Association of sedentary and physical activity time with maximal fat oxidation during exercise in sedentary adults

The present work examines the relationships of objectively measured sedentary time and physical activity (PA) with maximal fat oxidation during exercise (MFO) and the intensity of exercise that elicits MFO (Fat_max ), in young and middle-aged men and women. A total of 121 young sedentary adults (81 women, age 22.1 ± 2.1 years, body mass index 25.2 ± 4.9 kg/m² ) and 70 middle-aged sedentary adults (36 women, age 53.4 ± 4.9 years, body mass index 26.8 ± 3.8 kg/m² ) participated in this cross-sectional study. PA was objectively assessed using a wrist-worn accelerometer for 7 consecutive days. Wrist accelerations were classified into sedentary time and PA of different intensity (light, moderate, vigorous, moderate-vigorous), taking into account age-specific cut-offs. MFO and Fat_max were determined for all subjects by indirect calorimetry, using a walking graded exercise test. No association was found between any of the sedentary time-related variables and MFO (all P ≥ .05). However, several sedentary time-related variables were related to FAT_max in young men and women (all P ≤ .04). A positive relationship was also detected between PA and MFO in young and middle-aged women (P ≤ .05), although this became non-significant after adjustment for cardiorespiratory fitness (P > .05). The present results suggest that, depending on the characteristics of the study cohort, sedentary time, and PA time may be related to MFO and FAT_max during exercise. Future longitudinal and intervention studies are warranted to better understand the role of sedentary time and PA in metabolic flexibility during exercise.