FGF21 and type 2 diabetes mellitus: Higher levels of circulating FGF21 concentrations related to a high incidence of type 2 diabetes have been observed in mammals [93], which is analogous to how insulin infusion contributes to the upregulation of FGF21 expression in human skeletal muscle, indicating that FGF21 may be a myocyte factor involved in the regulation of glucose metabolism [34].