The influence of inulin on the absorption of nitrogen and the production of metabolites of protein fermentation in the colon

In the present study, the production and fate of bacterial metabolites in the colon were investigated in a direct way using two substrates labelled with stable isotopes: lactose [N-15, N-15]ureide as a source of labelled ammonia and egg proteins intrinsically labelled with [H-2(4)]tyrosine as a precursor of [H-2(4)]p-cresol. Both ammonia and phenolic compounds are believed to be carcinogenic. Stimulation of carbohydrate fermentation in order to prevent accumulation of these toxic metabolites was induced by inclusion of inulin in a test meal or by addition of inulin to the daily diet, allowing us to distinguish between changes induced by the actual presence of a fermentable carbohydrate and effects caused by a long-term dietary intervention. When a single dose of inulin was administered together with the labelled substrates, a significant increase in faecal N-15 excretion, accompanied by a proportional decrease in urinary N-15 excretion was observed, probably reflecting an enhanced uptake of ammonia for bacterial biosynthesis, since an increased concentration of labelled N in bacterial pellets was found. A statistically significant reduction of urinary [H-2(4)]p-cresol excretion was also noted. Upon supplementation of inulin to the daily diet during 4 weeks, however, only a tendency towards decreased urinary excretion of both labelled and unlabelled p-cresol was noted. Further studies are warranted to confirm these results in a larger cohort.