Meeting Abstract
Energy expenditure (EE) measurement can take place via direct or indirect calorimetry. With indirect calorimetry, O2 consumption and CO2 production rates are typically both required in order to calculate EE. This arises because the oxycaloric equivalent, required to transform O2 consumption rate to EE, varies with the respiratory quotient (RQ). In 1949 J.B. Weir derived an equation for calculating EE from O2 consumption and CO2 production rates, and had the key insight that CO2 dilution of the oxygen signal could counteract the inverse dependence of the oxycaloric equivalent on RQ. This insight was rediscovered sporadically in the coming years, but has not been widely used as its validity has not been rigorously demonstrated from first principles. Here, we present the first rigorous derivation of the formula for transforming O2 consumption rates, in the presence of CO2 dilution, into EE directly. We show that this formula applies only to ambient O2 levels close to normoxia, and also validate the formula using 168 individual mouse-days of EE data across a temperature range of 19 – 29C. Given the calibration uncertainties of CO2 calibration gases, this method provides a highly accurate calculation of EE, as it does not depend on CO2 analysis at all. However, if CO2 analyzers are used, it provides a method for validating their correct operation, given the 24h equivalence of food quotient and RQ (by Hess’s law of Constant Heat Summation). We further demonstrate that drying of the sample air is not necessary if water vapor pressure and barometric pressure are measured, allowing the dilution effect of water vapor to be removed using Dalton’s law of partial pressures.
