Skeletal muscle mitochondrial dysfunction is associated with aging and diabetes, which decreases respiratory capacity and increases reactive oxygen species. Lipoic acid (LA) possesses antioxidative and antidiabetic properties. Metabolic action of LA is mediated by activation of adenosine monophosphate-activated protein kinase (AMPK), a cellular energy sensor that can regulate peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), a master regulator of mitochondrial biogenesis. We hypothesized that LA improves energy metabolism and mitochondrial biogenesis by enhancing AMPK-PGC-1alpha signaling in the skeletal muscle of aged mice. C57BL/6 mice (24 months old, male) were supplemented with or without alpha-LA (0.75% in drinking water) for 1 month. In addition, metabolic action and cellular signaling of LA were studied in cultured mouse myoblastoma C2C12 cells. Lipoic acid supplementation improved body composition, glucose tolerance, and energy expenditure in the aged mice. Lipoic acid increased skeletal muscle mitochondrial biogenesis with increased phosphorylation of AMPK and messenger RNA expression of PGC-1alpha and glucose transporter-4. Besides body fat mass, LA decreased lean mass and attenuated phosphorylation of mammalian target of rapamycin (mTOR) signaling in the skeletal muscle. In cultured C2C12 cells, LA increased glucose uptake and palmitate beta-oxidation, but decreased protein synthesis, which was associated with increased phosphorylation of AMPK and expression of PGC-1alpha and glucose transporter-4, and attenuated phosphorylation of mTOR and p70S6 kinase. We conclude that LA improves skeletal muscle energy metabolism in the aged mouse possibly through enhancing AMPK-PGC-1alpha-mediated mitochondrial biogenesis and function. Moreover, LA increases lean mass loss possibly by suppressing protein synthesis in the skeletal muscle by down-regulating the mTOR signaling pathway. Thus, LA may be a promising supplement for treatment of obesity and/or insulin resistance in older patients.