People with advanced colorectal cancer who are overweight or obese may survive longer than their thinner counterparts, a new study suggests.

Researchers found that, on average, patients with a body mass index (BMI) of 25 or higher lived two and a half months longer after starting their treatment than patients with a lower BMI. People with a BMI of 25 or higher are considered overweight, and those with a BMI of 30 or higher are considered obese.

As BMI increased, the length of survival also went up, the researchers found.

"These results are surprising," Dr. Yousuf Zafar, the study's lead researcher and an associate professor of medicine at Duke University, said in a statement. "What we expected, based on prior evidence, was that those obese patients would do worse."

Obesity has long been considered a risk factor for both the development of colorectal cancer and the recurrence of the disease among those who have had it previously. In the new study, the researchers wanted to see if obesity also affected the length of survival in people with stage IV colorectal cancer (otherwise known as metastatic colorectal cancer) patients after chemotherapy.

In the study, the researchers gathered data from more than 6,000 patients with stage IV colorectal cancer who were enrolled in five cancer registry studies in the United States and Europe. All of the patients received the same drug, bevacizumab (known by the brand name Avastin) during their chemotherapy treatment. Patients were divided into four groups based on their BMIs, and the researchers measured how long patients survived after treatment. [10 Do's and Don’ts to Reduce Your Risk of Cancer]

The researchers found that patients in the lowest BMI category, with BMIs of 20 to 24.9 (considered a healthy weight), survived an average of 21.1 months after their treatment began. Patients in the next category, with BMIs of 25 to 29 (considered overweight), survived an average of 23.5 months. Patients with BMIs of 30 to 35 (considered obese) survived the longest — an average of 24 months. The correlation, however, appeared to drop off when patients' BMIs surpassed 35; patients in this category survived an average of 23.7 months after their treatment.

The researchers also noted how long patients went without additional tumor growth; however, they found that this was not linked with how long they lived.

The researchers cautioned that the results do not indicate that being overweight is protective for patients undergoing colorectal cancer treatment. Rather, the results suggest that there may be a biological factor at play that puts thinner patients at a higher risk for poor outcomes, said Zafar, who presented the findings in Barcelona on Wednesday (July 1) at a meeting of the European Society for Medical Oncology World Congress on Gastrointestinal Cancer.

There may be a link between BMI and the amount of 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," Zafar said in a statement.

Improving treatment for thin patients "may be where we can focus more attention on improving their outcomes," he added.

The next step in the research is to determine whether it's the biology of being underweight or the treatment itself that affects survival, he said. The researchers noted that the study was funded by the pharmaceutical company Genentech, which makes Avastin.

This isn't the first study that appears to link extra weight with better health outcomes. Previous research has found that obese patients with heart disease may survive longer than thinner patients, for example.

The phenomenon is known as the "obesity paradox," and while it may seem that extra weight can have a protective effect, there's likely another explanation — namely, that being underweight comes with its own set of risks, researchers say. In the heart disease finding, for example, the researchers said that underweight patients may develop heart disease for reasons such as genetic predisposition that are more difficult to treat than factors related to people's lifestyles.

Copyright 2015 LiveScience, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.