> They are still subject to the basic laws of physics. If you do a certain amount of work, but you take in less than what you have spent, the body has to use its stored energy to make up for the difference. How else would this work? The body cannot magically materialise energy out of nothing, and it cannot use more than what is available in the form of intake plus storage.

Your body temperature goes up and down, changing how much energy is spent by body. Your ability to perform goes up and down - you become sleepy and tired, your brain spends less energy and that is that (yes it has impact on your attention and performance). You cant stop it from happening. You may try to exercise the same and can easily become unable to.

Your body gets rid of calories in poo and piss, when sweating or not.

Your body can just turn off functions and generally malfunctions and you dont have direct control over it.

Basic laws of physics is a misnomer. If I consume gasoline which has calories in the physics sense (31,000kcal/gal), it doesn't affect my weight. CICO is a broken model because it doesn't account for this.

Calories on nutrition labels should account for potential metabolic energy, not total energy, so the example of gasoline isn't by any means a disproof of the model at all since gasoline is obviously not a food source you safely metabolize.

If we looked at total energy of your gasoline we'd need to consider mass to energy conversion as well... but your body can't do that either. Energy your body can metabolize is clearly a subset of total energy available. It's certainly an estimate and it's not perfect but it's useful.

Right, and what factors modulate the actual metabolic energy?

Well that's where all the biochemistry comes into play. There are pathways where various components are broken down through physical force, stomach acid, gut bacteria and enzymes, etc.

This looks like a digestible introductory guide: https://themedicalbiochemistrypage.org/digestion-and-digesti...

And a brief description of how all that helps inform calorie counts on nutrition labels: https://en.m.wikipedia.org/wiki/Atwater_system

People make entire career studying these sort of processes. Over time the information is refined. All of this ultimately helps inform how we measure and quantify things like calorie counts on food labels. Again, not perfect, but the process works and improves over time. It's a complex macroscopic system relative to the underlying chemistry and physics but if we assume larger systems are built on, composed, and governed by principles of constituent systems then we have a way of slowly understanding these complex processes and using that knowledge to our advantage.

Just as with software systems we rely on abstract processes below us to talk about things. That doesn't mean the high level abstractions are ignoring the details, it's just that there's value in talking about things simplistically like "calories in" and "calories out" without people looking to take very literal interpretations and trying to convert total energy possible into kcals. Heck, this is why they're "Calories" and not "calories" and why "Cal" has even been proposed to help avoid this sort of confusion.

Right, this is a very long way of saying that CI = modulating factors * CO which is the point I've been trying to make.