 Twelve obese patients approved for gastric bypass surgery will be randomized to either receive an MCT-based or LCT-based liquid diet for a 4 week period. These diets will be identical except for the quality of the fat. The intervention will be double-blinded. Six patients will receive a daily diet consisting of 5 liquid meals using the HMR 800 meal replacement product Health Management Resources, Inc; 160 caloriesshake with 44.5 grams MCT oil Life Enhancement Products, Inc., Petaluna, CA; 8.3 kcalgm added. Six patients will receive the same HMR diet but with 41 grams LCT oil corn oil; 9 kcalgm added. Patients in both groups will be given a list of supplemental foods that are suitable for the study. They may choose to eat up to an additional 700 kcal per day from this list and will be asked to complete food records for monitoring of their caloric intake. Patients will undergo gastric bypass surgery after 4 weeks of being on their diets. Patients in both groups will be seen weekly by a registered dietitian and a physician. Blood pressure and weight will be recorded, and any adverse events will be noted and cared for as is appropriate. Dietary instruction will be reviewed as needed. 