Dietary fiber and colorectal neoplasia

PURPOSE: Dietary fiber has been implicated in colorectal neoplasia, despite conflicting evidence. This is a review of the currently available data on the role of dietary fiber in colorectal carcinogenesis. METHODS: A literatu re search was conducted using the MEDLINE database. Ail case-control, longi tudinal, and randomized, controlled studies published in English between 19 88 and 2000 were identified, as were animal model studies in the period 198 6 to 2000. Data from the various studies were tabulated and systematically analyzed, with particular emphasis on the effect of dietary fiber on tumor incidence and luminal parameters such as short chain fatty acids. RESULTS: Epidemiologic correlation studies show a high intake of dietary fiber to be associated with a lower risk of colorectal neoplasia. Thirteen of the 24 c ase-control studies reviewed demonstrated a protective effect of dietary fi ber against colorectal neoplasia, and 16 showed a protective effect of vege tables or vegetable fiber. On the other hand, of 13 longitudinal studies in various cohorts, only 3 demonstrated a protective effect of fiber and 4 a protective effect of vegetables or vegetable fiber. The fn e published rand omized, controlled trials all investigated the effect of increased fiber in take on short-term adenoma recurrence; however, none showed any significant protective effect. Among 19 experimental studies in animal models, 15 show ed a protective effect of fiber against tumor induction compared with contr ols. Animal studies also showed that poorly fermentable fibers (e.g., wheat bran and cellulose) were more protective than soluble fibers (e.g., guar g um and oat bran), which sometimes enhanced carcinogenesis. No clear correla tion was found between luminal pH or short chain fatty acids and tumor indu ction. CONCLUSIONS: On the basis of current data, there is little evidence to support the use of dietary fiber supplements to reduce the risk of color ectal neoplasia. Lifelong and early exposure may be important but are diffi cult to study. Other risk factors interact with the effects of dietary fibe r.