A new study published in the Journal of the National Cancer Institute finds that soy protein might exacerbate breast cancer, particularly in women who already have the disease.

“This study doesn’t tell us anything about whether soy raises the risk of developing breast cancer,” said Dr. Jacqueline Bromberg, a breast cancer specialist at Memorial Sloan Kettering Cancer Center in New York City, according to Health Day. Instead, the study focuses primarily on how soy protein might possibly make matters worse for women who already have breast cancer.

Research has been done to examine the link between soy and breast cancer before, much of it “highly complex, controversial, and evolving,” according to Marji McCullough of the American Cancer Society. In some studies, soy has even been shown to reduce breast cancer risk in women who didn’t have the disease yet. But because soy foods can act like estrogen, which has been linked to “hormonally-sensitive” cancers in women, they might pose a slight risk for women who already have breast cancer. All of this, of course, is still uncertain and not fully researched.

The results of this new study are likewise complicated and quite hazy, with researchers unable to come to any clear conclusion. For the study, researchers assigned 140 women with early-stage breast cancer into two different groups. The first group took a soy protein supplement every day for one to four weeks — the equivalent of about four cups of soy milk per day — and the second group only took milk powder. Twenty percent of the women who took the soy supplement developed high blood concentrations of genistein, a soy phytoestrogen, which has previously been linked to genes that promote breast tumor growth. In these women who showed higher levels of genistein, some also had more activity in tumor-promoting genes.

But “[d]oes that necessarily mean the tumor is growing more rapidly?” Bromberg said. “No.” Instead, there wasn’t any solid evidence that tumor growth was happening any faster in women who took soy supplements. “All we can say is that two weeks of soy supplementation was enough to increase expression of genes related to tumor proliferation,” Bromberg continued.

Regardless, some breast cancer specialists still suggest that women with breast cancer avoid soy products to be safe — or eat soy foods like tofu and tempeh in moderation. As McCullough writes on the American Cancer Society's website:

Even though animal studies have shown mixed effects on breast cancer with soy supplements, studies in humans have not shown harm from eating soy foods. Moderate consumption of soy foods appears safe for both breast cancer survivors and the general population, and may even lower breast cancer risk. Avoid soy supplements until more research is done. So, enjoy your occasional tofu stir-fry or tofu burger — they are unlikely to increase your risk of breast cancer and, on balance, are some of the healthier foods you can eat!