Since the findings in this regard were not consistent and as we know, there is no previous review summarized these reports, we performed a systematic review and meta-analysis of epidemiologic studies to compare serum/plasma BDNF levels in individuals with T2D or other glycemic disorders with healthy controls and to evaluate the association between circulating BDNF levels and glycemic parameters, as well. Here, BDNF is linked to type 2 diabetes mellitus.