In general, many large epidemiological and case-control studies have resulted in data describing conflicting outcomes examining the relationship between colorectal cancer and fiber intake. A combined analysis of 13 case-control studies found relative risks of colon cancer to be 0.53 with high versus low fiber intakes. Similarly, a case-control study of 2000 cases found intakes high in fiber correlated with a low relative risk. In contrast, large epidemiological studies such as the Nurses’ Health Study(48) and the Health Professionals Study did not find any such relationships. Lupton (2000) suggests that a higher intake in fermentable fiber is more protective because it results in lower colonic pH and production of the short-chain fatty acid butyrate, a primary energy source for colon cells. The recent review from Obrador examined the evidence supporting the role of dietary fiber in the prevention of colorectal cancer. While most epidemiological and intervention studies analyzed in this review show little strength in terms of cancer prevention, meta-analyses on case control studies report reductions of up to 50% in the risk of colorectal cancer associated with higher intakes of dietary fiber. However, the evidence remains weak since case-control designs are often associated with various biases, namely recall bias and the influence of the subject’s health status on the validity of self-reporting.

Among factors contributing to the inconsistency in the data available are the source and form of fiber. While some studies have shown both soluble and insoluble sources to be protective, others state that insoluble fiber is the only form that could play a role in cancer prevention. The diet accompanying fiber intake could play an important role as well, as it is hypothesized that the type of fat consumed affects gut susceptibility to cancer. Namely, oils that induce apoptosis, such as fish oil, may be more protective. Confounding the effect of dietary fiber on colorectal cancer is the natural presence of phytochemicals in such food groups as fruits, vegetables and whole grains. In fact, polyphenols and others have shown great antioxidant, antiproliferative and apoptotic activities and could be responsible for the anti-carcinogenic effect of high-fiber foods.

Although there seems to be a significant body of evidence in favor of dietary fiber as a prevention of colorectal cancer, data from well-designed studies characterized by large sample sizes show no effect of dietary fiber on cancer development or progression. Confounding factors such as composition of meals accompanying fiber intake and antioxidant properties of fiber-rich foods have made the relationship between dietary fiber and colorectal cancer inconclusive, creating the need for further research to isolate the effect of fiber on cancer development. Moreover, it should be noted that cancer itself is a complex disease that can arise secondary to other health conditions, diet and lifestyle. Health claims pertaining to colorectal cancer prevention cannot be justified based on the current literature, however given the natural benefits of fiber containing foods, a potential health claim which states that regular consumption of fiber may decrease the risk of colon cancer could be appropriate in orienting the consumer towards healthier food choices. In conclusion, evidence shows that soluble fiber reduces the risk of cardiovascular disease, and that insoluble fiber protects against disturbances of the digestive tract. Although more extensive research is needed to confirm the beneficial role of dietary fiber in the prevention of colorectal cancer and diverticulosis, the fact remains that fiber is an important and safe component of a healthy diet and has been positively linked to lower incidences of these diseases.