Objective: In this study the researchers aim to investigate the influence of metformin (500 mg 1/day) on temperature in the colon, glucose uptake in the colon and energy expenditure in healthy lean (BMI < 24kg/m2) or obese subjects (BMI>28kg/m2). The investigators will measure 18F-FDG uptake in the colon, temperature in the colon, insulin sensitivity and energy expenditure before after using metformin.