Obesity has surprisingly been found to prolong survival in colorectal cancer patients, a new study has found.

The study by Duke University researchers that was recently presented at the European Society for Medical Oncology World Congress on Gastrointestinal Cancer in Spain revealed that obese and overweight patients survived for 2.5 months longer on the average compared with thinner patients.

Researchers were surprised by the results because they expected obese patients to respond poorly to treatments because of their increased risk of recurring disease.

In the study, more than 6,000 patients who had undergone previous treatment for metastatic colorectal cancer received Avastin, an anti-cancer drug, and chemotherapy.

The participants were then followed for two years. Patients with a body mass index (BMI) reading from 20 to 24.9—considered by BMI guidelines as healthy—lived on for an average of 21.1 months following treatment.

Those with a BMI reading from 25 to 29, classified as overweight, lived on for an average of 23.5 months.

Patients who have an obese BMI reading of 30 to 35 survived for 24 months on average, while those with readings higher than 35.1 survived for 23.7 months on average.

"Contrary to our hypothesis, patients who had the lowest BMI were at risk of having the shortest survival," said Yousuf Zafar, associate professor of medicine at Duke and lead author of the study.

Zafar noted that the study did not suggest that being overweight could protect patients receiving chemotherapy. The findings simply suggest that there may be an "aspect of biology" that makes thinner patients respond more poorly to treatment.

Studies have shown that obesity increases the likelihood of various diseases, particularly heart disease, type 2 diabetes, obstructive sleep apnea, certain types of cancer and osteoarthritis. Obesity is most commonly caused by a combination of excessive food energy intake, lack of physical activity and genetic susceptibility, although a few cases are caused primarily by genes, endocrine disorders, medications or psychiatric illness.