Hypothesis Fiber from Fruit and Colorectal Neoplasia

The role of fiber intake in the etiology of colorectal cancer and precursor adenomatous polyps has been controversial. Although a clear inverse association was observed in several case-control studies (reviewed in refs. 1, 2), large prospective studies subsequently found inconsistent associations between intake of dietary fiber and colorectal cancer (3-8). The lack of an influence was further supported by the findings from intervention studies of dietary modifications designed to increase total fiber intake (9) and of wheat bran (10) and ispaghula husk (11) fiber supplements on risk of recurrent colorectal adenomas. Taken together, the findings from the prospective studies and clinical trials cast doubt on the protective effect of dietary fiber. With the findings of two studies reported in Lancet in 2003, the controversy continues (12, 13). The latest observational findings come from two large studies conducted in the United States and Europe. Data from the Prostate, Lung, Colorectal, and Ovarian Screening Trial and the European Prospective Investigation into Cancer and Nutrition cohort were used to examine the association between dietary fiber and colorectal adenoma or cancer, respectively. Both studies observed statistically significant inverse associations of dietary fiber intake with colorectal neoplasia. Several hypotheses have been put forth addressing the question of why results from the fiber studies have been so inconsistent. Some of the reasons given include differences in study design, end points, study population characteristics, distribution of fiber intake, and the array of confounders that were controlled for (14). The role of confounding may be of particular importance. Fiber intake is associated with many healthy lifestyle choices, making it difficult to disentangle the effect of fiber from several modifiable factors. For example, people who eat diets rich in fiber also tend to eat less red meat, smoke less, drink less alcohol, and exercise regularly (15). Careful adjustment for these factors is essential in investigations of fiber and colorectal cancer. Different sources of dietary fiber preferentially consumed within cohorts of people may also contribute to the conflicting results found in investigations of overall dietary fiber (16). Dietary fiber primarily consists of plant cell walls. These plant cell walls vary tremendously in structure and composition (17). In experimental studies, fibers derived from different plant cell wall types have been found to have diverse effects on the potential for carcinogenesis (18, 19). The type or source of fiber consumed also may be important in conferring protection against human colorectal cancer. Therefore, we briefly examined the associations of different sources of fiber among several recent prospective cohort studies and clinical trials. Table 1 provides the estimates of association of dietary fiber with colorectal neoplasia by source of fiber. Relative risk estimates for total dietary fiber or fiber supplements have ranged from inverse associations to no association with the exception of an intervention study that reported a statistically significant elevated odds ratio (OR) of 1.67 (11). When investigating source of fiber within the prospective studies and the two recent large observational studies, modest inverse associations are found for fruit fiber in some of the studies (see Table 1). The relative risk estimates for fruit fiber range from 0.78 to 1.10, with all but two of the eight point estimates <1.00. Given the differences among the various studies in designs, populations, and assessment tools, these modest yet suggestive associations for fruit fiber may provide some insight into the etiologic role of fiber source and composition in colorectal carcinogenesis. A moderate to high proportion of fruit fiber from commonly consumed fruits is soluble fiber (largely pectin). Oats (largely gums), legumes (largely gums), and some vegetables (largely pectin), including carrots, cabbage, Brussels sprouts, squash, and broccoli, also contain higher proportions of soluble fiber, whereas wheat bran fiber primarily consists of insoluble fiber. For example, 58% of the fiber content of an orange and 44% of the fiber content of an apple are soluble fiber (20). Soluble fiber estimates for beans, oats, and vegetables range from 15% to 20% of total fiber (20). On the other hand, only 7% of the total dietary fiber in all-bran cereals are soluble fiber (21); the insoluble component is primarily cellulose. Potential mechanisms of action for soluble fiber that might be hypothesized to reduce the risk of colorectal neoplasia may be through its influence on insulin and glucose control or on the production of short-chain fatty acids. Soluble fiber slows the absorption of glucose from the small intestines, which may reduce hyperinsulinemia Cancer Epidemiol Biomarkers Prev 2004;13(8):1267 – 70