Although dietary fiber has been hypothesized to reduce the risk of colorectal cancer, the results of numerous previous studies have been inconsistent.

Ecological correlation studies and many case-control studies have found an inverse association between dietary fiber intake and risk of colorectal cancer.

However, most prospective cohort studies have found no association between dietary fiber intake and risk of colorectal cancer or its precursors (adenomas). Also, randomized clinical trials of dietary fiber supplementation have failed to show reductions in the recurrence of colorectal adenomas.

Nonsignificant Weak Link

Yikyung Park, ScD, formerly of the Harvard School of Public Health, Boston, and colleagues reanalyzed the primary data from 13 prospective cohort studies, including 725,628 men and women who were followed-up for six to 20 years across studies, to see if there was a connection between fiber intake and colorectal cancer risk.

During the follow-up, 8,081 colorectal cancer cases were identified. Among the studies, the median energy-adjusted dietary fiber intake ranged from 14 to 28 grams per day (g/d) in men and from 13 to 24 g/d in women.

The major source of dietary fiber varied across studies with cereals as a major contributor to dietary fiber intake in the European studies, and fruits and vegetables as the main sources in the North American studies.

In the age-adjusted model, dietary fiber intake was significantly associated with a 16 percent lower risk of colorectal cancer in the highest quintile compared with the lowest. This association was attenuated slightly but still remained statistically significant after adjusting for nondietary risk factors, multivitamin use, and total energy intake.

Additional adjustment for dietary folate intake further weakened the association, however. In the final model, which further adjusted for other dietary factors -- such as red meat, total milk and alcohol intake -- only a nonsignificant weak inverse association was found. Fiber intake from cereals, fruits and vegetables was not associated with risk of colorectal cancer.

Lower Risk of Other Chronic Conditions

"The association between dietary fiber intake and risk of colorectal cancer has been inconsistent among observational studies, and several factors may explain the disparity: potential biases in each study, the failure to adjust for covariates in the multivariate models, and the range of dietary fiber intake," note the authors.

Although they did not find support for the hypothesis that high dietary fiber intake has a major effect on the risk of colorectal cancer, the authors conclude that "a diet high in dietary fiber from whole plant foods can be advised, because this has been related to lower risks of other chronic conditions, such as heart disease and diabetes."

John A. Baron, MD, of Dartmouth Medical School examines the results of the pooled analysis in an accompanying editorial.

"The findings by Park et al … provide at least some indications that dietary fiber of some sort is related in some way to colon or rectal cancer risk," he says.

"Over the short term, wheat fiber or psyllium (soluble fiber) interventions do not seem to affect colorectal carcinogenesis," Dr. Baron acknowledges, "but understanding longer-term relationships with any type of fiber will require more work. Studies like that of Park et al provide valuable help, but unfortunately there is more to do."