ObjectiveTo compare the absorption of carbon-13 (C) acetate–enriched nutrients with D-xylose absorption.DesignProspective cohort observational study.SettingSurgical intensive care unit of a university hospital.PatientsA total of 24 critically ill patients requiring enteral nutritional support.InterventionThe patients were divided into three groups according to the route of C acetate administration: 1) gastric, 2) jejunal, and 3) intravenous. D-xylose was administered via the same route as enteral nutrition.Measurements and Main ResultsC acetate absorption and oxidation were reflected by pulmonary CO2excretion. Breath CO2isotopic enrichment was measured by mass spectrometry. C acetate absorption was rapid, and D-xylose absorption was depressed in all three groups, compared with the normal values (p< .0001). Breath CO2isotopic enrichment was similar after intravenous and jejunal administration but slightly delayed during the first 240 mins after gastric administration (p< .01). Enteral feeding was well tolerated: mean energy delivery amounted to 77%, 88%, and 86% of measured resting energy expenditure on days 1–3.ConclusionsGastric and jejunal C acetate are rapidly absorbed in critically ill surgical patients requiring enteral nutrition, contrasting with a depressed or delayed D-xylose absorption. CO2recovery kinetics was similar after jejunal or intravenous C acetate and slightly depressed after gastric administration. Further studies are required to determine the value of labeled nutrients to assess gastric emptying and intestinal absorption.