BDNF and bipolar disorder: BDNF was found to be decreased in both mania (SMD −1.16, 95% CI −1.5 to −0.81, P < 0.00001, I2 = 32%, k = 4, n = 252) and bipolar depression (SMD −1.35, 95% CI −2.11 to −0.6, P = 0.0004, I2 = 69%, k = 3, n = 251).