Fat Oxidation, But Not Subcutaneous Adipose Tissue Lipolysis, Differs Between Males and Females During a Treadmill-Based Heat Tolerance Test.

This study aimed to determine whether fat metabolism differs between males and females when exposed to extreme exercise-heat stress. Physically active males (n = 11, 23 ± 4 years, 81.7 ± 11.8 kg, body fat 16.4 ± 6.6%) and females (n = 13, 25 ± 4 years, 60.4 ± 7.1 kg, 24.4 ± 6.7%) completed a 2-h exercise-heat tolerance test (40 ° $\boldsymbol{{}^{\circ}}$ C, 40% relative humidity). Differences (pre-, post-, change, and mean difference (MD)) within and between groups were analyzed. The subcutaneous abdominal adipose tissue (SCAAT) interstitial glycerol concentration and adipose tissue blood flow (ethanol Output:Input (O:I)) pre-exercise, every 30 min (min) of exercise, and during recovery was measured with microdialysis. Non-esterified fatty acids, insulin, insulin-like growth factor, epinephrine and norepinephrine, and cortisol were measured in blood. Resting energy expenditure (REE) was measured pre- and post-exercise and exercising metabolic heat production (MHP) was collected during 30 and 90 min of the HTT. Despite no sex differences in systemic blood biomarkers, fat oxidation (g × min-1) was higher in males (M) (vs. Females (F)) at 30 min of HTT (3.92 ± 0.25 (M), 3.58 ± 0.38 (F), p = 0.021). SCAAT interstitial glycerol was similar across all time points between sexes (baseline glycerol ranges (mmol × L-1): 104.6-1260.0 (F), 165.0-775.6 (M)); however, females had a greater O:I ratio at 90 min of exercise (vs. M) (0.69 ± 0.33 (F); 0.44 ± 0.20 (M); p = 0.033). Pre- and post-exercise REE were 23% (MD: 447.5 kcal × day-1, p < 0.001) and 25% (MD: 485 kcal × day-1, p < 0.001) lower in females compared to males. Post-exercise RER (0.67 ± 0.06 (F); 0.72 ± 0.08 (M); p = 0.045) and mean MHP was higher in males (MD: 94W). Fat oxidation was higher in males compared to females at 30 min with no changes in SCAAT lipolysis or blood biomarkers before or after an acute bout of exercise-heat stress.