Georgetown Lombardi Comprehensive Cancer Center researchers have used animal models to reveal new information about the impact — positive and negative — that soy consumption could have on a common breast cancer treatment.
The scientists have uncovered the biological pathways in rats by which longtime soy consumption improves effectiveness of tamoxifen and reduces breast cancer recurrence. But they also show why eating or drinking soy-based foods for the first time while being treated with tamoxifen can, conversely, reduce effectiveness of the drug, and promote recurrence.
The study, published in Clinical Cancer Research, uncovers the molecular biology behind how soy consumption, especially its most active isoflavone, genistein, affects tamoxifen — both positively and negatively.
It also mirrors what has been observed in breast cancer patients, says the study’s senior investigator Leena Hilakivi-Clarke, PhD, professor of oncology at Georgetown Lombardi.
“There has long been a paradox concerning genistein, which has the similar structure as estrogen and activates both human estrogen receptors to a degree. Estrogen drives most breast cancer growth, yet high soy intake among women in Asian countries has been linked to a breast cancer rate that is five times lower than Western women, who eat much less soy,” she says. “So why is soy, which mimics estrogen, protective in Asian women?”
More than 70 percent of the 1.67 million women diagnosed with breast cancer worldwide in 2012 was estrogen-receptor positive, and tamoxifen and other endocrine therapies meant to reduce the ability of estrogen to promote cancer growth, are the most common drugs used for these cancers. Although endocrine therapies can be highly effective in preventing or treating breast cancer, about half of patients who use them exhibit resistance and/or have cancer recurrence.
Dr. Stegall’s Comments: This research is very important, as soy consumption has been debated for decades. In general, soy is thought to be estrogenic but certain soy derivatives such as genistein have been shown to have an anti-cancer effect. I do not typically recommend dietary soy consumption in my patients, but I do frequently include a genistein supplement in my protocols if sensitivity testing shows it to be effective against a patient’s circulating tumor cells.