Table 1. Characteristics of the included studies.
| Study | Region | Study design | Complication type | Age (y) | Sex (M/F) | BMI (kg/m2) | HbA1c | eGFR (mL/min/1.73 m) | Smoking (%) | Hypertension (%) | Exercises (%) | Drinking (%) |
| Gong 2010[35] | China | Case control | DR(+) | 56.95±12.66 | 1.67 | NA | 9.27±2.53 | NA | 25. 12 | NA | NA | NA |
| DR(-) | 49.85±10.62 | 2.73 | NA | 8.90±2.30 | NA | 26. 15 | NA | NA | NA | |||
| Han 2015[18] | Korea | Case control | DN | 58 | 1.11 | 25.3 | 7.3 | 90.4 | 31.63 | 58 | 13.71 | 10.79 |
| Sharma 2016[32] | USA | Case control | DN | 47 | 0.86 | 27.1 | NA | 100.3 | 25.37 | 23.08 | 79.26 | 81.74 |
| Ricardo 2015[33] | USA | Case control | DN | 41.5 | 1 | 26.4 | NA | NA | 51.6 | 20.4 | NA | NA |
| Song 2017[34] | Korea | Case control | DR(-) | 57±0.7 | 0.82 | 22.7±0.1 | 7.2±0.1 | 92.6 | 23.2 | 52.4 | 23.1 | 54.2 |
| DR(+) | 58.5±1.6 | 1.18 | 22.2±0.3 | 8.2±0.2 | 87.7 | 27.3 | 37.2 | 20 | 48.1 | |||
| Amiri 2014[19] | Iran | Case control | DR(-) | 55.69±8.49 | 0.42 | NA | NA | NA | NA | 28.17 | NA | NA |
| DR(+) | 56.97±10.46 | 0.46 | NA | NA | NA | NA | 26.87 | NA | NA | |||
| Nitta 2017[17] | Japan | Case control | DR/DN (+) | 55.1±9.3 | 1.72 | 24.0±3.9 | 8.7±1.8 | NA | 42.6 | NA | NA | NA |
| DR/DN (-) | 52.4±10.3 | 1.44 | 23.9±3.9 | 7.7±1.5 | NA | 38 | NA | NA | NA | |||
| Veena 2018[36] | India | Prospective cross-sectional study | DR(-) | 30-65 | 2.57 | NA | NA | NA | NA | NA | NA | NA |
| DR(+) |
NA: Not accessible; BMI: Body mass index; eGFR: Estimated glomerular filtration rate; HbA1c: Hemoglobin A1c.