Despite provision of adequate calories, defined formula diets in rats lead to bacterial translocation (BT), fatty infiltration of the liver, and an increased susceptibility to endotoxin. These deleterious effects may be due in part to a loss of intestinal barrier integrity resulting from bowel atrophy. Defined formula diets lack both glutamine and fiber, substances which may help maintain intestinal mass. To determine whether supplementation of defined formula diets with either glutamine or fiber might prevent bowel atrophy and, thus, BT, hepatic steatosis, and the altered response to endotoxin, Wistar rats were fed (1) defined formula diet ad libitum (DFD), (2) (DFD + 2% (w/v) glutamine, (GLUT), or (3) DFD + 2% (w/v) psyllium (FIBER). Rats given standard food isocalorically pair-fed to DFD were used as controls. Nutritional status was assessed by daily weight gain, as well as the ability to maintain serum albumin, hematocrit and white blood counts. After 2 weeks of these feeding regimens, animals were sacrificed, and organ weights and composition were determined, with rates of bacterial translocation determined by mesenteric lymph node, abdominal viscera, and cecal cultures. Additional animals receiving the same experimental diets were subsequently challenged with endotoxin and observed for mortality with rates of post-endotoxin BT and the responses of acute phase proteins and cytokines measured. All dietary regimens resulted in equivalent weight gain and other nutritional parameters. Both glutamine and fiber supplementation maintained small bowel mass, but only GLUT preserved normal jejunal mucosal architecture. Neither fiber nor glutamine supplementation prevented cecal bacterial overgrowth or BT, resulting from the DFD.(ABSTRACT TRUNCATED AT 250 WORDS)