Now, a new study published in the JNCI: Journal of the National Cancer Institute directly compared women who took soy supplements with women who took a placebo to determine what effect soy would have on the expression of genes associated with breast cancer. The researchers looked at 140 patients who had been recently diagnosed with invasive breast cancer. In the 2-3 weeks before their surgery (either a mastectomy or lumpectomy), these women were given either soy supplements (the equivalent of about four 16 oz. glasses of soy milk or four large servings of tofu) or a placebo every day. “The hypothesis was that soy food for a limited period of time could influence the behavior of already established breast cancer,” says study co-author Jacqueline Bromberg, MD, PhD, breast medical oncologist and researcher at Memorial Sloan Kettering. MORE: New Study Shows Diet is Crucial After Breast Cancer Diagnosis After a few weeks, they found that some women in the soy group had high levels of genistein, a metabolite of soy, while the women in the placebo group did not. And interestingly, patients with high levels of this metabolite saw an increase in the genes associated with tumor growth. However, it’s important to note that the tumors were not actually growing better in these women. Basically, taking a huge amount of soy in a short period of time led to changes in gene proliferation for some women with invasive breast cancer. But importantly, this didn’t lead to an actual change in the appearance or growth of a tumor, explains Bromberg. While the researchers definitely saw changes in the expression of these genes, they don’t know if this would lead to even greater changes if the women had been taking soy supplements for longer than 2-3 weeks. So does that mean you should limit soy or avoid it completely if you have breast cancer? Not necessarily. Like many things, soy seems to be safe in moderation, and there is definitely research showing it’s protective effects against a variety of cancers, including breast cancer. That said, this study does suggest that high amounts of soy may have an effect on women who have already been diagnosed. Bromberg notes that normal amounts of soy are probably fine for women with breast cancer, but she strongly advises against taking soy supplements. Moderation in a healthy diet is key, she says. MORE: How Your Weight Impacts Your Breast Cancer Risk “Our study simply says that [after taking] large amounts of soy for a short period of time, a subgroup of patients who had high levels of the soy metabolite genistein had an increase in the expression of genes involved in the growth of tumor cells,” says Bromberg. “But that did not translate to an increase in actual growth.” So there’s no need to ditch tofu and edamame—just make sure you’re not going overboard and talk to your doctor if you’ve been diagnosed with breast cancer and are worried about your consumption. For more information, check out these best (and worst) foods to protect yourself from breast cancer. MORE: New Research on the Birth Control-Breast Cancer Link
