So yes and no to it being that simple. For understandings sake, you can say that it is that simple, you burn the food, some energy is lost from the food as it turns into energy/matter in other forms(matter and energy can be converted to each other), the total amount of energy never changes, only where and how its stored. On the other end of the complexity spectrum, what actually is happening when you burn that food, is alot more meticulous, for anyone's liking, even the people who study things like molecular physics lol.