Validity of the doubly labeled water method for estimating CO₂ production in mice under different nutritional conditions

The doubly labeled water (DLW) technique is used to assess metabolic rate (MR) in free-living conditions. We investigated whether differences in the nutritional and body adiposity status affect validity of the assessment of CO2 production (rCO(2)) by the DLW technique. To serve this purpose, we compared calculated rCO(2) by the DLW method to actual CO2 production concomitantly measured in an indirect calorimetry setup over a 3-day period in mice fed with a low-fat (LF) diet or an obesogenic high-fat/high-sucrose (HF) diet. To uncover a potential effect of body composition on DLW accuracy, the HF-fed group was further subdivided in a diet-induced obesity-prone (DIO) and diet-induced obesity-resistant (DR) group. Furthermore, we assessed the influence of different sampling protocols, duration, and methodology of calculation. An excellent match was found between rCO(2) assessed by the two methods in the LF-fed mice (least discrepancy -0.5 +/- 1.1%). In contrast, there was a consistent overestimation of rCO(2) by the DLW technique in the HF-fed animals compared with actual CO2 production independent from body mass gain (least discrepancy DR + 15.9 +/- 2.2%, DIO + 18.5 +/- 3.2%). The least discrepancies were found when two-pool model equations and the intercept method were used to calculate the body water pool. Furthermore, the HF group presented different equilibration kinetics of H-2 and O-18 and a lower dilution space ratio between the two. We recommend particular caution when using the DLW method for MR assessment in HF-fed animals and potentially humans because of the overestimation of rCO(2).