FNDC5 and metabolic disease: Guo [29] demonstrated that irisin-knockout (irisin-KO) mice exhibited exacerbated age-induced muscle atrophy and progressive sarcopenic features, while chronic intraperitoneal administration of recombinant irisin effectively attenuated age-related muscle atrophy and metabolic disorders in aged mice, highlighting the critical role of irisin in maintaining muscle physiology and systemic energy homeostasis during aging.