Researchers at the Arizona Cancer Center, University of Arizona, looked at the relationship between physical activity, obesity, and levels of prostaglandins. Prostaglandins are substances found in cells that line the large bowel. Some types of prostaglandins are suspected of enhancing the development of colorectal cancer.

The researchers, led by Maria Elena Martinez, PhD, looked at whether higher levels of physical activity and lower body weight were associated with lower levels of prostaglandins in the rectum. Their study, which was published in a recent issue of the Journal of the National Cancer Institute, included 41 men and 22 women aged 42 to 78. All had a history of colorectal polyps.

Scientists excluded anyone who used aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), which many researchers believe can lower risk of colon cancer by suppressing prostaglandin production.

The researchers measured prostaglandin levels in rectal biopsy samples from study participants and assessed levels of physical activity, using a questionnaire filled out by study participants. According to Dr. Martinez and colleagues, "The results of our study show a significant reduction in rectal mucosal PGE2 (prostaglandin) levels associated with a higher level of leisure-time physical activity and a lower BMI (body mass index)."

Researchers found levels of prostaglandin E2 (PGE2) increased as people grew older, and levels were higher than normal for people who weighed more. In those who exercised, PGE2 levels were lower.

Dr. Martinez said PGE2 levels were reduced by as much as 28 percent in people who jogged 30 minutes or walked for an hour each day. She said, "It is never too late to start an exercise program and this does not have to involve strenuous or vigorous exercise. The majority of participants in this study were simply walking."

Michael Thun, MD, vice president of epidemiology and surveillance research for the American Cancer Society (ACS), called the study interesting even though it was small. "This single study is by no means conclusive. However, it suggests that several factors may help reduce the risk of colon cancer through overlapping mechanisms," he said.

Other studies are needed to learn more, said Dr. Martinez. "Since this is the first study to document this finding, the results need to be replicated by additional studies using different study populations. An important question is whether physical activity has the same effects on prostaglandin levels among users of nonsteroidal anti-inflammatory drugs," she said.

"Three factors that consistently tie in with lower risk of colorectal cancer in epidemiologic studies are being physically active, not being obese, and regularly using aspirin or related drugs," said Dr. Thun. He said more research is needed to understand all the effects of nonsteroidal anti-inflammatories.