Hypothalamic Function Before and After Bariatric Surgery

Inclusion Criteria: * Patients aged 35-65 years; * BMI \> 40 kg/m2; * BMI \> 35 and \<40 kg/m2 with co-morbidity, which is expected to improve after surgically-induced weight loss; * A history of longstanding obesity (\>5 years); * Proven failed attempts to lose weight in a conventional way, or primarily successful weight loss with eventual weight regain; * Intention to adhere to a postoperative follow-up programme. Exclusion Criteria: * BMI\> 50 or body weight \> 150 kg (because it is impossible to safely put people above this weight on a standard table for MRI); * Subjects with disease related obesity, i.e. Cushing or medication related obesity; * Use of medication known to affect glucose or lipid metabolism (i.e. prednisone) * Monogenetic diabetes: MODY, Mitochondrial diabetes; * LADA (adult-onset latent autoimmune diabetes); detected by c-peptide measurement on screening. * Impaired renal function (serum creatinine \> 176 μmol/L); * Leg ulcers, gangrene. * Any genetic or psychiatric disease (e.g. fragile X syndrome, major depression) affecting the brain * Any significant chronic disease * Renal or hepatic disease * Pregnancy * Recent weight changes or attempts to loose weight (\> 3 kg weight gain or loss, within the last 3 months) * Aerobic exercise more that 3 times 60 minutes a week * Alcohol consumption of more than 28 units per week at present or in the past * Recent blood donation (within the last 2 months) * Recent participation in other research projects (within the last 3 months), participation in 2 or more projects in one year * Contra-indications to operative procedures as regular in bariatric surgery * Contra-indications to MRI scanning