Clinical epidemiological investigation also observed that the serum BDNF level of diabetic patients were significantly lower than that of normal controls and were reversely correlated with blood glucose concentration independent of obesity, indicating the robust relationship of BDNF and glucose metabolism in that the hyperglycemic status of diabetic patients might have repressed the release of BDNF from the brain [39, 40]. Here, BDNF is linked to obesity due to melanocortin 4 receptor deficiency.