Hi,
Pre-op diet is a Liver Shrinking Diet (LSD). As bariatric surgery is carried out on the stomach and your liver lies partially over the top of the stomach, it needs to be lifted out the way for the surgery to take place. In order for this to safely happen the liver needs to be small and floppy.
All of us carry a lot of fat around the liver area which makes the liver brittle, so the LSD is designed to get rid of the fat and shrink the liver so it's safe for the op.
Hope that helps!