Sexual dimorphism of substrate utilization: Differences in skeletal muscle mitochondrial volume density and function

New Findings What is the central question of this study? Females rely to a greater extent than males on fat oxidation during exercise. Whether any difference in skeletal muscle mitochondrial phenotype and oxidative capacity contributes to this sexual dimorphism remains incompletely explored. What is the main finding and its importance? Female prioritization of fat during exercise occurs in parallel to augmented mitochondrial volume density and intrinsic fatty acid and lactate oxidation in skeletal muscle fibres compared with males, independently of aerobic exercise capacity. The enlarged metabolic machinery in skeletal muscle of females is associated with lower body size and leg mass. Abstract Fat oxidation during exercise is greater in females than in males. We sought to determine whether sex differences in substrate metabolism are paralleled by distinct skeletal muscle mitochondrial volume density and oxidative capacity. Whole‐body substrate (fat and carbohydrate) utilization during submaximal treadmill running was assessed, and skeletal muscle biopsies were taken to determine mitochondrial volume density and function in healthy young females ( n = 12) and males ( n = 12) matched by aerobic exercise capacity and exercise performance. Females presented a lower respiratory exchange ratio (0.87 ± 0.04 versus 0.91 ± 0.04, P = 0.023) and whole‐body carbohydrate oxidation (27.8 ± 8.3 versus 35.8 ± 6.5 mg kg −1 min −1 , P = 0.027), whereas fat oxidation was higher (8.7 ± 2.8 versus 5.9 ± 2.6 mg kg −1 min −1 , P = 0.034) during submaximal exercise compared with males. In skeletal muscle biopsies, females demonstrated augmented mitochondrial volume density (7.51 ± 1.77 versus 5.90 ± 1.72%, P = 0.035) and oxidative capacity for fatty acid 36.6 ± 12.8 versus 24.5 ± 7.3 pmol O 2 s −1 (mg wet weight) −1 , P = 0.009 and lactate 71.1 ± 24.4 versus 53.2 ± 14.6 pmol O 2 s −1 (mg wet weight) −1 , P = 0.040. No sex differences in respiratory exchange ratio, whole‐body fat oxidation and skeletal muscle variables were detected when adjusted for anthropometric variables including body mass or leg mass, which were lower in females. In conclusion, female prioritization of fat over carbohydrate oxidation during exercise is underpinned by augmented body size‐related mitochondrial volume density, fatty acid and lactate oxidative capacity in skeletal muscle fibres.