After an overnight fast, the patients are instructed to ingest Ensure® (1.06 kcal/ml; 22% fat, 64% carbohydrate, and 14% protein) at constant rate of 30ml/min, and complete a Fullness Rating Scale (FRS) at 3-min intervals (ie. 0min, 3min, 6min…). The test will stop when a score of 4, which is equivalent to unbearable fullness, is reached in FRS. The volume (calorie) of Ensure that is ingested will be recorded and it serves as surrogate marker of satiety function. Serial blood sample (2ml each) will also be taken at 0, 30, 60, 90, 120min during test for assay of ghrelin and gastrin profile. If the test is stopped before 120 minutes, the remaining blood will still be taken according to schedule.