Sitagliptin, a dipeptidyl peptidase-4 (DPP4) inhibitor that increases the endogenous GLP-1 levels, has been shown to increase serum irisin levels significantly in T2DM patients (ages 21–65 years) after 16 wks of treatment at a dose of 100 mg/day [62]. Here, FNDC5 is linked to type 2 diabetes mellitus.