Overweight colorectal cancer patients were 55 percent less likely to die from their cancer than normal-weight patients who have the disease, according to a new Kaiser Permanente study published today in JAMA Oncology.

Of cancers affecting both men and women, colorectal cancer is the second-leading cause of cancer death in the United States, according to the U.S. Centers for Disease Control and Prevention.

Research has shown that people with higher body mass indexes (BMI) are significantly more likely to be diagnosed with several types of cancer; however, once diagnosed their prognosis is often better than normal-weight patients.

This has been called the "obesity paradox."

"Overweight and obesity have been identified as risk factors for many health conditions, but for people with colorectal cancer, some extra weight may provide protection against mortality," said lead author Candyce H. Kroenke, ScD, research scientist with the Kaiser Permanente Division of Research in Oakland, California. "Our study, which represents the largest cohort of colorectal cancer patients with the most comprehensive data regarding patient weight before, at time of, and following diagnosis, supports the notion of the 'obesity paradox.'"

In this study, researchers examined the electronic medical records of 3,408 men and women diagnosed with stages 1 through 3 colorectal cancer between 2006 and 2011.

All were Kaiser Permanente members in Northern California.

Patients were compared for mortality risk based on their BMI at the time of diagnosis and 15 months following diagnosis.

The data were adjusted for socioeconomic and demographic factors, disease severity, pre-diagnosis BMI, smoking, and other factors.

Compared with patients who were low-normal-weight at diagnosis (BMI between 18.5 and less than 23), those who were underweight (BMI under 18.5) or obese (BMI greater than or equal to 35) had elevated risks of mortality.

By contrast, patients who were in the high-overweight category at diagnosis (BMI between 28 and up to 30) had a 48 percent lower risk of mortality overall and a 55 percent lower risk of mortality related to colorectal cancer when compared with patients in the low-normal-weight category.

Senior author Bette J. Caan, DrPH, research scientist with the Kaiser Permanente Division of Research, said that biological mechanisms for the obesity paradox are not known, and merit additional study.

"The current findings, and previous and ongoing research on the obesity paradox, suggest that recommendations for the ideal weight range associated with the best outcomes after a cancer diagnosis may not be the same as the ideal weight range to prevent cancer," Caan said. "And just as treatment differs by cancer, ideal weight recommendations may vary according to cancer site."

Source: JAMA Oncology