GDF15 and type 2 diabetes mellitus: Consistently, in this study, we also found that serum GDF15 levels were significantly increased in T2DM patients taking metformin compared with T2DM patients not taking metformin [851.31 (611.15–1237.18] vs. [1370.67 (723.22–1895.21), P = 0.034], but EV-GDF15 levels were not significantly different between the two groups [15.11 (9.60–21.07) vs. 10.77 (5.40–27.75), P = 0.293] Additional file 1: Fig. S6).