Extra pounds may raise the risk for recurrence among women with the most common kind of breast cancer, a new study shows.

Prior studies have found that being overweight or obese increases the risk of getting a number of cancers, including breast cancer. And smaller studies have noted that obese breast cancer patients tended to fare worse than those who are normal weight.

The new study takes a fresh look at data collected on more than 7,000 breast cancer patients who were enrolled in three government-sponsored clinical trials.

The trials were originally designed to test the effectiveness of different combinations of chemotherapy drugs. All the women in the studies had operable cancers without spread to distant organs and received cutting-edge treatment for their cancers.

Slideshow: A Visual Guide to Breast Cancer

As Body Weight Climbs, So Does the Risk

Despite that, a woman’s weight was still linked to the likelihood that she might beat the disease.

Among women with hormone-sensitive tumors, those who were overweight or obese when they were diagnosed were about 30% more likely than those with normal body weights to see their breast cancers come back after surgery and chemotherapy. They were also about 50% more likely to die over the course of the studies.

The risk appeared to increase in a step-wise fashion -- the higher a woman’s weight, the greater her odds of recurrence and early death.

The study doesn’t prove that too much body fat causes cancer directly. But as scientists learn more about fat, they think that’s a strong possibility.

Fat cells play a role in the production of hormones like estrogen and insulin that spur the growth of tumors. Fat also increases inflammation, another cancer driver, says researcher Joseph A. Sparano, MD, associate chairman of medical oncology at the Montefiore Einstein Center for Cancer Care in New York City. 

What it means, simply, is “the more you weigh, the worse the outcome,” says Stefan Gluck, MD, a professor of medicine and a breast oncologist at the University of Miami’s Sylvester Comprehensive Cancer Center. He was not involved in the research.

Advice to Patients

Beyond surgery and chemotherapy, “every patient asks, ‘What else can I do?'” Gluck says. “It’s always part of my counseling to explain that” too much body fat “leads to a more aggressive cancer behavior and you want to avoid it.”

But that’s not always easy to do. Many breast cancer drugs zap energy, making it harder to exercise.

Sparano says this study makes it important for patients and doctors to do what they can to try prevent those extra pounds.

“What we don’t know yet is whether weight loss in breast cancer patients will significantly reduce the risk of breast cancer recurrence or death,” he says.

Gluck says studies are under way to answer that question.