AbstractThe modifying effect of a dietary fiber, Fibeta (beet fiber), on experimentally induced colorectal cancer was studied in Wistar rats. The rats were fed a powdered semisynthetic casein‐based diet in which the carbohydrate pool was substituted with Fibeta as the sole source of fiber. Dimethylhydrazine dihydrochloride (DMH‐2HCI) was used as initiator in a dose of 20 mg/kg body wt and given by gavage once a week for 10 weeks. Throughout the experiment the rats were offered the diets with different levels of fiber in a preinitiation period of 8 weeks, during the initiation, or in a 30‐week postinitiation period. The study was terminated after one year.A protective effect of the fiber was not found at any stage of the colorectal carcinogenic process. Even though differences (not statistically significant) in tumor incidences were seen, these did not reflect any effect of the high or low fiber intake during the study.Analysis for volatile fatty acids in cecal content showed that continuous feeding with a fiber‐rich diet resulted in significant increase in most of the volatile fatty acids. The relative change was highest for butyric acid. These findings do not support the hypothesis that butyric acid has a protective effect on colorectal cancer. The tumor yield in the present study was low compared with that reported in the literature, and possible causes for this are discussed.