FNDC5 and sarcopenia: Irisin is a myokine that is released by skeletal muscles and is a potential biomarker for sarcopenia.[13] Myostatin inhibition has been shown to increase irisin levels in animal studies.[14] Furthermore, irisin has been associated with decreased body weight and increased insulin sensitivity.[15] In sarcopenic individuals, circulating irisin levels have been shown to be lower and therefore recommended as a potential marker for sarcopenia.[13,16]