BDNF and diabetes mellitus: Subgroup meta-analysis of serum BDNF levels in DM based on study design revealed a significantly lower levels of serum BDNF in patients with DM compared vs. controls in case-control and cohort studies (SMD = -1.03[-1.57, -0.48], P = 0.00023 and SMD = -0.65[-0.97, -0.34], P = 0.0006, respectively).