FNDC5 and osteoporosis: Interestingly, various tissue-specific factors secreted by the muscle tissue, including tumor necrosis factor-α, interleukin (IL)-6, IL-15, reactive oxygen species, irisin, and myostatin, are linked to the pathogenesis of sarcopenia and are also modulators of bone remodelling, and thus are related to osteoporosis [44, 46].