Another useful option would be to assess circulating BDNF in healthy and AD subjects divided according to the WHO body mass index (BMI) classes defined for the adult population (i.e., moderate/severe thinness, BMI ≤ 17 kg/m2; underweight, 17 < BMI < 18.5 kg/m2; normal weight, 18.5 ≤ BMI < 25.0 kg/m2; overweight, 25.0 ≤ BMI < 30.0 kg/m2; obesity, BMI ≥ 30.0 kg/m2; obesity class 3, BMI > 40.0 kg/m2) [132]. Here, BDNF is linked to obesity due to melanocortin 4 receptor deficiency.