Metabolic tests are carried out on two separate days in random order. On both days, patients are admitted to the Metabolic Clinic Research Center at 07:30 after an overnight fast. All studies are carried out in a quiet, temperature controlled (22° C) room. On one day an oral glucose tolerance test (OGTT) (75 g) is performed to assess beta cell function. On a separate day, a euglycemic insulin clamp is performed to assess insulin sensitivity.