Obesity is a clear risk factor for developing colorectal cancer, but a surprising new study has found being overweight may actually help prolong survival in patients who develop the disease.

The study, presented by Duke University researchers this week at the European Society for Medical Oncology World Congress on Gastrointestinal Cancer, found that overweight and obese patients lived an average of 2.5 months longer than thinner patients.

The results surprised researchers, who expected obese patients to respond more poorly to treatments for advanced colorectal cancer due to their increased risk of developing the disease and having it come back.

"Contrary to our hypothesis, patients who had the lowest BMI [body mass index] were at risk for having the shortest survival," said lead researcher Yousuf Zafar, M.D., associate professor of medicine at Duke. "In this case, patients with the lowest body weight — people who had metastatic colon cancer and a BMI of less than 25 — were at the highest risk."

The study involved 6,128 patients who had previously been untreated for their metastatic colorectal cancer. All received the anti-cancer drug Avastin along with chemotherapy in their treatment and were tracked for two years.

Patients with the lowest BMI from 20 to 24.9, which would be considered a healthy weight by BMI guidelines, survived an average of 21.1 months after starting treatment.

Patients with a BMI of 25 to 29, considered overweight, survived an average of 23.5 months. By comparison, patients with BMIs of 30 to 35, obese by the standards, survived an average of 24 months, while those with BMIs of 35.1 and higher survived an average of 23.7 months.

"There may be a relationship between having a lower BMI and how much treatment patients can tolerate," Zafar said "I would hypothesize that the lowest weight patients in our analysis received or tolerated less treatment, or received adequate treatment at first, but became too sick to receive additional therapy. That may be where we can focus more attention on improving their outcomes."