[lrei]

Eat (drink) 4000 calories of gasoline. wont make you fat. point is: only calories you can convert into "human fat" will make you fatter. that process can not happen without certain chemicals (hormones) in your body, like insulin mentioned above. it's not as straightforward as that which was op's point.

[koluft]

Are your confusing nutritional calories with physics calories? When people talk about diet, of course they are talking about calories obtained from digestion, not combustion engines.

[lrei]

Energy is energy, a calorie is a calorie in gasoline, food or electricity. However when talking about nutrition labels, those are generally an estimate of calories potentially extracted by humans (which I think is what you mean with nutritional calories). I'm not confusing anything. But some people seem convinced that the car analogy is exactly how the human body works - you put food into your mouth and it gets "burned" by exercise or stored (calories in, calories out) and no amount of evidence or science seems to convince them otherwise.