The problem you're sweeping under the rug is that calorie in and calorie out are interrelated. You have to account for the existence of complex control systems.

The problem you're sweeping under the rug is that calorie in and calorie out are interrelated. You have to account for the existence of complex control systems. The temperature in a room is also just a question of calories in / calories out, but when there is a thermostat in the room, opening the window or adding an extra heater might not work so well. If external factors are fucking with your thermostat, it's going to be more challenging to reach a target temperature. There are complex feedback systems controlling hunger and physical activity. In one study it was found that when participants were fed extra calories, only some of them gained weight. Some of them burned them off by unconsciously fidgeting more, others didn't. People can burn 350 cals per day from fidgeting. On the flipside, excercising makes people hungrier.