Calorimetric validation of13C bicarbonate and doubly labeled water method for determining the energy expenditure in goats | Kalorimetrische Validierung der13C-Bicarbonat- und der Wasser-Doppelmarkierungsmethode zur Bestimmung des Energieumsatzes bei Ziegen

The purpose of the present study was to validate the¹³C bicarbonate method (¹³C-M) and the doubly labeled water method (DLWM) for the estimation of the CO₂ production R(CO₂) in goats as a ruminant model. Indirect calorimetry was chosen as the reference method. Studies were carried out in 2 male African dwarf goats at 3 different developing stages (age: 5, 10, and 14 months, body mass: 14.6, 20.3, and 21.7 kg). Animals were fed a balanced feed 14 days before and during the studies. The isotope tracers (4 mg/kg NaH¹³CO₃, 120 mg/kg²H₂O, and 75 mg/kg H₂¹⁸O; 99 At.-%) were simultaneously given as a single pulse injection into the jugular vein. Thereafter, the animals were kept for 8 days in two respiration chambers (volume of chamber: 2.85 m³, air flow rate: 25 1 / min) for the estimation of CO₂ production and O₂ consumption. For the determination of R(CO₂) using the¹³C-M samples of exhaled breath were drawn from the respiration chambers. The¹³C enrichment and CO₂ concentration of breath samples were measured by means of an infrared isotope analyzer. In order to determine R(CO₂) by means of the DLWM, blood serum was used. The²H and¹⁸O enrichments were measured by an isotope ratio mass spectrometer. Urine samples were collected over 24 h to quantify renal water losses.The R(CO₂) was calculated by means of the¹³C-M using the area under the¹³C enrichment-time curve. The determination of R(CO₂) by means of the DLWM was based on the slopes of the²H and¹⁸O disappearance curves and the body water pool obtained from the zero time intercept of the isotope curves.The values of R(CO₂) resulting from the¹³C-M were found to be comparable with those from the calorimetric measurement. Smaller (not statistically significant) values of R(CO₂) - 92% from¹³C-M and 87% from DLWM - compared to the indirect calorimetry could indicate the incorporation of¹³C and²H into metabolites other than CO₂ and H₂O, respectively. The body water contents calculated from the zero time intercepts of the²H and¹⁸O disappearence curves amounted to 66% and 63%, respectively. The body water content was found to be not related to the age of animals. The renal water loss was calculated to be 35% of the total water loss (0.76 l/d).