Independent effects of fiber and protein on colonic luminal ammonia concentration

The potential interactive effects of protein and fiber on cecal and colonic surface areas, colonic luminal ammonia concentrations, luminal pH and blood indices of nitrogen metabolism were tested using two levels of protein (8% and 24%) and two types of fiber (8% pectin or cellulose). Pectin supplementation resulted in larger cecal surface areas and longer large intestines than those of rats fed fiber-free or cellulose-supplemented diets. All high protein diets resulted in total large bowel luminal ammonia (NH3 + NH4+) concentrations that were twice as high as their low protein counterparts (P less than 0.05). The effect of fiber on ammonia concentration depended on the fiber type. In the distal colon, pectin-fed animals had three times the ammonia concentration of the fiber-free animals, and 4-5 times the ammonia concentration of the cellulose-fed animals (P less than 0.001). Blood urea nitrogen values were higher in the high protein than in the low protein groups (P less than 0.05), and highest in the high protein/pectin animals (P less than 0.01). This study clearly demonstrates that luminal ammonia concentration is dependent upon both protein level and fiber type, and that a fermentable fiber (pectin), rather than decreasing colonic ammonia concentrations, actually increases them several-fold.