The13C bicarbonate method: an inverse end product method for measuring CO2production and energy expenditure

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Abstract
We reconsider the principle of the 13C bicarbonate (NaH13CO3) method (13C-BM) for the determination of the CO2 production to obtain an estimate of energy expenditure (EE). Its mathematical concept based on a three-compartmental model is related to the [15N]glycine end product method. The CO2 production calculated by the 13C-BM, RaCO2(13C) is compared to the result from the indirect calorimetry, RCO2(IC). In an interspecies comparison (dog, goat, horse, cattle, children, adult human; body mass ranging from 15 to 350 kg, resting and fasting conditions) we found an excellent correlation between the results of 13C-BM and IC with RCO2(IC) = 0.703 × RaCO2(13C), (R2 = 0.99). The slope of this correlation corresponds to the fractional 13C recovery (RF(13C)) of 13C in breath CO2 after administration of NaH13CO3. Significant increase in RF(13C) was found in physically active dogs (0.95 ± 0.14; n = 5) vs. resting dogs (0.71 ± 0.10, n = 17; p = .015). The 13C recovery in young bulls was greater in blood CO2 (0.81 ± 0.05) vs. breath CO2 (0.73 ± 0.05, n = 12, p < .001) and in ponies with oral (0.76 ± 0.03, n = 8) vs. intravenous administration of NaH13CO3 (0.69 ± 0.07; n = 8; p = .026). We suggest considering the 13C-BM as a ‘stand-alone’ method to provide information on the total CO2 production as an index of EE.

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