Mitochondrial dysfunction has been linked to development of insulin resistance in the muscle and hence to the pathogenesis of type 2 diabetes. Mitochondrial function can be studied non-invasively by 31 Phosphor Magnetic Resonance spectroscopy (31PMRS). In this study we will investigate changes in mitochondrial function using the combination of 31 PMRS and Near Infra-Red Spectroscopy (NIRS) after gastric bypass.