BETHESDA, Md--"Exercise appears to be one good way, among others, to lower the risk of colorectal cancer," Maria Elena Martinez, PhD, said at the American Society of Preventive Oncology meeting. "And you dont have to be a marathon runner to get this benefit."
Dr. Martinez, research assistant professor, Cancer Prevention and Control, Arizona Cancer Center, Tucson, spoke at a workshop on preventing colon cancer. She said that studies consistently show an inverse association between physical activity and colorectal cancers. The most sedentary segment of the population has an 80% greater risk of colorectal cancer than those who engage in frequent, vigorous exercise.
The mechanism by which exercise cuts colorectal cancer risk has not been identified, Dr. Martinez said, but there is enough evidence to posit a causal relationship. Several hypotheses have been suggested, including faster gastrointestinal transit time, improved immune function, and increased bile acid metabolism (although the bile acid hypothesis has since been discredited).
The prostaglandins have been implicated as well. Preliminary results from one study by Dr. Martinez show that reduced physical activity raises the levels of PGE2 in rectal mucosae (although not significantly), which may increase transit time.
There is also a positive association between body mass index and PGE2. "Perhaps body mass index is a marker for physical activity or perhaps there is a more direct connection," she said.
Yet another hypothesis centers on insulin resistance: Physical inactivity and abdominal obesity contribute to insulin resistance, leading to hyperinsulinemia and eventually to an increased risk of colon cancer.
Whatever the cause, Dr. Martinez suggested that a waist-to-hip ratio greater than 0.98 leads to a much higher risk in men. "This may be because when men gain weight, it is more likely to be focused in the central region, compared to women, who carry their weight lower down," she said.
There are conflicting data on the intensity of exercise required to lower colorectal cancer risk. Dr. Martinez said that low-intensity exercise has not been shown to be protective, but that moderate- and high-intensity exercise appear to be almost equally protective. Walking briskly just 6 hours a week, or jogging, swimming, or biking 3 hours a week, is sufficient. Even climbing stairs 10 minutes a day five times a week will fill one-third of the weeks exercise quota.