I agree that there are several factors involved in the process of absorption for "Calories in, calories out" to be taken as a sufficient guideline for losing weight. However, the point I think people try to make when they use it is that the raw amount of calories you consume are an UPPER BOUND for the amount of energy you have available.
I know that the actual values are hard even to estimate, but we only get energy from food: if you need amount X of energy to keep functioning and the raw total of energy contained in the foods you eat is less than X, you simply can't gain weight, "asymptotically" speaking. That's why people invoke thermodynamics here.
I know that the actual values are hard even to estimate, but we only get energy from food: if you need amount X of energy to keep functioning and the raw total of energy contained in the foods you eat is less than X, you simply can't gain weight, "asymptotically" speaking. That's why people invoke thermodynamics here.