Who Will Benefit From Bariatric Surgery for Diabetes?

Bariatric surgery group Inclusion Criteria: * Males and females planning to undergo RYGB * 18-80 years * Type 2 diabetes mellitus or prediabetes * Stable weight for at least 3 months * Obese (BMI ≥30kg/m2) * Eligible for surgery on the National Health Service (NHS) under The National Institute for Health and Care Excellence (NICE) 2014 criteria Exclusion Criteria: * Current pregnancy * Inability to give informed consent * Type 1 diabetes * Low fasting C-peptide * Secondary diabetes or absence of β-cell function * Unable to undergo DEXA, cirrhosis, ascites, or other condition that may modify body fat composition e.g. underlying malignancy * Current smoker * Participation in another (interventional) trial within the last 3 months * Unable to understand English Healthy volunteers Inclusion criteria: * Aged 18-80 years * Male or female * Body mass index 19 - 25 kg/m2 * Stable weight for at least three months Exclusion criteria: * Abnormal glucose tolerance and fasting glucose * History of any medical, or other condition, or use of any medications, including over-the-counter products, which, in the opinion of the investigators, would either interfere with the study * Without access at home to a telephone, or other factor likely to interfere with ability to participate reliably in the study * Pregnancy or breastfeeding * Unable to maintain adequate contraception for the duration of the study * Donated blood during the preceding 3 months or intention to do so before the end of the study * Current smoker * Participation in another trial within the last 3 months * Unable to understand English