Additional Study Information: The goal of this project is to understand why bariatric surgery is such an effective treatment for obesity with a focus on brain mechanisms. Cerebrospinal fluid neuropeptides, hormones, and protein levels will be measured in subjects before and after bariatric surgery as compared with subjects before and after diet-induced weight loss. Participants in the diet arm will follow a low-calorie liquid diet (Optifast) for approximately 12 weeks. They will have two lumbar punctures (spinal taps) and liquid test meal challenges, as well as weekly nutritionist counseling. These participants will receive $900 for completion of the study. Diet participants who have achieved 10% weight loss at 3 months will have the option of continuing to receive nutrition counseling and to be studied with a third lumbar puncture at 1 year if weight loss is sustained; they will receive an additional $400 in compensation. Participants in the surgery arm will begin before their bariatric surgery and be followed for 1 year. They will complete three lumbar punctures and liquid test meal challenges. These subjects will receive $1350 for completion of the study.