Validation of the doubly-labelled water technique in the domestic dog (Canis familiaris)

We validated doubly-labelled water (DLW) by comparison to indirect calorime try and food intake-mass balance in eight Labrador dogs (24-32 kg) over 4 d . We used several alternative equations for calculating CO2 production, bas ed on the single- and two-pool models and used two alternative methods for evaluating the elimination constants: two-sample and multiple-sampling. In all cases the DLW technique overestimated the direct estimate of CO2 produc tion. The greatest overestimates occurred with the single-pool model. Using two samples, rather than multiple samples, to derive the elimination const ants produced slightly more discrepant results. Discrepancies greatly excee ded the measured analytical precision of the DLW estimates. The higher valu es with DLW probably occurred because the dogs were extremely active during the 1 h in each 24 spent outside the chamber. Estimates of CO2 production from food intake-mass balance, which include this activity, produced a much closer comparison to DLW (lowest mean discrepancy 0.3 % using the observed group mean dilution space ratio and an assumption that the mass changes re flected changes in hydration for all except one animal). We recommend an eq uilibration time of 6 h and use of the two-pool model based on the observed population dilution space for future studies of energy demands in dogs of this body mass.