The serum irisin concentration was lower in both the MetS and T2DM groups (6.6 ng/mL [2.8-18.0 ng/mL] and 6.8 ng/mL [2.2-23.2 ng/mL], respectively), compared with the controls (30.3 ng/mL [24.6-57.1 ng/mL]) (p = 0.004) (Figure 1). Here, FNDC5 is linked to type 2 diabetes mellitus.