Your surgeon wants you on a low/no carb, high protein diet to shrink your liver. Most morbidly obese have an enlarged liver due to the amount of carbs eaten. The liver is next to the stomach and shrinking your liver gives him more room laparoscopically and less risk to you.
Rule of thumb is don't eat anything white which contain simple carbs - sugar, bread, pasta, rice, potatoes, etc. This does not mean that you have to take in your protein pre-op through supplements though finding a protein supplement that you like is important; however, your taste may change post-op.
Usually clear liquids are given while you are in the hospital. Once released you are in Phase 1, a liquid diet for 2 weeks post-op which does not mean clear liquids or clear protein but includes protein shakes/smoothies, bullion, blenderized soups, jello, sugar free Popsicles, etc. Phase 2 is soft foods for 4 weeks whi*****ludes Phase 1 foods plus eggs, seafood, cheese, yogurt, beans, etc. Phase 3 is "normal" healthy food for the rest of your life.
HW: 392 lbs SW: 377.5 lbs CW: 215 lbs GW: 190 lbs