Comparison of Ramp and Step Graded Cardiopulmonary Exercise Tests for the Estimate of Substrate Oxidation Rates in Recreationally Active Individuals

Fat oxidation rates are key determinants of exercise capacity and metabolic health, evaluated by indirect calorimetry during step graded exercise test. We sought to verify whether indirect calorimetry applied to ramp graded exercise test provides comparable results to the reference test and to identify the physiological bases of possible dissimilarities. To this aim, 14 healthy volunteers performed two cardiopulmonary exercise tests with concomitant stress echocardiography according to standard protocols until limit of tolerance on separate days, in randomized order. Compared at matched exercise intensities, expressed as percentage of estimated maximal rate of oxygen uptake (%V˙O2max), indirect calorimetry applied to ramp provided identical kinetics of macronutrient oxidation, with a negligible (7%, p > .05) constant overestimation of fat and underestimation of carbohydrate oxidation rates. Despite identical hemodynamic and ventilatory parameters throughout the tests, we observed a minute downward shift of the Δ V˙CO2/Δ V˙O2 intercept with ramp in comparison to step (-0.05 ± 0.09 L/min; p = .077) due to delayed alignment of gas exchange to work rate, which explains the differences in substrate oxidation values between the two tests. Ramp macronutrient oxidation rates were fully normalized by correcting the stoichiometric equations for the Δ V˙CO2/Δ V˙O2 intercept difference. In conclusion, cardiopulmonary and metabolic responses are dependent on exercise intensity and not on the protocol used. Indirect calorimetry applied to ramp protocols correctly identifies the kinetics of macronutrient oxidation while introducing minimal differences in the absolute values due to different gas-exchange response that can be mathematically corrected by applying the Δ V˙CO2/Δ V˙O2 correction factor.