Risk after colon cancer higher for the very fat and very thin

For patients with stage II or stage III colon cancer, the difference in long-term survival for leaner patients compared to those with a body mass index (BMI) of 35 or greater — which physicians refer to as “very obese” — was comparable to the difference between those who had surgery followed by chemotherapy and those who had only surgery.

The very thin, those with a BMI less than 18.5, were also at increased risk of death, primarily from other cancers, including respiratory cancers possibly connected to smoking, as well as non-cancer causes.

“Given the increasing proportion of Americans with a BMI greater than 35 and the fact that these individuals are over-represented among colon cancer patients, we need to find out why extra weight has such a harmful impact and come up with new ways to counter that,” said study author James Dignam, Ph.D., a biostatistician and assistant professor in the Department of Health Studies at the University of Chicago.

“One first step would be to investigate whether modifying diet and exercise habits for patients after treatment would have a positive impact on colon cancer outcomes,” he said. “Other studies beginning to explore the effect exercise in colon and other cancers have been promising.”

The researchers studied data from 4,288 patients with stage II or stage III colon cancer who enrolled in either of two multi-center clinical trials between July 1989 and February 1994. Both trials, administered by the National Surgical Adjuvant Breast and Bowel Project, headquartered at the University of Pittsburgh, compared different regimens of chemotherapy following surgery. Median follow-up from surgery to last contact with patients who were still alive was 11.2 years.

More than half of the patients in the trial (54%) were overweight and 5.5 percent were very obese. About three percent were underweight.

The researchers focused on the relationship between body mass index at the time of diagnosis and long-term prognosis. They found that the risk of death was increased “at both ends of the adiposity spectrum.”

Patients who were very obese were most likely to die from colon cancer recurrence. Patients who were underweight had twice the risk of death, but their increased risk was from non-colon cancer-related causes.

The mechanism connecting excess weight to cancer recurrence and death “is not completely understood,” the authors note. Previous studies have suggested a role for insulin, insulin-like growth factors, the hormones that regulate hunger and other obesity-related pathways. Another possibility is the presence of concurrent illnesses associated with obesity, which may interfere with the ability to deliver effective treatment.

“This study provides us with an easy way to predict that certain patients have a higher risk of recurrence and thus might benefit from closer surveillance and additional interventions,” said Dignam.