WASHINGTON - A daily walk may cut a woman's risk of colon cancer in half, a study finds.
"Increasing physical activity levels may be an effective approach for reducing the burden of colon cancer in our society," the Harvard researchers wrote in the Journal of the National Cancer Institute.
Walking at a normal or brisk pace for one hour per day is associated with a 46 percent reduction in risk of the No. 3 cancer killer of women in the United States, the study said.
And women who do only half this can still reduce their risk by a quarter, said Dr. Graham A. Colditz, the study's senior author.
The findings add to the reasons, including reduced risk of heart disease, for women to do more exercise, Colditz said: "Our data are sufficient, in the context of everything else, to support the recommendation that women increase their activity."
The findings were based on 1986-92 data from the Nurses' Health Study, a leading database on women's health. The researchers analyzed activity levels reported, among other data, by 67,802 participants every two years.
They computed the amount of energy spent in such activities and noted 212 cases of colon cancer among the study participants. Walking was the most common activity, reported by 70 percent of the participants.
Women in the upper 20 percent in energy expenditure per week, which required at least brisk walks totaling more than seven hours, had 54 percent less colon cancer risk than did women in the bottom 20 percent on exercise, the study found. As the amount of energy used fell, so did the amount of risk reduced.
The benefit is similar to what had already been found in men, Colditz said.
It was the men's studies in large part that led to the Surgeon General's report last summer which concluded that exercise can be protective, said epidemiologist Carl Caspersen of the Centers for Disease Control and Prevention. The report was prepared under the CDC's scientific lead.
The report's authors were able to extend the recommendations to women because they did not expect to see pertinent sex-based differences in risk, Caspersen said.
However, although the paper is probably correct in concluding that exercise reduces risk, "this one article is not going to tell you definitively," commented Dr. Lee S. Rosen, director of cancer therapy development at the Jonsson Comprehensive Cancer Center at UCLA.
"Exercise in general is a good thing to prevent a lot of illnesses that are diet- or lifestyle-induced, and colon cancer is one of them," Rosen said.
Exercise is probably best started young, because colon cancer has a long lead time, developing from noncancerous intestinal growths over maybe 20 years, Rosen said.
Exactly how exercise reduces the risk is unclear. But one theory is that it increases the speed at which material moves through the bowel. This would reduce the time in which the bowel lining is in contact with chemicals that can cause cancer, according to the American Cancer Society.