FNDC5 and type 2 diabetes mellitus: Serum irisin concentrations were significantly higher in subjects with normal glucose tolerance compared to subjects with pre-diabetes or T2D (2.16 ± 0.59 μg/mL in normal glucose tolerance and 1.77 ± 0.91 μg/mL in pre-diabetes or T2D, P < 0.001; data not shown).