High Intake Of Dietary Fiber Not Associated With Reduced Risk Of Colorectal Cancer

Dietary fiber has been hypothesized to reduce the risk of colorectal cancer, according to background information in the article. However, the results of numerous epidemiological 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. But most prospective cohort studies have found no association between dietary fiber intake and risk of colorectal cancer or adenomas (precursors of colorectal cancer), and randomized clinical trials of dietary fiber supplementation have failed to show reductions in the recurrence of colorectal adenomas.

Yikyung Park, Sc.D., formerly of the Harvard School of Public Health, Boston, and colleagues evaluated the association between dietary fiber intake and risk of colorectal cancer by reanalyzing the primary data from 13 prospective cohort studies (Pooling Project of Prospective Studies of Diet and Cancer). The pooled analysis included 725,628 men and women who were followed-up for 6 to 20 years across studies.

During the follow-up, 8,081 colorectal cancer cases were identified. Among the studies, median (midpoint) energy-adjusted dietary fiber intake ranged from 14 to 28 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. 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.

"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," the authors write.

"In conclusion, we did not find support for a linear inverse association between dietary fiber intake and risk of colorectal cancer in a pooled analysis of 13 prospective cohort studies. Although high dietary fiber intake may not have a major effect on the risk of colorectal cancer, 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," the researchers write.

(JAMA.2005; 294:2849-2857. Available pre-embargo to the media at www.jamamedia.org)

Editor's Note: The study was funded by research grants from the National Institutes of Health and by the National Colorectal Cancer Research Alliance. Dr. Park is now with the National Cancer Institute.

Editorial: Dietary Fiber and Colorectal Cancer - An Ongoing Saga

In an accompanying editorial, John A. Baron, M.D., of Dartmouth Medical School, Lebanon, N.H., examines the results of the pooled analysis.

"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. ... Over the short term, wheat fiber or psyllium [soluble fiber] interventions do not seem to affect colorectal carcinogenesis, 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," Dr. Baron writes.

(JAMA.2005; 294:2904-2906. Available pre-embargo to the media at www.jamamedia.org)