To me the main issue with the calories in/out way of thinking is that while generally correct your body deals with different types of calories very differently.

To me the main issue with the calories in/out way of thinking is that while generally correct your body deals with different types of calories very differently. Obviously high quality protein and processed sugar cause very different hormonal responses in the body. The calories in/out way of thinking sort of brushes this under the rug. The weight regain phenomenon you mention can be explained in part by the hormone ghrelin. I’m no nutritional scientist but from my understanding ghrelin is involved in meal initiation and is a driving factor in the weight regain after a period of weight loss. Conversely the hormone leptin acts to suppress food intake and would cause someone to lose weight after a brief period of weight gain. Together when functioning correctly these work to “stabilize” ones weight. Obese individuas are thought to be leptin resistant. As you mention, hormonal effects are king and you want to take great care to not overly stress or break your body’s systems, for example, by becoming insulin or leptin resistant.