Regular, moderate-to-vigorous aerobic exercise significantly reduces a risk factor associated with the formation of colon polyps and colon cancer in men, according to a study led by researchers in the Public Health Sciences Division. The findings, from the first randomized clinical trial to test the effect of exercise on cancer biomarkers in colon tissue, appear in the September issue of Cancer Epidemiology, Biomarkers and Prevention.
"In men who met the study's exercise prescription of an hour of aerobic activity per day, six days a week for a year, we saw a substantial decrease in the amount of cellular proliferation in the areas of the colon that are most vulnerable to colon cancer," said lead author Dr. Anne McTiernan, an internist and epidemiologist who directs the Center's Prevention Center. "However, we found that even four hours or more of exercise weekly was enough to produce a significant benefit," she said.
Specifically, the researchers saw a decrease in the number of actively dividing cells, or cellular proliferation, within the colonic crypts — tiny tube-like indentations in the lining of the colon, or epithelium, which help regulate the absorption of water and nutrients. "A certain amount of cellular proliferation at the bottom part of the crypt is normal. But when these cells start dividing too quickly, they can migrate up the sides of the crypt to the surface and eventually form a polyp," she said. While most polyps are benign, over time some types can become malignant.
The researchers found an inverse relationship between the amount and intensity of exercise and the levels of cellular proliferation, as measured by how far the migrating cells traveled from the base of the crypt and up the sides toward the surface of the epithelium.
A significant decline in cellular proliferation was observed among men who worked out an average of four hours a week or more and in those whose cardiopulmonary fitness was most robust. The greatest decrease in cellular proliferation was seen in men who exercised more than five hours a week. No such decrease was seen among sedentary men or those who exercised infrequently.
"Proliferation in the upper section of the colon crypt decreased among those exercising for a mean 250 minutes per week or greater, which is important because this pattern of proliferation is most associated with risk for colon cancer," the researchers reported.
Body weight did not appear to have an impact on the effect of exercise on cellular proliferation. "These effects were independent of weight. Vigorous exercise was helpful for men of any size, as long as they worked out nearly every day," said McTiernan, who is also a faculty member at the University of Washington.
So while men of all shapes and sizes seemed to benefit from frequent, vigorous workouts of at least four hours a week, the investigators saw no notable changes in markers of cellular proliferation in their female counterparts. "This finding supports previous epidemiological studies that also have suggested that regular exercise reduces the risk of colon cancer in men more than in women," McTiernan said. "It's not a finding that we really wanted to see, but at least our results are consistent with those of previous population-based, epidemiological studies."
The mechanism behind the null effect in female exercisers is unknown. Possible explanations, the researchers hypothesize, include the fact that exercise lowers the level of naturally occurring estrogen, a hormone that protects the colon. Another possible explanation is that the men worked out more vigorously and more often than did the women. "On average, the men in the study met their physical-activity goal of an hour a day, six days a week, whereas the women met about 80 percent of their goal. Also, the men spent more time jogging or running compared to the women," McTiernan said. "The women still did very well in this exercise intervention, but it may not have been enough to protect the colon."
The study, which was funded by the National Cancer Institute and the National Institutes of Health, involved 202 healthy, sedentary Seattle-area men and women between the ages of 40 and 75. All had undergone a colonoscopy within three years of participating in the yearlong intervention to confirm the absence of colon cancer. Before and after completion of the study, the participants also underwent a flexible sigmoidoscopy, a procedure that allows for visual inspection of the rectum and lower colon, and the collection of tissue samples from the mucosal lining of the colon.
Half of the participants were randomly assigned to an exercise group and half were randomly assigned to a comparison, or control group. The exercisers were asked to engage in moderate to vigorous activity six days a week for a year, both on their own and at one of several exercise facilities (including one located in the PHS Division's Prevention Center). They were also asked to maintain their regular eating habits for the duration of the study. Those in the control group were asked to maintain their activity level and diet for a year, after which they had an opportunity to exercise for two months at no cost with a personal trainer at one of several study facilities.
Daily exercise logs indicated that 80 percent of the exercisers met more than 80 percent of their six-hour-a-week goal.
A major strength of the study was its randomized, controlled, clinical-trial design, which enabled the researchers to minimize the impact of confounding factors, document exercise activity and examine the direct effects of exercise on colon tissue.
"I think that this study really underscores the new activity recommendations from the United States Department of Agriculture and the Institute of Medicine, both of which advise people to exercise an hour a day, six days a week for weight control and general health," McTiernan said.
Researchers from the UW School of Medicine, Veterans Affairs Puget Sound Health Care System and Virginia Mason Medical Center in Seattle collaborated on the study. The Seattle Foundation and Precor Inc. of Bothell donated exercise equipment for the Center's state-of-the-art exercise facility.
More than 500 Seattle-area participants are sought for the National Cancer Institute-funded Nutrition and Exercise for Women (NEW) Study. The study, led by Dr. Anne McTiernan, examines the effects of exercise and nutrition on breast-cancer risk factors.
The researchers seek healthy, overweight and sedentary postmenopausal women (ages 50 to 75) who live in the Seattle area and are willing to travel to the Hutchinson Center for the yearlong exercise or nutrition intervention.
Eligibility requirements include being a nonsmoker, not using hormone therapy for the past six months, getting less than an hour of moderate activity per week, and being above a certain weight threshold, depending upon height.
Those who qualify must be willing to not participate in any other exercise or weight-loss programs during the 12-month study enrollment and must be willing to be randomly assigned to one of the following four groups:
The study will be conducted at the Prevention Center, located in the Arnold Building. Designated parking for study participants is free of charge. For more information, call the NEW Study information line at (206) 667-6444, e-mail firstname.lastname@example.org or visit www.thenewstudy.org.
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