Since the original suggestion by Denis Burkitt that dietary fiber might protect against colon cancer,48 numerous case-control and a few cohort studies have investigated this relationship. Most, but not all, have supported the original hypothesis of a protective role for fiber in the etiology of colon cancer. A recent review reported that 11 out of 17 case-control studies found an inverse relationship between dietary fiber intake and colorectal cancer.49 Another review and meta-analysis (based on 5255 patients with colorectal cancer) also found a significant inverse relationship between dietary fiber and colorectal cancer risk. An overall relative risk of 0.53 comparing those in the highest quintile with those in the lowest quintile was calculated; it persisted after controlling for adjustment for other nutrients and for vitamin C and beta-carotene).50
The few cohort studies of this question, however, have offered less convincing evidence. Although both the Iowa Women's Health Study51 and the Nurses Health Study52 have demonstrated some reduction in risk of colorectal cancer with increasing fiber intake (relative risks on the order of .8-.9), the Health Professionals Study found no protection after controlling for potential confounding.53 In addition, neither a study among Dutch Civil Servants54 nor the Hawaiian study of Japanese American men55 found any convincing reduction in risk with increasing fiber consumption.
The conflicting results from case-control and cohort studies are difficult to reconcile. Recall bias is a serious limitation in the conduct of case-control studies, and the overall protective role of dietary fiber in colorectal cancer may be more modest than suggested by these case-control studies.
There are few studies on the relationship between dietary fiber intake and cancer risk for cancers other than colon/rectum. Studies on breast cancer show mixed results for fiber,56-58 but a more consistent, protective role for fiber appears evident in regard to pancreas cancer.59
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