We compared the BDNF level between T2DM and HC groups (P < 0.001, I2 = 99%), and between the T2DM with or without cognitive impairment groups (P < 0.001, I2 = 90%) using a random-effect model since the heterogeneity test showed the I2 value >50%. The gene discussed is BDNF; the disease is type 2 diabetes mellitus.