The aim of this study was to evaluate the effect of fiber-enriched defined formula diets (DFDs) on radiation-induced enteropathy. Forty-five male Sprague-Dawley rats were assigned randomly after abdominal irradiation to one of three groups (15 in each group): a fiber-free DFD group, a non-soluble fiber-enriched DFD group, and a soluble fiber-enriched DFD group. They kept their diets respectively for seven days. On day eight, the mesenteric lymph nodes were harvested for bacterial translocation, and segments of jejenum and colon were sampled for microscopic examination. The rats in the fiber-enriched DFD groups lost significantly less body weight than the rats in the fiber-free DFD group. The intestinal structure was the worst in the fiber-free DFD group, intermediate in the soluble fiber-enriched DFD group, and the best in the non-soluble fiber-enriched DFD group with significantly higher measures of villous height and jejunal mucosal thickness. These findings suggest that fiber-enriched DFD may effectively protect intestinal structure against radiation-induced damage by improving mucosal integrity. (C) 2003 Elsevier Science Inc. All rights reserved.