The debate over soy and its relation to breast cancer has been a long—and confusing—one. For many years, it was thought that soy might increase the risk of breast cancer, due mainly to its concentration of isoflavones, naturally occurring chemicals that mimic the effects of estrogen in the body. Research never bore out this supposition, however, and most experts agreed that eating moderate amounts of soy foods posed no risk to women, even breast cancer survivors.
Now, new research from an international team co-directed by scientists at the Johns Hopkins Kimmel Cancer Center has found that not only are soy isoflavones relatively harmless, but they may actually reduce the risk of breast cancer recurrence among women who’ve already had the disease. The study was published in the February 2024 issue of JNCI Cancer Spectrum.
Scientists in Australia, Denmark, England, Norway, and the U.S. reviewed 22 published observational studies that examined the impact of diet on breast cancer recurrence and mortality, as well as all-cause mortality.
The studies specifically focused on the effects of a variety of foods thought to contain health benefits, including soybeans, lignans (compounds found in a variety of plants including seeds and nuts), cruciferous vegetables, and green tea. This included 11 studies of soy isoflavones, three of cruciferous vegetables, two of green tea, three of lignans, and three of enterolactone, which is a compound formed in the gut when lignans are digested.
According to the Johns Hopkins team’s meta-analysis of six studies following 11,837 breast cancer survivors, soy isoflavones were associated with a 26 percent reduced risk of recurrence. The results were most notable among post-menopausal survivors, and the greatest risk reduction was seen at 60 milligrams of isoflavones per day. This is the equivalent of two to three servings of soy per day, with one serving being a cup of soy milk, three ounces of tofu, or a half-cup of cooked soybeans. The effect of soy consumption on mortality risk, however, was statistically insignificant.
In addition, the meta-analysis found that:
- Consumption of green tea reduces the risk of breast cancer recurrence by 44 percent in women with stage I or II breast cancer. The greatest effect was seen from consuming three to five cups per day.
- Enterolactone reduced the risk of breast cancer-specific mortality by 28 percent, and death from any cause by 31 percent, particularly in post-menopausal women (35 percent reduction in death from any cause).
It is not possible to calculate the effective dose of lignans in the diet from these enterolactone findings because the gut microbiome that plays a role in lignan metabolism varies among individuals. That said, the findings themselves are significant. “These findings were graded probable, which means there is strong research showing that they contributed to the results we are seeing,” says lead study author Diana van Die, PhD, of NICM Health Research Institute at Western Sydney University, Australia.
While the study highlighted several promising avenues for further study, the researchers cautioned against reading too much into the results.
“It is critically important to stress that these studies were conducted on women who received medical and/or surgical treatment for breast cancer, and that these foods and phytonutrients should not be considered as alternatives to treatment,” says senior study author Channing Paller, MD, an associate professor of oncology at Johns Hopkins.
“This research highlights the need for more robust studies in this area looking at the most effective dosages of these compounds, and whether starting to consume them after diagnosis has the same effect as a lifelong dietary habit before diagnosis. This is what patients are looking for,” Paller added.