Exercise calorimetry in sedentary patients: procedures based on short 3 min steps underestimate carbohydrate oxidation and overestimate lipid oxidation

Abstract

Objectives Among exercise calorimetry tests designed for calculating the respective part of carbohydrates and lipids oxidized at exercise, some use 6 min steps and others use 3 min steps. Is this last method, which has been validated in healthy subjects, still accurate in very sedentary patients, who need more time to reach a steady state in respiratory gas exchanges? Methods We compared data obtained with calorimetry (RER and indicators of substrate oxidation) performed on the 2nd–3rd min and the 5th–6th min of each step of a protocol using four 6-min submaximal steps in 17 sedentary subjects (mean age: 51 years) including seven type 2 diabetics and six obese persons. Results Respiratory exchange ratio (RER) measured with the 3 min steps procedure are well correlated with the 6 min procedure in sedentary patients ( r = 0.928). However, a Bland–Altman analysis indicated an average underestimation of RER with 3 min steps (–0.0138). Moreover, we observed an average underestimation of carbohydrate oxidation rates of 70.1 mg/min with the 3 min steps procedure. On the contrary, as to lipid oxidation, we measured an average overestimation of 16.2 mg/min. Furthermore, carbohydrate and lipid oxidation rates measured with the 3 min steps procedure are well correlated with the 6 min steps procedure. Moreover, there was an average overestimation of the point at cross over with 3 min steps (+3.29 Watts). For lipox max point (power at which the increase in lipid oxidation induced by the increasing workload reaches a maximum), we observed an average underestimation with 3 min steps (–1.88 Watt). Although the differences between respectively mean values in cross over point and lipox max point between the two protocols are weak, a Bland–Altman analysis indicated more relevant discrepancies in many subjects between the two protocols. Conclusion In very sedentary patients undergoing such tests for targeting exercise prescription, the 3-min procedure appears to be too short for performing an accurate calorimetry and we rather recommend the protocol using 6-min steps.