Surprisingly little information is available on the natural abundance of the minor isotope of carbon, 13C, in common foodstuffs in the British diet. This study therefore aimed to examine the 13C natural abundance of foodstuffs from a small cross‐section of the British diet. The isotopic abundance, δ per mil, was calculated by measurement of the isotope ratio 13C:12C by isotope ratio mass spectrometry. Results from this study were also compared with results from a North American study to highlight the difference in isotopic abundance between Northern European foodstuffs and North American foodstuffs. Such data should prove useful to those planning tracer studies using the stable isotope 13C where enrichment is measured against a large and variable natural abundance in the body. Minimisation of this basal variation, for example in breath CO2, can be achieved by controlling dietary intake of foods naturally abundant in 13C. Copyright © 2000 John Wiley & Sons, Ltd.

Objectives: Lactose [ 13 C]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 [ 13 C]ureide ( 13 C LU) and glucose [ 13 C]ureide ( 13 C 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) 13 C GU, (4) 13 C LU after predosing with unlabelled lactose ureide and (5) 13 C 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 13 CO 2 recovery and urine was analysed for total 13 C recovery, 13 C urea recovery and 13 C GU recovery. Results: The profiles and extent of tracer recovery in breath and urine were similar when either 13 C GU or 13 C LU was used, suggesting similar handling of these substrates by the gut. 13 C GU was the major 13 C-enriched species recovered in the urine even when 13 C LU was consumed. Predosing with either lactose ureide or glucose ureide increased the rate of appearance of tracer, but did not alter transit times.

Modeling of 13CO2 enrichment curves after ingestion of 13C-enriched wheat flour is an attractive means to estimate the contribution of the upper and lower gut to starch digestion and fermentation.

Lactose ["C]Ureide as a marker for colonic fermentation and the deconvolution of a complex "CO2breath test curve.

There is a growing interest in the use of (13)C-enriched substrates to investigate metabolic processes in humans. The non-invasive nature of (13)C breath tests makes them attractive to clinicians, particularly because they can be safely used in children. The availability of suitable (13)C-enriched substrates can limit the application of this biotechnology. We have used isotope ratio mass spectrometry to assay the chemical purity and isotopic enrichment of substrates that were synthesised to study gut transit and colonic fermentation. Lactose ureide and lactose [(13)C]ureide were synthesised by acid-catalysed condensation of lactose and urea or (13)C urea, respectively. Glucose ureide and glucose [(13)C]ureide were synthesised by similar methods but required an additional purification step to remove urea of crystallisation. Substrates were analysed by standard analytical techniques and combustion isotope ratio mass spectrometry for carbon and nitrogen content and (13)C-enrichment. Monitoring the C/N ratio proved to be a sensitive assay of chemical purity. Analysis of the percentage composition of C and N (and hence O + H) suggested that lactose ureide crystallises as the dihydrate. It was synthesised with approximately 99% chemical purity and with the theoretical enrichment. Glucose ureide was synthesised with approximately 98% chemical purity but with lower than theoretical enrichment.