High Dietary Fiber Intake May Not Reduce The Risk Of Colon Cancer
Since the early 1970's when it was first proposed, the theory that dietary fiber prevents colon cancer has become popular. Studies attempting to determine if dietary fiber can prevent colon cancer in humans have relied on observational, correlative studies. These are studies of large numbers of individuals for whom the intake of dietary fiber is correlated with the rates of colon cancer. Such studies cannot prove or disprove that dietary fiber prevents colon cancer. Nevertheless, if fiber does indeed prevent colon cancer, then higher intakes of fiber should be associated with lower rates of colon cancer.
In fact, studies in humans have been inconclusive. Some studies have shown an association of higher fiber intake with a reduction in colon cancer. Others have not. In numerous animal models for studying colon cancer, several types of fiber have clearly been shown to prevent the formation of cancer. The difference is that it is relatively easy in animals and difficult in people to establish that dietary fiber prevents colon cancer using sound, experimental, scientific methods.
The largest study examining the association between the incidence of colon cancer and dietary fiber consumption was published (New England Journal of Medicine 1999; 340:169-76). The authors report that they found no such correlation in a study of 89,000 US nurses. This finding suggests that the hypothesis that dietary fiber prevents colon cancer is false or, at least, that the effect of fiber is too insignificant to be discerned. Moreover, there was no association between fiber intake and the development of colon polyps, which are believed to be precursors of colon cancer.
Should these findings be accepted? The study was very well executed and probably could not have been better conducted. The potential problems lie not in the quality of the study, but the complexity of the issue.