FNDC5 and type 2 diabetes mellitus: In addition, free fatty acids and glucose, in turn, could be the direct mediators of decreased irisin expression, as indicated by a study conducted by Kurdiova and colleagues in a cohort of 99 European sedentary adults, divided into lean (n = 29), obese (n = 29), prediabetic (n = 25), and T2DM (n = 16) subjects.