Fat Oxidation, But Not Subcutaneous Adipose Tissue Lipolysis, Differs Between Males and Females During a...

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 $°$ 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.