Obesity is an important risk factor for breast cancer. Now, a study reveals that the way obesity alters breast tissue might be to blame.

Specifically, obesity enhances the stiffness of breast fatty tissues — which indirectly stimulates tumor growth, a study published today in Science Translational Medicine shows. Researchers found that precancerous cells grown in obese fat tissues are more likely to turn into malignant tumors than cells grown in lean fat tissues. However, these effects might not be permanent; by putting obese mice on a diet, the researchers showed that some of the changes that took place in the fatty tissues were reversible.

The prognosis for breast cancer patients who are obese tends to be worse than that of women whose weight falls within the normal range. Despite this link, few breast cancer treatments specifically target women who are obese. That's why figuring out exactly what's going on at the cellular level is so important; it could lead to new drugs, new forms of treatment, and better breast cancer outcomes for women in general.

"Now that we know what the changes are, this study provides a point of attack where researchers could identify new ways to completely reverse those changes to help treat women with breast cancer," says Charlotte Kuperwasser, a breast cancer researcher at Tufts University who didn't work on the study.

To learn more about the link between obesity and breast cancer, researchers studied the breast tissues of lean and obese breast cancer patients, as well as tissues belonging to obese mice. Thanks to this approach, they were able to determine that increased stiffness in breast fat tissue changes chemical communication between cells, which in turn causes more aggressive cancer cell behaviors. Putting obese mice on a diet reversed some of these effects — but not all.

Given that the diet part of the study was done on mice and not humans, it's not entirely clear whether the findings can be replicated in women. A lot more work will need to be done to make sure that these findings are relevant for humans, too. In the meantime, however, the study hints that, in some cases, losing weight might be able to improve health outcomes for breast cancer patients who are obese.

The findings could also have an impact on breast reconstruction surgeries that involve the implantation of obese fat tissues, says Claudia Fishbach, a biomedical engineer at Cornell University and one of the co-authors of the study. Using those tissues could inadvertently "activate residual breast cancer cells after mastectomy," which might "promote recurrence."

This is "an important new piece [of information] that helps integrate what we know about obesity and breast cancer into a unified picture," Kuperwasser told The Verge. "We should care because understanding how obesity can promote and accelerate breast cancer will help in finding additional ways of preventing it."