Concordance between (13)C:(12)C ratio technique respect to indirect calorimetry to estimate carbohydrate and Fat oxidation rates by means stoichiometric equations during exercise. A reliability and agreement study

Indirect calorimetry is a tool used routinely by sport/exercise physiologist to assess the metabolic response to training and to nutritional interventions. There are different stoichiometric equations to estimate fat (Fat(OxR)) and carbohydrates (CHO(O) (xR)) oxidation rates, however there are not enough information in literature about what are the most accurate equations. The purpose of this study was to determine the concordance between indirect calorimetry and a method of reference for stoichiometric equations used to estimate Fat(OxR) and CHO(O) (xR). Concordance between indirect calorimetry and the method of reference ((13)C to (12)C ratio ((13)C:(12)C ratio) technique) for key stoichiometric equations was assessed in well‐trained triathletes. Subjects carried out a carbohydrate depletion‐repletion protocol, labeling the glycogen stores with (13)C, and a laboratory test to assess the (13)C metabolic response during a wide range of aerobic intensities during exercise. All the equations showed a narrow agreement interval (Δ) (CHO(O) (xR) nPC (protein component negligible): −0.308, 0.308, CHO(O) (xR) PC (protein component): −0.268, 0.268, Fat(OxR) nPC and PC: −0.032, 0.032 (g·min(−1))). Fat(OxR) showed a similar concordance (28–32%) with CHO(O) (xR) nPC ranging from 55% to 75%, and for CHO(O) (xR) PC between 51% to 71%. None of the stoichiometric equations met a perfect agreement with the method of reference. The Jeukendrup and Wallis equation showed the best concordance for CHO(O) (xR) nPC whilst the Frayn and Ferrannini (Glu) equations had the best agreement for CHO(O) (xR) PC. All Fat(OxR) equations showed similar concordances and they are able to be used indistinctly.