The inventors show that apelin plays a role in energy metabolism and particularly in energetic mechanisms in mitochondria. rnThey show that apelin treatment increases complete fatty acid oxidation (FAO), glucose transport, mitochondrial oxidative capacity and biogenesis in muscle of insulin-resistant mice. rnFurthermore, they show that skeletal muscle appears as the major tissue target for apelin action, where it mediates increased fuel consumption. Thus apelin could be used in diseases related to problems in energetic mechanism in mitochondria.rnUsing apelin KO mice and wild type mice chronically-treated by apelin, the inventors identified that markers of sarcopenia like myostatin and myogenin were respectively down or up regulated in old mice. Thus, apelin should be used to treat dysfunction associated with aging and particularly in sarcopenia.rnrnThus, the invention relates to an APJ receptor agonist or an apelinomimetic for use in the treatment or the prevention of a dysfunction associated with aging.