FNDC5 and type 2 diabetes mellitus: The optimal levels of circulating FNDC5/irisin in humans are not established, however, some evidence indicates that individuals with type 2 diabetes mellitus (T2DM) (6,7) and metabolic syndrome and insulin resistance (IR) (8) have lower levels of this peptide, indicating that a larger amount of FNDC5/irisin could be a protective factor against these diseases.