Large intestinal pH and ammonia in rats: dietary fat and protein interactions.

Mature male Sprague-Dawley rats were assigned to a 3 x 3 factorial experiment in which they were fed AIN-76A diets supplying 8, 16 and 32% of energy as protein and 12, 24 and 48% of energy as fat. During the 5 mo of feeding, 10 in vivo measurements of intracolonic pH were recorded on each rat with a flexible electrode. The pH ranged from 7.8 to 8.0 near the anus and declined to 7.4 to 7.5 at 12 cm from the anus. The mean percentages of dry matter in the contents of the colon, divided into three approximately equal segments, were as follows: proximal colon, 35; middle colon, 45; distal colon, 58. Ammonia concentrations in luminal fluid rose significantly with higher protein intake in the cecum, proximal colon and distal colon. The concentrations in the distal colon ranged from 39 to 74 mmol/L, depending upon protein intake. Thymidine incorporation by distal colon mucosal cells was higher in rats fed 32% of energy as protein and 48% of energy as fat compared with rats fed 8% of energy as protein and 12% as fat. The evidence suggests that increased intestinal cell proliferation in rats fed the high protein, high fat diet was due to greater concentrations of ammonia in the large intestine resulting from the high protein intake and greater concentrations of non-ionized ammonia resulting from the higher pH associated with increased fat intake. The actual determinations and calculations of ionized to non-ionized ammonia concentrations were compatible with the assumption that the large intestinal cells absorbed more ammonia at higher fat intakes.