I wonder where/when this calories-in-calories-out trope ever made sense. Metabolism is so complex that the energy lost during reactions between and into metabolites cannot be measured - they happen inside cells mostly, at an insane rate. The mere presence of increased active thyroid hormone (T3) will elevate body temperature, leading to increased heat loss. Muscles become more efficient when used often. Muscles load glycogen after sports. Energy is conserved by warm clothing. Low room temperature leads to extra energy loss. Etc
How is one supposed to even approximate the energy balance without a fully body composition (dexa) scan every week?
due to conservation of energy, energy lost during reactions between and into metabolites becomes body heat, which can be measured; also virtually all of it consumes inhaled oxygen, which can also be measured
Body heat can't be accurately measured. You can just measure heat loss via the surface. But there's variations on the cell level. Since heat can be stored inside the body for a longer period, measuring heat loss over a short period of time is not representative.
Why would that be? Eg Organs are warmer than hands etc - there certainly is a gradient. The heat from metabolic processes will not become obvious immediately because it’s redistributed before it’s lost to the environment.
Besides heat loss, there’s also kinetic energy (muscle contractions, peristalsis etc), various pumps (eg sodium/potassium), fluid flow (blood, lymph etc) and weird things like biophotons.
I think calories in/out is just propaganda used to help the average person not overeat. Like most of the medical stuff were taught, it's a half truth because the systems are so complex that even the experts don't truly know.
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
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.
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.
Here's something that's always puzzled me. It seems almost inescapable that if two people of the same size have radically different metabolism, their body temperatures must be different. If anything, a skinny person should consume less energy because they have less skin.
Granted, this is me, a physicist, thinking simplistically. But it's what's always made me skeptical about metabolism.
There are too many aspects. Like gut flora and even enzymes present. Those could affect how and to what input is broken down. And then there are hormone responses and how body responses to those. In the end we are probably very far away of having complete understanding what is going on. And differences might be minute, but will add over years.
How is one supposed to even approximate the energy balance without a fully body composition (dexa) scan every week?