It's an extremely simple idea, but not quite a tautology. It's a necessary idea for debunking the worst of the fad diets. The second-order effects you allude to

It's an extremely simple idea, but not quite a tautology. It's a necessary idea for debunking the worst of the fad diets. The second-order effects you allude to as the "interesting questions" are helpful for understanding things like how one diet can be easier to adhere to than another, or why the calorie counts on nutrition labels aren't 100% accurate in all circumstances, but the basic calories in/calories out idea still explains weight gain vs loss in a quantitative way.