Patients with depression had significantly lower levels of BDNF than the participant in the non-depressed group (SMD [95%CI] = − 0.60 [− 1.10 to − 0.10], p-value < 0.001, I2 = 88%, p-value < 0.001) (Fig. 5). This evidence concerns the gene BDNF and depressive symptom measurement.