FERMENTABLE CARBOHYDRATE EXERTS A UREA-LOWERING EFFECT IN NORMAL AND NEPHRECTOMIZED RATS

The influence of nondigestible carbohydrate on intestinal fermentation s and on the route of nitrogen excretion has been investigated in norm al rats and in unilaterally nephrectomized rats. Rats were adapted to 10% casein diets, either fiber free or containing different levels of two fermentable carbohydrates, inulin or crude potato starch. Ingestio n of fermentable carbohydrate led to a considerable enlargement of the cecum because of hypertrophy of the cecal wall and an increase in cec al contents. Cecal digesta contained elevated concentrations of short- chain fatty acids, resulting in acidic pH. Diets containing fermentabl e carbohydrate enhanced fecal nitrogen excretion, which was more than doubted at the highest level of inulin or potato starch. In parallel, urinary nitrogen excretion was significantly decreased by fermentable carbohydrate. Although these changes were similar in all animals, ther e were quantitative differences in the response of nephrectomized anim als to fermentable carbohydrate. In nephrectomized rats, plasma urea c oncentrations were more than 2.5 times higher than in normal rats (5.8 mM compared with 2.2 mM). Plasma urea concentrations were reduced by similar to 50% when normal rats were fed diets containing 7.5-15% inul in or 10-20% resistant starch. In nephrectomized animals fed the highe st level of fermentable carbohydrate, plasma urea concentrations were also significantly decreased, but only by 30%. In nephrectomized rats, the concentration of nitrogen cycling in the cecum was greater (urea nitrogen transfer into the cecum was 50-60% greater and ammonia flux f rom the cecal lumen to the blood was two times higher than in normal r ats), but fecal nitrogen excretion was equivalent in normal and nephre ctomized animals. When expressed as a percentage of total nitrogen exc retion, fecal nitrogen excretion was <20% in animals fed fiber-free di ets, compared with 45-50% in normal animals and 40% in nephrectomized animals fed fermentable carbohydrate.