Role of Fermentable Carbohydrate Supplements in the Course of Uremia

During the past few years, considerable attention has been given to the impact of nutrition on kidney disease. Interventions that restrict protein intake lower the plasma urea concentration, alleviate adverse clinical symptoms, and may slow the progression of chronic renal failure (CRF). Although these studies provide a logical explanation for a relationship between a low protein diet and altering the progression of functional renal deterioration, this beneficial effect is often accompanied by some muscle wasting and malnutrition. The question arises whether the effect of a moderate dietary protein restriction could be reinforced by enrichment of the diet with fermentable carbohydrates, since these carbohydrates may stimulate the extra-renal route of nitrogen (N) excretion, via the fecal route. At a physiological level, there is two routes to eliminate N: urine and feces. Our results show that it is possible to increase the N fecal route excretion by reducing the protein supply and increasing fermentable carbohydrate availability. This additional dietary manipulation seems to be very interesting in case of renal deficiency in animal models and in humans. The modification of the urea N enterohepatic cycling by fermentable carbohydrates is likely to promote proliferation of the large intestine microflora, which could be an interesting approach to deviate a part of the N excretion to feces as a consequence of N utilization by the bacteria. Conversely, a large proportion of the N released by urea hydrolysis in the digestive tract may be recycled, via the reabsorption of ammonia, and could contribute thus to reduce the need of N. In the domain of preventive nutrition, the first recognized effect of dietary fibers was to regularize the digestive transit, especially in the case of constipation. However, during the past decades, some other interesting effects have been reported, especially lipid-lowering effects, improvement of blood glucose control, reduction of colon cancer risk and increase of the availability of some cations. In contrast, the efficacy of the dietary fibers in the treatment of CRF has been less investigated although the availability of dietary fibers should profoundly alter the microflora metabolism and proliferation, and as a result N metabolism. The aim of this chapter is to review, for the course of CRF, the capacity of an additional manipulation of diet in increasing fermentable carbohydrates which can stimulate the transfer of urea into the large intestine and shift N excretion from the urine to the feces.