Bulk and compound specific analysis of stool lipid confirm that the "missing" 13C in the mixed triacylglycerol breath test is not in the stool.

The 13C-mixed triacylglycerol (MTG) breath test is used to measure intraluminal fat digestion. In normal digestion 20% to 40% of the ingested 13C label is recovered in breath CO2. We aimed to identify the proportions of ingested label excreted in stool as well as breath following ingestion of 13C-MTG by children with impaired exocrine pancreatic function and healthy controls. 13C enrichment of breath samples was measured by continuous flow isotope ratio mass spectrometry (IRMS) and the cumulative percent dose recovered (cPDR) in 10 hours was calculated. Total 13C of a fecal fat extract from each stool was measured by combustion-IRMS, and 13C enrichment and concentration of the tert.-butyldimethylsilyl (TBDMS) derivative of octanoic acid was measured by gas chromatography-mass spectrometry (GC/MS) after hydrolysis of the fat extract. Stool 5-day cPDR was calculated. Mean breath cPDR was 33% for children with cystic fibrosis and 45% for controls. Mean cPDR in stool by combustion-IRMS and GC/MS, respectively, was 0.7% and 0.3% for children with cystic fibrosis and 1.4 and 4.2% for controls.