 The purpose of this study is to investigate the short term and long term effects of Roux-en-Y gastric bypass RYGB surgery on energy expenditure, gastrointestinal and appetite regulating hormone levels, and appetite sensation. We hypothesize that following RYGB surgery, metabolism will be elevated in comparison to patients who have not yet had RYGB but who are losing weight simultaneously using a low calorie diet. We further hypothesize that this higher metabolism will be associated with alterations in fasting and postmeal levels of gastrointestinal and appetite regulating hormones. Long term 1.5 years after RYGB, we hypothesize that differences in metabolism, body composition, and hormone levels will distinguish between patients who have maintained their weight loss after RYGB vs those who have regained weight. 