Evaluation of the necessity of induction for lactose-[15N, 15N]- ureide to study the colonic ammonia metabolism

Objective. Stable isotope labelled glycosyl ureides have been described as non- invasive markers for measurement of gastrointestinal processes. Lactose-[C-13]- ureide is used for the evaluation of orocaecal transit time ( OCTT), whereas lactose-[N-15, N-15]- ureide is used to study the fate of the NH3 metabolism in the colon. Induction with unlabelled lactose ureide is necessary for the determination of the OCTT. In the present study, the effect of a preceding induction on the fate of 15 NH3 in the colon was evaluated. Material and methods. Ten healthy volunteers performed two tests: the first test without induction, the second test one week later with induction, i. e. administration of 1g lactose ureide the evening before the test. Each test consisted of a fractionated 24-h urine collection after the subjects had received a pancake test meal labelled with 75 mg lactose-[N-15, N-15]- ureide. All samples were analysed for [N-15]- content by combustion IRMS ( isotope ratio mass spectrometry) and results were expressed as a percentage of the administered dose. Results. No significant differences were found in the percentage dose 15N excreted in the different urine fractions between the test without and with preceding induction. The cumulative excretion rates of the N-15- isotope after 24h were 44.35% without induction and 44.27% with induction. Conclusions. The results in this study show that predosing with unlabelled lactose ureide is unnecessary for the evaluation of the ammonia metabolism in the colon by means of lactose[(15N), N-15]- ureide.