Interactions between brown adipose tissue activity and exercise modality on metabolic kinetics: a crossover study in trained individuals

This study aimed to investigate the interactions between basal brown adipose tissue (BAT) activity and exercise modality on energy expenditure and substrate oxidation kinetics in trained individuals. Thirty-five trained males were stratified into high (HBAT, n = 15: 26.5 ± 4.3 years, 25.5 ± 1.8 kg/m²) and low (LBAT, n = 20: 27.0 ± 4.1 years, 26.0 ± 1.7 kg/m²) BAT activity groups based on infrared thermography evaluation. In a crossover design, each participant performed two experimental sessions: continuous aerobic exercise (AERO: 30 min at 70–80 % HR_reserve) and high-intensity interval exercise (HIIE: 4 × 4 min at 90–95 % HRmax with 3-min active recovery), separated by 7 days. energy expenditure and substrate oxidation including carbohydrate, lipid and protein were continuously measured by indirect calorimetry during exercise and throughout a 30-min recovery period. Polynomial regression modeling of temporal trajectories revealed a significantly higher total energy expenditure in the HBAT group compared to the LBAT group (+25 %, p < 0.01), with the peak energy expenditure rate observed during HIIE in the HBAT group (3.7 ± 0.2 kcal min⁻¹ versus 2.9 ± 0.3 kcal min⁻¹ in AERO-LBAT; p = 0.004). Carbohydrate oxidation displayed a characteristic biphasic decline but showed no significant differences between groups. In contrast, lipid oxidation peaked later and was substantially higher in the HBAT groups, with the most pronounced effect following HIIE. Protein oxidation remained minimal and consistent across all conditions. These findings demonstrate that elevated BAT activity is associated with increased exercise-induced energy expenditure and enhanced lipid utilization during the latter stages of exercise, with HIIE showing the most robust metabolic associations in individuals with high BAT activity.