SOD2 and proliferative diabetic retinopathy: DR: diabetic retinopathy; NPDR: nonproliferative diabetic retinopathy; PDR: proliferative diabetic retinopathy; T2DM: type 2 diabetes mellitus; T2DCON: type 2 diabetes mellitus without complications; Mg: magnesium; Zn: zinc; Mn: manganese; Se: selenium; Cr: chromium; MnSOD: manganese superoxide dismutase (gene polymorphism); HbA1c: glycated hemoglobin; FBS: fasting blood sugar; PPBS: postprandial blood sugar; BMI: body mass index; NA: not available; NS: not significant