Severe hypoglycemia (low blood sugar)after gastric bypass surgery is an increasingly recognized condition, characterized by symptoms of hypoglycemia after eating and inappropriately elevated insulin concentrations that occur at the time of hypoglycemia. Severe hypoglycemia can be dangerous and debilitating and can also impact cognitive function. At the moment no medical therapies have been developed for this disorder. Determining why some but not other patients develop this condition would allow for improved prediction, prevention, and treatment approaches. The purpose of the study is to understand the physiological changes observed in those patients who undergo gastric bypass and develop symptomatic hypoglycemia.