The thing is that "energy in-energy out" works if you make some changes while keeping everything else constant.... and we're really, really bad at keeping every

The thing is that "energy in-energy out" works if you make some changes while keeping everything else constant.... and we're really, really bad at keeping everything else constant. Add extra activity, and the natural effect will be an increased appetite that'll nudge you to eat more calories - and it's not a given the effect of that extra activity will be a calorie deficit, the increase in appetite can easily be more than what you spent in those exercises. If you try to reduce caloric intake, that's a very different beast than actually reducing caloric intake - if you try to reduce caloric intake by skipping carbs at lunch, it can result in reduced activity (so less calories out) and a strong nudge to "cheat" at other meals or snacks; and again it's plausible to get to a situation where the unintended compensation is larger (and opposite) than the behavior modification that you made. That is the problem with the "energy in-energy out" model - one does not simply change the 'energy in' or 'energy out' part and expect that other things will stay constant either magically or through sheer willpower.