FNDC5 and sarcopenia: For this review, we performed a literature search using the PubMed/MEDLINE database with a combination of the keywords myokines and osteoporosis (studies concerning irisin in postmenopausal osteoporosis were included; those concerning the effect of other myokines in postmenopausal osteoporosis were excluded) and myokines and sarcopenia (only studies concerning the study of irisin in sarcopenia were included; those concerning all other myokines in sarcopenia were excluded).