Fermentable carbohydrate and digestive nitrogen excretion.
BACKGROUND
Interventions that restrict protein intake lower plasma urea concentration and may slow the progression of renal failure. The question arises whether the effect of a dietary protein restriction could be reinforced by enrichment of the diet with fermentable carbohydrate because these carbohydrates may stimulate the extra-renal route of nitrogen (N) excretion through the digestive route.


METHODS
The influence of fermentable carbohydrate and moderate protein restriction on N metabolism was investigated in a rat model of renal failure with ablation of 70% of renal mass compared with control rats with intact kidneys. Animals were adapted to diets varying with respect to nondigestible fermentable carbohydrate (0% or 10% fructooligosaccharide [FOS]) and with respect to protein content (10% or 18% casein).


RESULTS
Feeding FOS led to a considerable enlargement of the cecum (increase in contents, wall thickness, and blood flow). These changes resulted in a concomitant enhancement of urea N uptake into the cecum and a decrease in plasma urea concentration (-30%). The extent of urea uptake by the cecum was influenced by plasma urea level that was determined by the dietary protein level and by the renal function. Thus, compared with control rats, the rate of urea uptake by the cecum and the total N excreted by the uremic rats was greater under all nutritional conditions. It is noteworthy that, when expressed as a percentage of total N excretion, fecal N excretion nearly doubled in rats adapted to the low-protein diets containing FOS. These effects occurred in both control rats and in uremic rats, in which a 22% decrease in urinary N was recorded as a result of FOS in addition to the low-protein diet. Globally, decreasing the amount of protein in the diet and adding a fermentable carbohydrate led to a decrease in urinary N excretion of more than 65% in uremic rats.


CONCLUSION
These results suggest a possible usefulness for combining fermentable carbohydrate, such as FOS, with a low-protein diet to increase N excretion through the digestive route in detriment of the renal route. This may represent an efficient preventive measure to relieve the renal function in case of renal failure.