The 3rd year biochemistry course I just took focused on metabolism in the last month. My teacher, who I trust to know her stuff, said that there are medical cases were severely obese people have lived under hospitalized conditions on nothing but water alone. That is, there body has accept the fact that now carbohydrates, additional fats or proteins is coming their way to be digested and absorbed and switch to utilizing fatty acids that have been stored in adipose (fat) tissue. With just water intake to keep them properly hydrated, they have lived off of their own body fat for one year.
Most of us live to eat. I know I sure do, but if you eat to live, that is taking in only what is essential for your body to function you would find the calorie count to be a lot lower. There are stretch receptors in your digestive system that detect a full stomach and signal the brain that you have reach a saturation point. I don't know what people feel like who have had the surgery, but I imagine that they feel fuller sooner because the receptors will be triggered alot sooner in the small stomach sac than in others.
Speaking of eating, have takin my first metabo ultramax, now I am going for lunch