Note: This has been in my draft "hopper" for a while. I thought I might as well dust it off and post it. ;-)
I've been thinking for a while about the whole caloric balance arguments and theories of obesity, etc. I keep coming back to a simpler model of it all to which there really can be no argument. That is that matter is neither created nor destroyed. What goes into a vessel must come out of that vessel or the vessel will weigh more.
Intake: macronutrients, water, oxygen, minerals and other micronutrients -- mostly oral ingestion and inhaling.
Outgoing: excretory products -- mostly expiration and urine, but feces (containing mostly unabsorbed intake) and perspiration as well.
Metabolism, bioenergetics, whatever ... it all boils down to the rearranging of atoms and molecules for some purpose. In the end, forget the purpose and just look at the atoms themselves. We cannot convert an atom of oxygen to an atom of carbon. If we take in XX carbon atoms in the form of fat, carb or protein, they are either excreted or they stay in some form in our carbon-based lifeforms. Same for everything else.
As it turns out, it is the macronutrient mass balance that pretty much "rules". Once we ingest a carbohydrate, fat or protein -- it is broken down into simpler carbs, free fatty acids and amino acids and absorbed. Anything that passes through undigested/absorbed is a freebie. Once it gets into "circulation", you own it. Our bodies are ruthlessly efficient and even recycle things we temporarily excrete!
But once we "own" something, it will add to our net mass/weight unless we get rid of it somehow. How do we get rid of carbon we don't need/use for structure? We exhale most of it as CO2, and some more in urea that also serves as an export route for nitrogen through urine.
In any case oxidation/breakdown is the primary route *OUT*. So here is where mass balance ties in with the caloric balance. If we don't oxidize intake, we retain it and gain weight. Plain and simple. The "little stuff" like H2O and CO2 can get out. The former is mostly our solvent in which all our bodily chemicals are dissolved, etc. The latter is the byproduct of oxidation. But what of bile and concurrent secretions from the liver into the intestine? The vast majority are recycled (re-absorbed), so this is not a major excretory route. Ketones? I'll give you that one, but this has been determined to be a small amount in the non-adapted person, and even more negligible in someone who is keto-adapted.
Hormonally driven theories of fat accumulation fall flat when one considers mass balance. What goes in must come out or be accounted for. IF a zero carber consumes 2 pounds of only rendered pork fat one day, those fatty acids MUST go somewhere. Those carbon atoms, etc. cannot be created nor destroyed. Therefore, no amount of hormonal response or lack thereof can render the mass balance LAW moot.
But CS, what of futile cycling and uncoupling?
1. Futile cycles: We have many of these in our bodies that are seemingly futile -- IOW there does not seem to be a purpose to do work. But they have a purpose, often to maintain body temperature. However such cycles consume ATP which was made in ETC, Krebs, glycolysis, beta-oxidation ... IOW "burning" macronutrients. If a futile cycle uses a certain amount of ATP, this will be reflected in expired CO2 = mass out.
2. Uncoupling: This is our body's "pressure relief valve" if we're running a bit too fast for our ultimate needs. It's like expending energy to drive your car up a hill, but if there are too many cars up there, we release the parking brake on a few and let them roll back down. How do we account for this? ATP is used to in the metabolic equivalent of driving up the hill. This is "wasted" when we release the brake. But the wasted ATP is accounted for in the manner described in #1. When we need more cars back on top of the hill, more ATP will be expended to do that, corresponding to more expired CO2.
Bottom line, these two processes are "mass dissipating", but there's nothing magic about it. They both seemingly "waste" ATP, but I contend this is part and parcel of keeping our engines running smoothly and is largely accounted for in our basal metabolic rate (that, incidentally, is determined by our CO2 expiration rate).
So ... how do we get fat? We ingest a greater mass of carbon based macromolecules than we excrete on a chronic basis. If we don't store that excess as triglycerides in adipose tissue as we are supposed to, the lipids will either remain elevated in circulation, or accumulate elsewhere.