The problem isn't metabolic or thermodynamic or anything like that. The problem is measurement. We cannot actually measure calories in or out in a direct way.

The problem isn't metabolic or thermodynamic or anything like that. The problem is measurement. We cannot actually measure calories in or out in a direct way. If I eat an apple, I know that X grams of apple will be about Y calories. Usually. For typical apples. I cannot actually measure the apple I ate. It's a destructive process. Both the measuring and the eating. Similarly, I cannot measure the caloric expenditure of everything I expel. Either through breath, sweat, heat, waste, etc. It's difficult. We have approximations. And it's hard to get that information because we're always breathing, drinking, eating, sweating, urinating, defecating, etc. So if were to weigh myself right now, fast for the entire day, then come back in 24 hours and weigh myself again, I'd still only have an approximation of how many calories I expended in that time. When we talk about how things are "processed" what we are really doing is talking about whether or not they are processed. A calorie that gets expelled through waste is not really a calorie consumed. It just passed through you. Look back to Olestra. It was marketed as a sort of 0 calorie oil because it wasn't processed by the body. It would pass right through you. Which kind of was the problem with it. But there are other ways to cause your body to reject calories ingested. It's the mechanism behind bulimia. You can cause yourself to vomit or take laxatives to cause yourself to defecate. Whereas anorexia modifies the CI part of things. End of the day, you find out roughly how many calories you burn a day and then eat roughly that many calories a day, you'll hover in your current weight range. Want to lose weight? Eat less than that. Gain? Eat more than that.