FNDC5 and sarcopenia: Irisin reduced mRNA and protein levels in muscle tissue during aging, while intraperitoneal administration of irisin (2 mg/kg body weight, three times per week) in aging (14‐month‐old) or aged (22‐month‐old) mice significantly improved sarcopenia, as indicated by increased strength (+18.42% or +13.88%) and muscle weights (+9.02% or + 16.39%).120