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Fiber May Not Guard Against Cancer After All

December 13, 2005
That fiber we’ve been eating for all these years to lower the risk of colorectal cancer may not have made a difference after all.

In an analysis combining data from 13 studies, high intake of dietary fiber was not associated with reduced risk of colorectal cancer, according to a report in the December 14 issue of JAMA, the Journal of the American Medical Association.

Dietary fiber has long been thought to reduce the risk of colorectal cancer. But that belief didn't hold up to sustained analysis.

"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," the researchers write.

The researchers were quick to add that there are still plenty of reasons to eat a high-fiber diet.

"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," they conclude.

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, and randomized clinical trials of dietary fiber supplementation have failed to show reductions in the recurrence of colorectal adenomas, which can be precursors of cancer.

Yikyung Park, Sc.D., 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.

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.

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 increased 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 an accompanying editorial, John A. Baron, M.D., of Dartmouth Medical School, Lebanon, N.H., writes. "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 adds.

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