Dietary fiber intake showed no effect on the risk of colorectal cancer in an analysis that pooled data from 13 prospective cohort studies, reported Dr. Yikyung Park of Harvard School of Public Health, Boston, and associates.
Pooling the results of studies netted 8,081 cases of colorectal cancer, giving the analysis a level of statistical power such that “a substantial effect of fiber is unlikely to have been missed,” the researchers said (JAMA 2005:294:2849–57).
The pooled analysis was undertaken because the results of many epidemiologic studies and randomized clinical trials have been conflicting, and there is still much debate over whether dietary fiber decreases colorectal cancer risk.
Each of the 13 prospective cohort studies Dr. Park and associates identified for their analysis included at least 50 cases of colorectal cancer and adequately assessed dietary fiber intake. The data yielded over 7,300,000 person-years of follow-up (6–20 years of follow-up across the studies), and included incident colorectal cancer cases in 2,776 men and 5,305 women.
The initial age-adjusted analysis showed a significant link between fiber intake and colorectal cancer, with the highest levels of intake associated with a 16% lower risk than the lowest levels. However, after the data were adjusted for potentially confounding risk factors including nondietary factors, milk and red meat intake, and alcohol consumption, “only a nonsignificant weak inverse association was found,” they said.
The findings were similar after the studies were combined and analyzed as a single data set. Likewise, dietary fiber showed no effect on cancer risk when the data were analyzed by subjects' body mass index and by sources of fiber.
Although the findings indicate that dietary fiber “may not have a major effect on the risk of colorectal cancer,” a diet rich in whole plant foods can still be advocated because it may reduce risks of other disorders. “A true association between dietary fiber intake and risk of colorectal cancer may be underestimated in our study” because the analysis was limited by possible errors in measuring fiber intake, they noted.