Relationships between insulin action and oxidative capacity in skeletal muscle have been suggested in humans, particularly in insulin-resistance-related metabolic disorders, such as obesity and diabetes. The regulatory action of insulin on fuel metabolism in insulin-sensitive tissues such as skeletal muscle is well documented, but the effect of insulin on muscle mitochondrial functions and oxidative capacity have yet to be defined in humans. In a recent article, Stump et al. investigated the stimulatory action of insulin on mitochondrial activities in key cellular events, such as ATP production, enzyme activity, mitochondrial protein synthesis, and mRNA expression of transcripts encoding mitochondrial proteins. To analyze the impact of insulin resistance on mitochondria, Stump et al. conducted their study in subjects with type 2 diabetes mellitus and in non-diabetic matched controls. Their results reveal that as well as being the predominant postprandial anabolic hormone for substrate utilization, insulin is also a major regulating factor of mitochondrial oxidative phosphorylation in human skeletal muscle.