March 9, 2011 — Exercise might reduce the risk for colon cancer by reducing the risk of developing precancerous polyps, according to a new study published online March 2 in the British Journal of Cancer.
"We've long known that an active lifestyle can protect against bowel cancer, but this study is the first to look at all the available evidence and to show that a reduction in bowel polyps is the most likely explanation for this," said lead author Kathleen Wolin, MD, from the division of public health services at the Washington University School of Medicine in St. Louis, Missouri.
Earlier work by the same group, as well as other research, has shown that exercise can reduce the risk for colon cancer by 25%.
This new study, based on a meta-analysis of 20 studies, focused on colon adenomas, the precancerous polyps that are removed during sigmoidoscopy or colonoscopy. The researchers found that regular physical exercise was associated with a 16% decrease in the risk of developing colon polyps, and with a 30% decrease in the risk of developing polyps that were large or advanced, and thus more likely to become cancerous.
"Exercise has many benefits, including boosting the immune system, decreasing inflammation in the bowel, and helping to reduce insulin levels — all factors that we know are likely to have an effect on bowel polyp risk," Dr. Wolin said in a statement.
"This study adds weight to the evidence showing that regular exercise can substantially cut the risk of bowel cancer," said Sara Hiom, director of health information at Cancer Research UK, in the same statement, which was issued by the charity to publicize the study. "We recommend doing at least half an hour's moderate exercise a day," she added.
The American Cancer Society (ACS) also recommends at least 30 minutes of at least moderate activity on 5 days or more per week, and says that 45 to 60 minutes of intentional physical activity is preferable. The protective effect of exercise on colon cancer risk is "one of the most consistently reported relationships," according to a recent ACS report, Colorectal Cancer Facts & Figures 2011-2013.
For the new study, Dr. Wolin and colleagues reanalyzed data collected in 20 clinical trials that reported on physical activity levels (obtained mainly from questionnaires) in individuals who had undergone sigmoidoscopy or colonoscopy (both symptomatic and screening). Most studies did not specify the reason for undergoing the procedure.
Together, these trials involved more than 250,000 individuals.
Overall, there was a significant inverse association between physical activity and colon polyps (fixed-effect relative risk [RR], 0.87; 95% confidence interval [CI], 0.83 to 0.91; random-effects RR, 0.84; 95% CI, 0.77 to 0.92).
The risk reduction was similar in men and women.
"There was a tendency for the effect of physical activity to be restricted to large or advanced adenomas, and not low-grade ones," the researchers note.
"Our meta-analysis found the effect was stronger, though not significantly so, for large or advanced adenomas than for the overall effect," they add.
The risk reduction (RR, 0.83) was "largely unchanged" when the analysis was restricted to the 18 studies in which the results for adenomatous polyps were separated from all polyps (i.e., hyperplastic, malignant polyps), they report.
Involved in Whole Carcinogenic Process
Dr. Wolin and colleagues note that some earlier reports failed to show an association between physical activity and colon polyps; this has been interpreted as suggesting that physical activity might be more important in the "adenoma to carcinoma" sequence than in adenoma development.
However, they report that this comprehensive meta-analysis — together with numerous previous reports demonstrating that physical activity can reduce the risk for colon cancer — suggests that physical activity plays a role across the whole carcinogenic process.
Several mechanisms have been proposed for such effects, including enhanced immune function, decreased inflammation, reduced insulin levels and insulin resistance, and higher vitamin D levels, Dr. Wolin and colleagues note.
The researchers have disclosed no relevant financial relationships. One of the coauthors, G.A. Colditz, MD, DrPH, from the Washington University School of Medicine, was supported by an American Cancer Society Clinical Research Professorship.
Br J Cancer. 2011;104:882-885. Abstract
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Cite this: Exercise Reduces Risk for Colon Polyps, Resulting in Less Colon Cancer - Medscape - Mar 09, 2011.