We piss and shit out calories. Our temperature goes up and down. Bacterias in our guts eat calories, making us absorb more or less. There are multiple health issues that will make you loose or gain weight quickly with zero change in lifestyle or food. Oh, and we become tired, weak and passive which makes us spend less energy then if we are feel good.

The assumption that body resembles simple closed physical model is absurd.

OK then let's lock a bunch of us in a vacuum thermos like in that "DEVS" show, have them eat a reasonable diet of 2k calories a day, have them walk on hamster wheels and hope they will produce > 2k calories worth of energy. Scale out, profit! Spoiler alert, this will not work.