While gastric bypass surgery (GBS) is known to be highly effective in achieving significant weight loss, it is also associated with other biologic changes that occur in the body when weight is lost. In this study, subjects undergoing gastric bypass surgery will be followed throughout the weight loss period. They will undergo four detailed medical evaluations to assess changes in several biological systems that occur in the body when weight is lost.

The initial assessment will be performed before the surgery (testing period 1). After surgery is completed, subjects will attend clinic visits at the Rockefeller University outpatient clinic. During these visits, weight and leptin levels will be monitored. Two additional assessments will be performed during weight loss, when subjects lose 10% and 20% of their initial weight (testing periods 2 and 3 respectively). A final evaluation will be performed after weight is stabilized, about 18 months after the surgery is completed (testing period 4). Each testing period will be performed over a 2 week period in an inpatient setting at the Rockefeller University Hospital. During testing periods subjects will undergo a series of metabolic, behavioral, hormonal, immune and molecular tests to evaluate changes that occur in the body after weight loss. Subjects will receive monetary compensation for participating in the study.

Inclusion Criteria:

• 18 - 65 years old
• Subjects approved for gastric bypass surgery

Exclusion Criteria:

• Subjects not approved for gastric bypass surgery