How Does Soy Consumption Impact Breast Cancer Outcomes?

A new study published by Cancer shows that consumption of isoflavones may reduce all-cause mortality among women with breast cancer. Isoflavones are estrogen-like compounds that are most commonly found in soy products.

In the analysis, which included 6235 patients, the authors discovered that intake of isoflavones was linked to a 21% decrease in mortality. These findings were only observed among patients with hormone-receptor-negative cancers, and in those not treated with endocrine therapy, such as tamoxifen.

"At the population level, we see an association between isoflavone consumption and reduced risk of death in certain groups of women with breast cancer,” said lead researcher Fang Fang Zhang, MD, PhD. “Our results suggest, in specific circumstances, there may be a potential benefit to eating more soy foods as part of an overall healthy diet and lifestyle.”

Due to the preliminary nature of these findings, the authors are uncertain of the overall effect of isoflavone supplements.

"Since we only examined naturally occurring dietary isoflavone, we do not know the effect of isoflavone from supplements,” Dr Zhang said. “We recommend that readers keep in mind that soy foods can potentially have an impact, but only as a component of an overall healthy diet.”

In past studies, isoflavones were observed to slow the growth of breast cancer cells in cultures. Human studies have indicated that East Asian women with increased isoflavone consumption had reduced mortality.

However, some studies have suggested that the estrogen-mimicking isoflavones can impact the efficacy of endocrine therapy, which has led to uncertainty in whether it should be recommended or avoided by patients with breast cancer.

In the current study, patients were sorted into 4 groups based on isoflavone consumption, which was self-reported in questionnaires. Patients in the highest cohort consumed at least 1.5 milligrams of isoflavones per day. Patients were followed up with for a median of 9.4 years.

Compared with the cohort with the lowest intake of isoflavones, patients with the highest intake were observed to have a 21% reduction in all-cause mortality. This difference was most pronounced among patients without estrogen or progesterone receptors, according to the study.

The link was weaker, but significant among those who did not receive endocrine therapy. The authors did not discover any association for patients with hormone-receptor positive cancers or those treated with endocrine therapy.

The researchers caution that the individuals may have underestimated isoflavone intake when completing the questionnaires.

"The comparisons between high and low consumption in our study are valid, but our findings should not be interpreted as a prescription," Dr Zhang said. "However, 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. Especially for women with hormone-receptor-negative breast cancer, soy food products may potentially have a beneficial effect and increase survival."

Due to the diverse group of patients included, investigators were able to account for various factors and observe risk among various subtypes of breast cancers.

There was a correlation between isoflavone intake and socioeconomic and lifestyle factors, which could impact mortality. Asian Americans, young, physically active, not overweight, educated, non-smoker, non-drinkers were more likely to consume high levels of isoflavones, according to the study.

Additional studies should be conducted before recommending that patients with certain subtypes of cancer increase intake of or avoid isoflavones.

"Whether lifestyle factors can improve survival after diagnosis is an important question for women diagnosed with hormone-receptor negative breast cancer, a more aggressive type of breast cancer,” concluded researcher Esther John, PhD. “Our findings suggest that survival may be better in patients with a higher consumption of isoflavones from soy food.”