Review

¹³C-breath tests to measure fat and carbohydrate digestion in clinical practice

This study describes experiments using simple helmets to collect breath samples from individual birds for measurement of ¹³ CO₂, H₂, and CH₄, which form the basis for various diagnostic tests for intestinal dysfunction in humans. Peak enrichment in ¹³ C in breath CO₂ occurred between 5 and 30 min postingestion by 18-d-old chickens administered a gelatin capsule containing approximately 3.6 mg of ¹³ C-octanoic acid dissolved in vegetable oil. For 25-d-old chickens given 10 mL of homogenized cooked corn by oral gavage, peak enrichment occurred 60 to 90 min postingestion. In fully fed 25-d-old chickens, H₂ and CH₄ concentrations in breath ranged from 7 to 115 ppm and from 0 to 5.5 ppm, respectively. Following an overnight fast, H₂ and CH₄ concentrations in breath ranged from 0.5 to 7.5 ppm and 0 to 3.0 ppm, respectively, in the same chickens. Ranges in H₂ (1.0 to 56.5 ppm) and CH₄ (0 to 8.0 ppm) concentrations widened considerably 3 h after oral gavage with approximately 130 mg of lactulose (an indigestible disaccharide) dissolved in 5 mL of water. The results from these investigations indicate that collection of re-breathed air samples from chickens is plausible, which opens the way for development of noninvasive methods for evaluating gastrointestinal functions in chickens.

There is a need for non-invasive tests of gastrointestinal and nutritional function. Clinical problems peculiar to infancy and childhood require prompt diagnosis, and methods that are invasive or involve the use of radioisotopes are often impractical or ethically unacceptable. What the pediatrician and clinical scientist seek are tests that are simple, repeatable, and unequivocal in their result for diagnosis, to assess the effects of treatment, and to measure the development of gastrointestinal function during early life. Stable isotope breath tests offer a ready and attractive answer to these needs. They involve the ingestion of substrates labeled with the non-radioactive isotope of carbon (¹³C), followed by the collection of serial breath samples for analysis of the enrichment of ¹³CO₂, the end product of substrate metabolism. Their non-invasive nature recommends them for use in infancy and childhood, and they can be performed in the ward, clinic, laboratory, and home. In this article I discuss to what degree stable isotope breath tests fulfill the pediatrician’s and scientist’s needs. I have chosen two examples from the work of myself and my colleagues to illustrate the principles and use of ¹³C breath tests to detect Helicobacter pylori infection and to measure fat digestion in infancy and childhood.

The use of ¹³C-labelled compounds to study lipid metabolism is increasing. Typically less than 40% of the orally administered label is recovered in breath CO₂. The remainder must be either absorbed and not oxidised or not absorbed and remain in the faeces. Two methods of determining how much tracer passes through the body, and is present in the stool, were compared. Compound specific analysis of tert.-butyldimethylsilyl [¹³C]hexadecanoic acid by gas chromatography–mass spectrometry (GC–MS) with electron impact ionisation was compared with bulk analysis of whole stool and lipid extract by continuous flow isotope ratio mass spectrometry (CF–IRMS) with a combustion interface. The mean difference between the IRMS and GC–MS methods was −0.02 mmol ¹³C d⁻¹ with a mean excretion of 14.2 mmol ¹³C d⁻¹. Combustion IRMS is both simpler and cheaper, when the objective is to determine how much administered dose appears in stool, and information about the form of the label is not required.