A very simplifed answer is we mostly burn carbs, which are different variation of sugars. Sugar is mostly made of Carbon atom linked to Hydrogen atom and Oxygen atoms. You breath in extra oxygen, and breath put the carbon atoms in the form of CO2. The remaining hydrogen and oxygen form water (H2O) that come out as pee and humidity in the breath (which is why you can exhale white fog in cold weather).
This is also the reason why you don’t shit as much mass as you eat, most of the mass when out in your breath and in your pee.
Fats are just Carbon and hydrogen, same principle, just adding more oxygen from breathing and producing more water then carbs (camels’ back are full of fat, which gives them water when consumed).
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