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.

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.