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

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.