The effect of lactulose, pectin, arabinogalactan and cellulose on the production of organic acids and metabolism of ammonia by intestinal bacteria in a faecal incubation system.

An in vitro faecal incubation system was used to study the metabolism of complex carbohydrates by intestinal bacteria. Homogenates of human faeces were incubated anaerobically with added lactulose, pectin, the hemicellulose arabinogalactan, and cellulose, both before and after subjects had been pre-fed each carbohydrate. Fermentation of added substrate was assessed by the production of short-chain fatty acids (SCFA) and suppression of net ammonia generation over 48 h of incubation. Control faecal homogenates to which carbohydrate was not added yielded an average increment of SCFA of 43 mmol/l, equivalent to 172 mmol/kg in the original stool. The addition of lactulose, pectin and arabinogalactan each increased the yield of SCFA by a similar amount, averaging 6.5 mmol/g carbohydrate or 1.05 mol/mol hexose equivalent; organic acid yield was not increased by pre-feeding these substances for up to 2 weeks. Acetate was the major SCFA in all samples at all times and, after pre-feeding with extra carbohydrate, butyrate concentrations exceeded propionate in all samples. Faecal homogenates incubated with cellulose showed no greater SCFA production than controls over the first 48 h, but there was a slight increase when samples from two subjects pre-fed cellulose were incubated for 14 d. Net ammonia generation was markedly suppressed by addition of lactulose to faecal incubates with an initial period of net bacterial uptake of ammonia. Pectin and arabinogalactan also decreased ammonia generation, but the reductions were not significant unless subjects were pre-fed these materials; cellulose had no effect on ammonia generation.

The contribution of endogenous urea to faecal ammonia in man, determined by 15N labelling of plasma urea.

To establish the role of endogenous urea as a source of faecal ammonia, the plasma urea of two healthy men was labelled with 15N at a constant level for several days and its 15N enrichment was compared with that of faecal ammonia and total nitrogen. Faeces collected after one complete gastrointestinal transit from the onset of plasma labelling had ammonia 15N enrichments which were only 8.5 +/- 1.2% and total nitrogen enrichments which were 6.8 +/- 0.7% of the plasma urea 15N enrichment. These results show that endogenous urea is not the main precursor of faecal ammonia, which is probably derived by bacterial deamination from the protein of dietary residues, intestinal secretions and shed epithelial cells. The minor contribution of endogenous urea to faecal ammonia suggests that the lumen of the large bowel is not the main site of endogenous urea hydrolysis. The similar labelling of faecal total nitrogen and ammonia nitrogen supports other evidence that these faecal nitrogen fractions are in a constant state of exchange.

Treatment of chronic renal failure with dietary fiber.

We have tested the hypothesis that dietary fiber, by inhibiting colonic bacterial ammonia generation and increasing fecal nitrogen excretion, might decrease hepatic urea synthesis and thereby reduce plasma urea in patients with chronic renal failure. Six and 8 week courses of two different hemicelluloses, arabinogalactan and ispaghula, reduced mean plasma urea in uremic subjects by 11% and 19% respectively. Ispaghula also reduced the rate of rise of plasma creatinine to zero and, in one formal balance study, increased fecal nitrogen excretion by 39%. Experiments in vitro showed that ispaghula depressed anaerobic fecal bacterial net ammonia generation by 30%, and adsorbed neither urea nor ammonia. The reduction in plasma urea caused by dietary fiber is likely to be due to inhibition of colonic bacterial production of ammonia; such therapy could conceivably alleviate some of the symptoms of uremia and postpone dialysis in patients with endstage renal disease.

Ammonia production by intestinal bacteria: the effects of lactose, lactulose and glucose.

Ammonia production by eight groups of intestinal bacteria was measured, and the effect on ammonia production of lowered pH and ambient ammonia concentration was determined. Endogenous ammonia production from bacterial protoplasm was also examined. To examine the mechanisms by which fermentable substrates reduce ammonia formation in a faecal incubation system, the effect of lactose, lactulose or glucose on ammonia release by pure cultures of intestinal bacteria was studied. The largest amounts of ammonia were generated by gram-negative anaerobes, clostridia, enterobacteria, and Bacillus spp. Gram-positive non-sporing anaerobes, streptococci and micrococci formed modest amounts, and lactobacilli and yeasts formed very little ammonia. All groups of bacteria formed less ammonia at pH 5.0 than at pH 7.0 and production of ammonia was not inhibited when 30 mmol ammonia/litre was included in the medium. Small amounts of ammonia were formed due to endogenous metabolism of bacterial cells. Washed cell suspensions of four isolates of Bacteroides, one clostridial isolate and two streptococcal isolates formed less ammonia from alanine, methionine or histidine after growth in the presence of either lactose or lactulose. In contrast, the Bacteroides isolates formed more ammonia from aspartate than from either lactose or lactulose. Also, cultures of gram-negative anaerobes and enterobacteria, and to a lesser extent clostridia and streptococci, formed significantly less ammonia in nutrient broth when lactose, lactulose or glucose was included in the medium. This decrease in ammonia formation was not due to a fall in pH of the medium. Ammonia production by gram-positive non-sporing anaerobes was not affected by carbohydrate fermentation. These results suggest that gram-negative anaerobic bacteria make a major contribution to ammonia generated from peptides and amino acids in vivo, and that ammonia may be formed from bacterial cells in the colon. Fermentation of lactose and lactulose may repress the formation and inhibit the activity of enzymes responsible for ammonia release. In the human colon these substrate effects may decrease the amount of ammonia available to exert a toxic effect on the host, and thus contribute to the beneficial effects of lactulose when it is used in the treatment of portosystemic encephalopathy.