 Phase I, 6 weeks: Intensive weight loss 42 obese subjects with type 2 diabetes mellitus or impaired fasting glucose FBS greater than 100 mgdl and a patient at the Nutrition and Weight Management Center at Boston Medical Center will be randomized to receive either an MCT-based or LCT-based liquid diet for a 6 week period Phase I to establish compliance followed by a second 6 week period Phase II. These diets will be isocaloric 1083 kcalday and identical except for the quality of the fat. The intervention will be double blinded. The MCT diet will consist of 5 liquid meals using a commercial product plus 34 grams MCT oil Life Enhancement Products, Inc. Petaluna, CA; 8.3 kcalgm added per day to the HMR shakes. The LCT diet will utilize 5 HMR shakes plus 31.5 grams LCT oil corn oil; 9 kcalgm per day. 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 400 kcal per day from this list and will be asked to add these to their food records for monitoring of their caloric intake. A dietitian will instruct subjects at a baseline visit on behavior modification and a moderate physical activity program. Baseline testing includes the following: Body composition by DEXA Hologic; Subcutaneous abdominal adipose tissue biopsy; blood work. Subjects who do not lose 5 of their baseline weight by the end of Phase I will be withdrawn from the study. If subjects do not lose 5 in Phase I, they will be considered inappropriate for a liquid diet, and therefore for the study, and will be discontinued from the study. 