Yes. All patients must go on the Optifast shake diet for 2 weeks prior to surgery in order to shrink the liver and reduce fat in the abdomen. In order to perform a bariatric procedure, the liver must be lifted to access the stomach so the pre-surgery diet assists with surgery and makes it safer for the patient. The length of this diet is determined by your BMI. This is evidence based, see reference below.
(1) Lewis MC, Phillips ML, Slavotinek JP, Kow L, Thompson CH, Toouli J. Change in liver size and fat content after treatment with Optifast very low calorie diet. Obesity Surg 2006 06;16(6):697-701.