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 calories/shake) with 44.5 grams MCT oil (Life Enhancement Products, Inc., Petaluna, CA; 8.3 kcal/gm) added. Six patients will receive the same HMR diet but with 41 grams LCT oil (corn oil; 9 kcal/gm) 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.Both diets will be 1170-1870 kcal/day, consisting of 36% CHO, 26.2% protein, and 37.6% fat. The MCT diet will contain 30.2% of total calories at MCT oil. The percentages given are based on the shake and oil consumption only (1170 kcal/day). The remainder of the diet will vary according to patients' selections from the list of permitted foods, up to an additional 700 kcal/day (for the total 1870 kcal/day). All subjects will receive a multi-vitamin each day. Subjects will also receive two Fibercon capsules per day to prevent possible bowel changes associated with being on a full liquid diet. During surgery, biopsies of visceral and omental adipose tissue, as well as subcutaneous abdominal adipose tissue, will be obtained. Tissue will be analyzed to determine mRNA levels of key enzymes in fatty acid esterification, lipolysis, and oxidative disposition. This preliminary phase will be conducted to determine whether visceral, omental, and subcutaneous abdominal adipose tissue behave similarly when exposed to an MCT-based diet.