Obesity, insulin resistance, type 2 diabetes mellitus, and aging are associated with impaired skeletal muscle oxidation capacity, reduced mitochondrial content, and lower rates of oxidative phosphorylation. Several studies have reported ultrastructural abnormalities in mitochondrial morphology and reductions in mitochondrial mass in insulin-resistant individuals. From lower organisms to rodents, mitochondrial membrane structure, function, and programmed cell death are regulated in part by the balance between the opposing forces of mitochondrial fusion and fission, suggesting they may also play an important role in human physiology.