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The effects of different fiber types and processing on putative protective mechanisms for colorectal cancer were evaluated. Rats were fed diets of similar nutrient balance containing either no added fiber or 10% fiber from various sources. The rate of distal colonic epithelial proliferation, measured by the metaphase arrest method, was dependent on fiber type; ranking of fibers from highest to lowest yielded the following order: methylcellulose > coarse wheat bran > fine wheat bran approximately parboiled and extruded rice brans > no fiber (P = 0.012). Effect on stool output ranked identically. Ranking of effect on fecal pH, from most to least acidic was as follows: coarse wheat bran approximately the rice brans > fine wheat bran > no fiber approximately methylcellulose (P = 0.00001). Coarse wheat bran gave significantly higher fecal butyrate concentrations than did the rice brans, which in turn gave higher levels than fine wheat bran, methylcellulose and the no-fiber diet. Proximal colon epithelial proliferation was unaffected by diet although cecal short-chain fatty acid concentrations and pH were affected. Different fibers have different effects on events in the fecal environment and distal colonic epithelium. Putative protective events (increased output, low fecal pH, high butyrate, low proliferation) are not equally affected and are unlikely in themselves to allow prediction of the protective effect of a fiber.

The amount and type of dietary fibre ingested influences colonic luminal characteristics, especially the concentration of carbohydrate fermentation products such as butyrate. This study aimed to assess whether diets supplemented with fibres of differing fermentability (delivering different amounts of butyrate to the colon) influence mucosal activities of urokinase and brush border hydrolases, and epithelial turnover. Groups of five rats were fed one of four diets containing low (2%), highly fermented (guar 10% or oat bran 10%) or slowly fermented fibre (wheat bran 10%) for 4 weeks. Activities of urokinase, alkaline phosphatase, dipeptidyl peptidase IV and maltase were measured in mucosal homogenates of proximal and distal colon and from rectum. Proliferative kinetics were assessed in distal and proximal colon by the metaphase arrest technique. Hydrolase activities were similar across all four dietary groups but a significant difference was found for urokinase (P = 0.014). This was due to a reduction in urokinase activities of > 30% at the three sites in the wheat bran group compared with the other groups. Of proliferative indices, only crypt column height differed across the groups (P = 0.038) and was highest in rats fed wheat bran and lowest in those fed the low fibre diet (P = 0.047). The proportion of mitoses in the top one-fifth of the crypt also differed across groups (P = 0.038) due to the high values in the distal colon of the low fibre group. Thus, addition of a slowly fermented (but not highly fermented) fibre to the diet of rats reduces net urokinase activity in large bowel mucosa and increases the life span of colonic epithelial cells without changing activities of brush border hydrolases.