There's a very easy explanation, (well two): 1. The commonly known variation of metabolic processes means some people are just more fidgety or have their bodie

There's a very easy explanation, (well two): 1. The commonly known variation of metabolic processes means some people are just more fidgety or have their bodies generating more waste heat. You could reasonably argue this constitutes "calories out", but if it's unconscious, unmeasurable, and varies arbitrarily it doesn't really fit the paradigm of "It's simply a matter of behavioral difference" 2. The part that everyone forgets: human waste contains a non-zero amount of calories, and this varies between people. If your body thinks it doesn't have enough calories (obviously an oversimplification) it will work harder to extract more calories from what you eat. If your body thinks it has enough calories, it'll ignore calories it doesn't want and simply excrete them. The body can tune the amount of calories it gets from food based on its understanding of your environment, and so it's possible to cut down on the amount of calories you ingest and still net more calories from your food than you started with. Thus, calories in calories out is as much of a fallacy as telling people without money that all they're missing is a simple paradigm of cash in cash out, as if loans can't be adjusted based on the fed's interest rate, their landlord can't decide to charge them more, gas and food can't get more expensive, and they can't lose their job, all outside of their control. This is going to be a poor metaphor, because the human body isn't capable of this, but if a society was super reliant on energy efficiency, tried sticking uranium in a furnace and concluded that it's a poor, useless fuel, they'd be missing that through another process you can get far more energy from uranium. But that's how we look at food: caloric content is measured by literally burning it and measuring how much energy it gives off, as if that's a reasonable analogy for the human body.