FNDC5 and sarcopenia: Negatively correlated with age (Huh et al., 2012; Rana et al., 2014). Increase with age (Mahmoodnia et al., 2016; Ruan et al., 2019). Use as a biomarker to predict pre-sarcopenia and sarcopenia (Chang et al., 2017). Positive association with bone mineral status and low irisin level is related to increased hip fracture risk (Yan et al., 2018).