GSTM1 and type 2 diabetes mellitus: Concerning the combination effects of GSTM1, GSTT1, and GSTP1 IIe105Val polymorphisms, considerably increased T2DM risk was observed (model 1: OR = 2.81, 95% CI = 1.02–3.22, model 9: OR = 1.75, 95% CI = 1.00–3.07, model 10: OR = 2.24, 95% CI = 1.32–3.80, Table 7) when we excluded low-quality studies.