Background/Objectives: To investigate acute caffeine (CAF: 375 mg, ≈4.8 mg/kg body mass) effects on energy expenditure (EE) and substrate kinetics during high-intensity interval exercise in individuals with high (HBAT) versus low (LBAT) brown adipose tissue activity using time-trend polynomial modeling. Methods: This is a randomized, double-blind crossover study in which 35 highly-trained males [HBAT-CAF, HBAT-PLA (Placebo), LBAT-CAF, LBAT-PLA] performed 30-min treadmill HIIE. Infrared thermography (IRT) assessed BAT activity by measuring supraclavicular skin temperature (SST). Breath-by-breath ergospirometry measured EE (kcal/min) and carbohydrate (CHO), lipid (LIP), and protein (PTN) oxidation. We applied second- and third-order polynomial regression models to depict the temporal trajectories of metabolic responses. Results: HBAT groups showed 25% higher sustained EE versus LBAT (p < 0.001), amplified by CAF. CHO oxidation exhibited biphasic kinetics: HBAT had 40% higher initial rates (0.75 ± 0.05 vs. 0.45 ± 0.04 g/min; p < 0.001) with accelerated decline (k = −0.21 vs. −0.15/min; p = 0.01). LIP oxidation peaked later in LBAT (40 vs. 20 min in HBAT), with CAF increasing oxidation by 18% in LBAT (p = 0.01). HBAT-CAF uniquely showed transient PTN catabolism (peak: 0.045 g/min at 10 min; k = −0.0033/min; p < 0.001). Conclusions: BAT status determines EE magnitude and substrate-specific kinetic patterns, while CAF exerts divergent modulation, potentiating early glycogenolysis in HBAT and lipolysis in LBAT. The HBAT-CAF synergy triggers acute proteolysis, revealing BAT-mediated metabolic switching.

Aims: to measure whether subjects with high (HBAT) and low activation of brown adipose tissue (LBAT) showed acute differences in energy expenditure during and after 30-minutes of high intense interval training. Methods: Forty professional soccer athletes were invited to participate in the study, of which 37 started the protocol and 35 completed it. This was an acute, double-blind, case-control, quasi-experimental study design. Activation of BAT was estimated to be using infrared thermography. All participants were divided into four groups and compared by condition: (a) HBAT-CAF; (b) HBAT-PLA; (c) LBAT-CAF; and (d) LBAT-PLA. They performed 30 min of high-intensity interval training (HIIT) which included four sets of 4-minute of running in a speed corresponding at 90–95% of HRmax, each followed by a 3-minute active recovery interval at 60–70% of HRmax and 30 min of passive recovery. During exercise and recovery, breath-to-breath spirometry was performed and energy expenditure (EE), carbohydrates (g/day), fats (g/day) and proteins (g/day) were measured. Results: HBAT showed higher EE at all measurement times (p≤0.01) in the CAF condition and at all measurement times in the PLA condition (p≤0.001). For fat catabolism, the CAF condition showed a significant difference between conditions, with CAF showing higher catabolism at 60 min [36.5 g/day (0.8; 137.2); p=0.047] compared to PLA. For PTN catabolism, the HBAT-CAF condition, minutes 40, 50, and 60 showed a significant difference compared to the other measurement times (p≤0.023 for all comparisons). Conclusion: After high-intensity interval exercise with CAF supplementation, both HBAT and LBAT subjects experience a significant increase in EE during exercise and recovery compared to baseline. Basal BAT activity did not affect the rate of CHO and FAT oxidation, although a higher rate of PTN utilization was observed in the HBAT-CAF group.