Meta-regression based on mean duration of diabetes (reported only in 9 of 14 studies) showed that this variable did not significantly related to serum/plasma BDNF values in cases vs. controls (β = − 0.162, P = 0.64, I2 residual = 99.38%, Adjusted r2 = − 10.72%) and could not decrease the observed heterogeneity. This evidence concerns the gene BDNF and diabetes mellitus.