Validation of the 13C-bicarbonate tracer technique for determination of CO2 production and energy expenditure in ponies by indirect calorimetry

Four Shetland ponies were used to validate the 13C-bicarbonate technique (13C-BT) against indirect calorimetry (IC) for determination of CO2 production and estimation of short term energy expenditure (EE), when a single bolus of 13C-bicarbonate was given either as an oral or intravenous (IV) dose. The study was divided into two experiments. In experiment 1 the ponies were placed in respiration chambers making it possible to compare the 13C-BT with IC, and to find a suitable respiratory quotient (RQ) and recovery factor (RF) of 13C in breath CO2 needed for the calculations of EE. In experiment 2 the ponies were measured in the stall and breath samples were collected with a mask and breath bags. There was no effect of the methods used in experiments 1 and 2 (IC, 13C-BTIC or 13C-BTstall) on the measured CO2 production (P>0.05) and the estimated EE (P>0.05). There was no effect (P>0.05) of administration route (IV or oral) on the RQ-value (RQ=0.794), but there was an effect (P=0.026) of route on the RF (RFIV=0.690; RFOral=0.760). The average RQ and the respective RF for IV and oral administration of 13C-bicarbonate were used for the calculations. This validation study against IC showed that the 13C-BT can be used to determine CO2 production for estimation of EE under resting conditions in ponies, independent of administration route of 13C-bicarbonate. The results from IC were similar to measurements performed in the stall under normal resting conditions, where samples were taken with a mask and breath bags.