That news comes from researchers including Anne McTiernan, MD, PhD, of Seattle's Fred Hutchinson Cancer Research Center.
"Vigorous exercise was helpful for men of any size, as long as they worked out nearly every day," McTiernan says, in a news release.
McTiernan's team studied 102 men and 100 women aged 40-75 years (average age: mid-50s) who had had a colonoscopy within the past three years.
When the study started, participants were healthy but sedentary. They took a treadmill test to gauge their maximum heart rate.
Then the researchers gave half the group heart rate monitors and an exercise prescription:
- Get an hour of aerobic exercise six days weekly for a year.
- Workouts should be moderate to vigorous, at 60% to 85% of maximum heart rate.
For comparison, the researchers didn't assign the other participants to exercise.
All participants were asked to not change their diets.
The researchers designed the study to see whether participants in the exercise group followed orders.
Those participants wore pedometers that counted every step they took. Every week, they also turned in logs of their at-home workouts. Workouts at study facilities were also logged.
Participants in the exercise group got monthly progress reviews, newsletters, incentives (such as water bottles), and group social events.
"Adherence to the program was excellent overall," judging by the exercise logs, the researchers write.
Those logs show that men averaged slightly more than six hours of weekly workouts, while women averaged nearly five hours of weekly exercise.
After a year, participants got flexible sigmoidoscopy tests.
In flexible sigmoidoscopy, doctors guide a thin, flexible tube with a tiny camera through the lower colon to check for abnormal growths, such as polyps, which might become cancerous.
The procedure is similar to colonoscopy, except that colonoscopy examines the entire colon, not just the lower part of the colon.
Using flexible sigmoidoscopy, the researchers checked cell growth (proliferation) in colon crypts, which are folds in the colon's lining.
They found substantially less evidence of cell proliferation in those colon areas for men who had followed the researchers' exercise prescription, compared with the study's other men.
Treadmill tests showed that the exercisers had improved their aerobic fitness. Men with the biggest gains in aerobic fitness had the least cell proliferation in the colon crypts.
What About Women?
Women in the exercise group didn't show any difference in cell proliferation in similar colon areas.
"It's not a finding that we really wanted to see," McTiernan says, adding that those results echo findings from other studies.
The reasons for the gender gap aren't clear.
Exercise lowers estrogen, a sex hormone that's higher in women before menopause. Perhaps lower estrogen levels reduce colon protection, the researchers write.
But that's not certain. The researchers didn't check women's estrogen levels.
There might be a simpler explanation: The women in McTiernan's study reported shorter, lighter workouts than the men.
"The male exercisers may have put more sustained intensive effort into their exercise sessions, which could explain the greater exercise effect on outcome in men versus women," the researchers write.
Ready to start an exercise program? Remember to check in with your doctor first, if you've been on the sidelines for awhile.
SOURCES: McTiernan, A. Cancer Epidemiology, Biomarkers and Prevention, September 2006; vol 15: pp 1588-1597. News release, Fred Hutchinson Cancer Research Center.
By Miranda Hitti
Reviewed by Louise Chang