Inclusion Criteria:

• Normal weight, overweight and obese subjects with BMI> 18 Kg/m2 residing in Olmsted County, MN: Otherwise healthy individuals who are not currently on treatment for cardiac, pulmonary, gastrointestinal, hepatic, renal, hematological, neurological, endocrine (other than hyperglycemia not requiring medical therapy) and unstable psychiatric disease.
• Age: 18-65 years
• Gender: Men or women. Women of childbearing potential will have negative pregnancy test within 48 h of enrollment and before each radiation exposure.

Exclusion Criteria:

• Weight exceeding 300 pounds or 137 kilograms (due to limitations regarding SPECT imaging studies).
• Abdominal surgery other than appendectomy, Caesarian section or tubal ligation.
• Positive history of chronic gastrointestinal diseases, systemic disease that could affect gastrointestinal motility or use of medications that may alter gastrointestinal motility, appetite or absorption e.g., orlistat (Xenical).
• Significant psychiatric dysfunction based upon screening with the Hospital Anxiety and Depression Scale [HADS] self-administered alcoholism screening test (substance abuse) and the questionnaire on eating and weight patterns (binge eating disorders and bulimia). If such a dysfunction is identified by a HADS score >8 or difficulties with substance or eating disorders, the participant will be excluded and given a referral letter to his/her primary care doctor for further appraisal and follow-up.
• Intake of medication, whether prescribed or OTC medication (except multivitamins) within 7 days of the study. Exceptions are birth control pill, estrogen replacement therapy, and thyroxine replacement.
• Concomitant use of MAOI inhibitors and other centrally acting appetite suppressants (since this would make them ineligible for sibutramine treatment).
• Hypersensitivity to sibutramine (since this would make them ineligible for sibutramine treatment).