How do we actually 'lose weight' - what is 'lost'?

I never realised how we actually ‘lose weight’

As a dummy I just assumed in the process of converting food eg fat into energy weight just... disappears.
But Science knows otherwise - and according to one of its rules;
The Law of Conservation of Mass - mass (weight) cannot just disappear.

So if I diet how do I 'lose' weight?

Amazingly most it is lost through breath.
Seriously.

Specifically by the addition of the body’s waste product - carbon - to oxygen - creating carbon dioxide.

To make energy in the body:
* You breathe in oxygen (O₂) through your nose or mouth.
* The oxygen goes into your lungs.
* Your blood carries the oxygen to all parts of your body.
* Your cells use the oxygen as a reactant to release energy from food.
* This process makes water and carbon dioxide (CO₂) as waste.
* Your blood carries CO₂ back to the lungs.
* You breathe the CO₂ out.

A practical illustration

To make the point
If you stop eating - but still drink water:
* Your body still needs energy.
* After using stored glycogen, it mainly burns fat stores.
* The carbon in that fat combines with oxygen you breathe in.
* Most of the lost mass leaves your body as carbon dioxide (CO₂) in your breath.

The process is essentially:

Fat + O2 → CO2 + H2 O + energy

So the fat does not “turn into energy” directly in a way that removes mass/weight.
The mass leaves physically as:
* mostly CO₂ you exhale
* some water in urine, sweat, and breath

The actual fat mass loss mostly exits through your lungs.
That’s one of the surprising facts about biology:

Your lungs are the main organ for fat loss.
Am I the only person that didn’t know this?

References

Meerman R, Brown AJ. "When somebody loses weight, where does the fat go?" British Medical Journal (BMJ), 2014. DOI: 10.1136/bmj.g7257.
Full article (open access)
PubMed

UNSW News (University of New South Wales). "When we lose weight, where does the fat go?" December 2014.
Link

BBC News. "Fat 'breathed out' of body via lungs, say scientists." December 2014.
Link

Merritt EK. "Why is it so hard to lose fat? Because it has to get out through your nose." Exercise and Sport Sciences Reviews, 2021. DOI: 10.1152/advan.00006.2021.
PubMed