T2DM: type 2 diabetes mellitus; DR: diabetic retinopathy; NPDR: nonproliferative diabetic retinopathy; PDR: proliferative diabetic retinopathy; Mg: magnesium; Zn: zinc; Mn: manganese; Se: selenium; Cr: chromium; RCT: randomized controlled trial; NHANES: National Health and Nutrition Examination Survey; OR: odds ratio; CI: confidence interval; FBS: fasting blood sugar; PPBS: postprandial blood sugar; MnSOD: manganese superoxide dismutase (gene polymorphism). This evidence concerns the gene SOD2 and non-proliferative diabetic retinopathy.