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
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
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.