IGF1 and sarcopenia: The causes of sarcopenia are varied and complex; they include disuse of muscles as a result of malnutrition, vitamin D deficiency, cerebral infarction, heart failure, and osteoarthritis; age-related changes in levels of hormones such as testosterone, estrogen, insulin-like growth factor 1, and insulin; apoptosis, denervation, inflammation, and changes in immunity involving interleukin- (IL-) 1, IL-6, and tumor necrosis factor-α; and social and mental causes, such as decline in cognitive function or decreased social activity [33–37].