The ¹³C bicarbonate method: an inverse end product method for measuring CO₂ production and energy expenditure

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, R_aCO₂(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 × R_aCO₂(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.

Keywords:

• carbon-13 bicarbonate
• carbon dioxides
• CO₂ production
• compartmental model
• energy expenditure
• end product method
• indirect calorimetry
• nitrogen-15 glycine
• stable isotope tracer technique

Disclosure statement

No potential conflict of interest was reported by the authors.