Dietary fiber stimulates the extra-renal route of nitrogen excretion in partially nephrectomized rats

The purpose of this study was to determine the effect of an indigestible carbohydrate/dietary fiber (the oligosaccharide/fiber blend, or O/F blend) on extra-renal nitrogen excretion in nephrectomized rats. The O/F blend provided a diversified source of soluble and insoluble fibers: 41.2% fructooligosaccharides, 26.4% insoluble oat fiber, 17.7% soy polysaccharides, 10.3% gum arabic, and 4.4% carboxymethylcellulose. Forty partially nephrectomized rats were randomized to one of four dietary treatments that varied in protein and O/F blend content. Dietary treatments were administered for 17 days tan adaptation phase of IO days, followed by an experimental phase of 7 days) and included the following treatment groups: (1) 8% casein, 0% O/F blend, (2) 8% casein, 8% O/F blend, (3) 14% casein, 0% O/F blend, and (4) 14% casein, 8% O/F blend. Compared with nonnephrectomized normal rats (n = 10), the nephrectomy procedure induced a marked renal insufficiency, with significant increases in plasma urea and creatinine concentrations (90% and 44%, respectively). In this model of renal insufficiency, feeding the fermentable fibers had the same effect as in previous studies in normal rats: The O/F blend increased cecal weight and cecal bloodflow, leading to accelerated diffusion of blood urea into the cecal lumen (by threefold), urealysis to ammonia and protein synthesis by the microflora, and increased fecal excretion of nitrogen. The efficiency of O/F blend in stimulating the extra-renal route of nitrogen excretion was greatly enhanced by the simultaneous use of a low protein rate: Fecal nitrogen excretion accounted for 23% of total nitrogen excretion in rats fed a 14% casein diet, compared with 45% in those receiving the 8% casein diet. The increase of fecal nitrogen excretion was accompanied by an equal and significant decrease in urinary nitrogen excretion and a decrease in blood urea nitrogen. In conclusion, addition of an O/F blend to a low protein diet exerts a potent urea lowering effect in renally insufficient rats, suggesting that these dietary conditions could help delay the progression of renal failure or prevent its consequences. (C) Elsevier Science Inc. 1998.