The food types are: proteins, carbohydrates, fat and alcohol. Each of them have a specific role in your body. The proteins usually go to your muscles, the carbohydrates are the fuel of the body, the fat and the alcohol do not have any special use.
What you have to know is that the body consumes the carbohydrates, when the carbohydrates are lost, he transform fat and proteins into carbs for energy.
The fat will not go directly on your ass, he will be kept in the short time fuel reserves, but at the end of the day it will be deposited into fat reserves in your body. The unused fat and carbohydrates are deposited, the proteins not. The proteins will be deposited as muscles only if you do physical exercises.
The proteins and carbohydrates have about 4 calories. Fat and alcohol have about 7.