How soy plays a role in breast cancer risk and recurrence is one of the most common questions we get asked. A large body of human research suggests eating tofu, soy milk and other soy foods in moderation safe. Now an animal study that may help explain what is seen in human research, shows that eating soy foods when young boosts the immune response against tumors, reducing cancer recurrence.
The study is being presented at the American Association for Cancer Research (AACR) annual meeting, and is not yet published.
Soy contains compounds called isoflavones that mimic the effect of estrogen. This raised concerns that it would stimulate breast tumors fueled by estrogen and may interfere with anti-estrogen treatment, such as tamoxifen. Early animal studies did find a link between isoflavones increasing risk of breast cancer. According to the news release, one reason may be that this early animal research used animals that do not have certain immune cells called cytotoxic T cells. These are among the cells that act against breast tumors.
A previous study by these researchers found that rats consumed genistein throughout their life responded better to anti-estrogen treatment than did control rats. They also had reduced risk of cancer recurrence.
In this study, the researchers found that rats fed genistein before puberty had activated T cell immune response before they started treatment with tamoxifen, along with other beneficial changes in their immune system.
“Our results suggest that genistein’s ability to activate anti-tumor immune responses and reduce expression of immunosuppressive mechanisms may explain why lifetime genistein intake reduces risk of breast cancer recurrence,” says Leena Hilakivi-Clarke, PhD, professor of oncology at Georgetown Lombardi in a press release.
Some human research points to soy’s protective role for breast cancer survivors. Among Asians — who eat soy foods throughout life — postmenopausal women who ate the most soy foods (about two to three servings a day) had the lowest risk of recurrence.
Hilakivi-Clarke is currently conducting an AICR-supported related study on soy and breast cancer recurrence.
Soy contains numerous healthy compounds that can be an important part of a plant-based cancer-protective diet.
AICR does not recommend relying on supplements to protect against cancer. Research on soy showing no increased risk of breast cancer for women — and possibly offering protection — is based on soy foods, not supplements. For breast cancer patients, population studies suggest there is no harmful effect of soy foods – not supplements – and anti-estrogen medications.
To read the research and what a moderate amount of soy is, visit our Foods that Fight Cancer: Soy.
The study was funded by National Cancer Institute and philanthropic support from Sara and Gabriel Solomon.