a detrimental effect of soy food intake among women who were treated with endocrine therapy,” said Dr. 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.”
More than 20 percent of all new breast cancer cases with known estrogen and progesterone receptor status are receptor-negative, and they have poorer survival rates than hormone receptor-positive cases.
“Whether lifestyle factors can improve survival after diagnosis is an important question for women diagnosed with this more aggressive type of breast cancer. Our findings suggest that survival may be better in patients with a higher consumption of isoflavones,” added senior author Esther John, of the Cancer Prevention Institute of California.
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 tumour survival and growth.
This latest research was published in the journal Cancer.