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.
Dietary fiber encompasses a wide range of vegetable and fruit substances. It is quite possible that some subtypes of fiber may indeed prevent colon cancer. It was not possible in the study reported in the New England Journal of Medicine to analyze for associations with fiber subtypes. Moreover, in the study, the group of individuals with the highest fiber intake was ingesting only an average of 25 grams of fiber a day, a large amount compared to most persons in the US, but still a relatively small amount compared with persons consuming the highest fiber diets. Perhaps higher amounts of fiber would be protective. Finally, there is the issue of the timing of exposure to fiber, which was not possible to study. Specifically, it may be that fiber intake during childhood affects the development of colon cancer at a later date. Since it was the fiber intake of the nurses as adults that was studied, an association of cancer with fiber intake in childhood may have been missed.
So, what conclusions can be reached? Does the study prove that fiber does not prevent colon cancer? No, it does not. Are the results important? Yes, they are, because the study was as well done as any study of this type could be, and the number of subjects was very large. The results are indeed a setback to the theory which positively linked fiber consumption to a reduced risk for colon cancer, though it is not a fatal setback.
Colon cancer is a multi-factorial disease, that is, there are many genetic, dietary, and environmental promoters and inhibitors of the formation of colon cancer. It is unlikely that any one factor, such as fiber, plays a dominant role. Dietary guidelines that target obesity as well as fat and calcium intake could be more important than fiber. For example, a recently-published, scientifically strong study (New England Journal of Medicine 1999;340:101-7) demonstrated that calcium supplements prevent the formation of colon polyps, which are believed to be the precursors of colon cancer.
Finally, in view of the other demonstrated benefits of higher fiber diets with respect to heart disease, high blood pressure, diabetes, and colonic diverticular disease (diverticulosis), higher dietary intakes of fiber can continue to be recommended. If fiber also helps prevent colon cancer, all the better.