Gastrointestinal handling of glycosyl [13C]ureides

Objectives: Lactose [C-13] ureide has been proposed as a noninvasive marker for oro-caecal transit time in adults and children. The present study investigates the handling of lactose [C-13] ureide (C-13 LU) and glucose [C-13]ureide (C-13 GU) by the gastrointestinal tract and describes the metabolic fates of these substrates and describes the extent of tracer excretion by different routes. Study Design and Subjects: Four subjects underwent five studies in which they ingested a test meal plus (1) no substrate, (2) (13) C LU, (3) C-13 GU, (4) C-13 LU after predosing with unlabelled lactose ureide and (5) C-13 LU after predosing with glucose ureide. Subjects were studied at home with at least 1 week between tests and they all completed the study. Breath was analysed for (CO2)-C-13 recovery and urine was analysed for total C-13 recovery, C-13 urea recovery and C-13 GU recovery. Results: The profiles and extent of tracer recovery in breath and urine were similar when either C-13 GU or C-13 LU was used, suggesting similar handling of these substrates by the gut. C-13 GU was the major C-13-enriched species recovered in the urine even when C-13 LU was consumed. Predosing with either lactose ureide or glucose ureide increased the rate of appearance of tracer, but did not alter transit times. Conclusions: C-13 LU is hydrolysed to C-13 GU in the small intestine with the fraction of C-13 GU appearing in the urine probably limited by small intestinal permeability. Either C-13 LU or C-13 GU can be used to measure oro-caecal transit time.