Primary dietary prevention: is the fiber story over?

Colorectal cancer is a major cause of morbidity in developed countries, and epidemiological and experimental research suggests that environmental factors, particularly diet, may play a key etiologic role. Among the various dietary factors that have been proposed to affect the risk and progression of colon cancer, dietary fiber has been of greatest interest, due to the effects of fiber on the function of the large bowel. Dietary fiber is a heterogeneous group of compounds, consisting of the remnants of plant cells resistant to hydrolysis by human alimentary enzymes. Several case-control studies and a few cohort studies have linked higher fiber intake to reduced risk for colorectal cancer, although the results of these observational studies have been inconsistent. In the large European Prospective Investigation into Cancer and Nutrition observational study, higher dietary fiber from foods was associated with an estimated 25% reduction in risk for large bowel cancer. However, no significant relationship between fiber intake (or major food sources of fiber) and risk for colorectal cancer was observed in a recently reported large pooled analysis of several cohort studies. Well-known limitations of observational studies, particularly relating to the collection and interpretation of dietary data, constrain conclusions from these studies. To date, intervention studies testing the relationship between dietary fiber and colon cancer have focused on whether fiber supplementation or diet modification can affect the risk for adenoma recurrence and growth in individuals with a history of adenomatous polyps. In four of these intervention studies, subjects in the intervention arm were prescribed dietary fiber supplements, and beneficial effects on adenoma recurrence were not observed over 3-5 years of follow-up. In a large randomized U.S. study, the Polyp Prevention Trial, the effect of prescribing diet modification (increased fiber and reduced fat intakes) was tested, and no effects on adenoma recurrence were observed, although dietary biomarker data suggest that the change in dietary intakes in the intervention arm was not substantial. The effect of increased dietary fiber intake on risk for colorectal cancer has not been adequately addressed in studies conducted to date. Longer-term trials and higher levels of fiber intake are strategies that have been suggested to increase knowledge in this area. Also, laboratory and clinical studies that continue to provide insight into biological mechanisms may help to better target intervention efforts.