Bulk and Compound Specific Analysis of Stool Lipid Confirm that the “Missing” 13 C 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.