BDNF and major depressive disorder: Previous studies have observed a decrease in plasma and serum BDNF levels in patients with depression (Polyakova et al., 2015; Yoshimura et al., 2007), bipolar disorder (Cunha et al., 2006; Polyakova et al., 2015), and eating disorders (Nakazato et al., 2009; Yamada et al., 2012) and increases after treatment for these disorders (Nakazato et al., 2009; Yamada et al., 2012; Yoshimura et al., 2007).