BDNF and Impaired glucose tolerance: Abbreviation: =: no change; ↗: increase; ↘: decrease; Aex: Aerobic training; BDNF: Brain Derived Neurotrophic Factor; CVD: Cardiovascular Disease; CT: Combined training; FGF-21: Fibroblast Growth Factor-21; HIIT: High Intensity Interval training; IGT: Impaired glucose tolerance; IL-6: Interleukin-6; IR: Insulin Resistance; MA: Meta-analysis; MS: Metabolic Syndrome; NA: not available; NAFLD: Non-alcoholic fatty liver disease; NGT: Normal Glucose Tolerance; PM: Post-menopausal; RCT: Randomized controlled trials; RT: Resistance training; SR: Systematic Review; w: with; w/o: without.