GSTM1 and type 2 diabetes mellitus: When we restricted only high-quality studies, notably increased T2DM risk was found the combinations of GSTM1 and GSTT1 polymorphisms (model 3: OR = 3.40, 95% CI = 2.42–4.79; model 4: OR = 2.00, 95% CI = 1.54–2.59; model 5: OR = 1.39, 95% CI = 1.12–1.74; model 6: OR = 2.21, 95% CI = 1.40–3.50, Table 4).