A prospective cohort study of 31,783 American women has found no significant association between levels of physical activity and colon cancer incidence.

Because exercise has been shown to accelerate gastrointestinal transit time, upregulate immune function, and alter the production of prostaglandins and insulin, experts have theorized that it might also inhibit colon cancer.
Over forty epidemiologic studies have examined the association with varying results. Many observed a reduction in colon cancer risk for people reporting high levels of activity, compared to those who did not.
Several studies have shown a stronger inverse relationship for men, compared to women.
Other studies have found no association at all for women.

To better understand the relationship between physical activity and colon cancer risk in women, researchers led by Brook A. Calton, now at the University of California-San Francisco ( UCSF ), conducted a prospective study of a large cohort of post-menopausal women in the United States. Using the Breast Cancer Detection Demonstration Project Follow-up Study, they collected information about physical activity and colon cancer cases for 31,783 women.
There were 243 cases of colon cancer during the ten-year study period from 1989 to 1998.

The researchers analyzed reported physical activity for all women in the cohort using a Cox proportional hazards regression, and observed no association between physical activity and the subsequent risk of colon cancer.

" The results of this large prospective cohort study among women do not support the hypothesis that physical activity is related to a lower incidence of colon cancer," the authors report.

They suggest that the null findings may be due to imprecisions related to physical activity assessment. These might mask a weak or modest association. They point out that some household activities, like washing clothes and scrubbing floors, may have been classified as more vigorous than they actually are.
Given that over 58 percent of women in the study reported engaging in at least 30 minutes of vigorous activity per day, the authors suggest that the inclusion of these various household activities may have led to overestimated amounts of absolute vigorous activity.
They also note, however, that "household activity may be of particular significance to women," and should be included as part of a physical activity assessment to gain a more accurate picture of total physical activity.

Still, the authors conclude, "our results do not support the hypothesis that physical activity confers significant protection against the overall development of colon cancer in women. Our findings point towards the need for conducting further research, particularly among women, that uses well-formulated, accurate measures of physical activity and distinguishes between different types of physical activity ( i.e. recreational, occupational, and household activity ) in relation to colon cancer risk."

Source: International Journal of Cancer, 2006


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