FWIW, I thought the same thing about my body for a long time, most of my life. I have usually been slightly overweight, a bit chubby, but pretty active in terms of sports and outdoor activities and exercise. Then I tried counting calories, and it totally worked, and I realized I’d forever been mentally resisting the idea of budgeting my food. My theory is that my regulating system isn’t gut absorption, it’s just how full I feel, and the threshold where I choose to stop eating, on average. My threshold is slightly higher than what I need to be skinny, and counting calories taught me what it feels like to eat the correct amount for me, which leaves me just a tiny bit hungry. I’m not currently counting calories, and I’m gaining weight little by little. Hard to say what happens when you have a one-off over-eating meal that’s insanely high calorie. You probably don’t absorb all the calories of an abnormally high-calorie meal, but that may change if you always eat a high calorie meal. Under-eating on occasion might have a different effect than over-eating. But yeah, it is easy to measure, if you can bring yourself to do it. I developed mental tricks so that it was easier to count calories and be fine being a little hungry, I feel like it’s important to avoid framing it as a self-control problem, or as overcoming hunger, and find ways to actually want to stick to the budget.