[foobarian]

Like sibling said, energy must be conserved. No matter how complicated the system is, calories in = calories out - the problem is that measuring (especially) the calories out part is tricky. What if you poop out your food mostly undigested? What if you spend your days half naked in a freezing cold unheated home? Etc

[giantg2]

And because not even the experts know all the possible variables, from a practical perspective it's a half truth... so what there's conservation of energy, that doesn't give us any meaningful information to use for the application at hand.

[andreareina]

There is if you expand eating less and moving more to being a surrogate endpoint instead of just being dials. If simply eating less causes a spontaneous reduction in activity that’s a signal. If changing what you eat lets you consume fewer (or desire more) calories for the same appetite/satiety that’s a signal. Paying attention to CICO is like having a gyroscope and accelerometer to help with navigation: leading indicators of where you’re heading until actual measurements (i.e. position/weight) confirm where you actually are. In fact it’s even better in this case because of how noisy weight is and needing multiple days (at least) to reliably judge the effect of an intervention.

[giantg2]

For many people, those signals can be wrong and shouldn't be listened to.