The Interrelations of Faecal Ammonia, pH and Bicarbonate: Evidence of Colonic Absorption of Ammonia by Non-Ionic Diffusion

Abstract

1. Ammonia, bicarbonate and pH were measured in samples of faecal dialysate from thirteen healthy subjects taking free diets. To observe the effect of marked changes in faecal pH, three subjects were also studied while taking 25 mmol/day of MgCO, or Na,S04 by mouth. Both salts increased stool weight without causing diarrhoea, but stool pH was significantly increased by MgCO, and decreased by Na2S0,.2. The total ammonia concentration and pH of faecal dialysate were very variable, but showed a highly significant negative correlation similar to that already established in man between urinary excretion of ammonia and urine pH. This relationship was more marked when individual subjects were studied while faecal pH was deliberately varied by administration of MgCO, and Na,SOa.3. Faecal bicarbonate concentrations were positively correlated with pH. Faecal Pco, was usually in the range 40-120 mmHg, the higher Pco, values being found in the more acid samples. Faecal total ammonia concentrations were negatively correlated with faecal bicarbonate. 4. These findings suggest that passive non-ionic diffusion is the main mechanism by which ammonia is absorbed by the colon, but do not exclude a minor contribution from diffusion of ionized ammonium. Colonic secretion of bicarbonate facilitates non-ionic diffusion of ammonia by providing an anion which is also absorbed by non-ionic diffusion, so maintaining an alkaline intraluminal reaction that continues to generate unionized ammonia.

Lactulose and Combination Therapy of Hepatic Encephalopathy: The Role of the Intestinal Microflora

“…However, the changes in pH that were nec essary to show large changes in ammonia transport were much greater [20] than seen after the administration of lactulose. Fur thermore, the increase in fecal ammonia seen after luminal acidification alone was trivial (approximately 22 mg/day) [21] and could account for only 1% of the total in crease in fecal nitrogen output caused by lac tulose [22], These findings indicate that oth er mechanisms are of much greater impor tance in explaining the effect of lactulose on ammonia metabolism.…”

“…Certainly lactulose did acidi fy the colonic lumen, and alterations in pH could influence ammonia transport [20,21]. However, the changes in pH that were nec essary to show large changes in ammonia transport were much greater [20] than seen after the administration of lactulose.…”

Influence of ammonia on sodium absorption in rat proximal colon

“…Ammonia 1 is produced physiologically in the large intestine of mammals by bacterial hydrolysis of urea and by degradation of protein that has escaped digestion in the small intestine [8,39,40,41]. Concentrations of up to 40 mmol l -1 have been measured in the luminal fluid of the hindgut [29,40].…”

Bacterial Short Chain Fatty Acids: Their Role in Gastrointestinal Disease

“…The clinical benefit from the use of lactu lose in chronic portal systemic encephalopa thy is attributed to colonic fermentation, either due to increased nitrogen incorpora tion into bacteria [77] or its effect on ammo nia metabolism, as well as the low luminal pH induced by SCFAs which traps ammonia [78,79], Diets of vegetable protein have also been shown to be effective in chronic portal systemic encephalopathy [80], This may at least partly be attributed to the effects of colonic fermentation and SCFA generation.…”