system
Given the following post, respond as if you're commenting on Reddit:
human
When you lose weight through healthy diet and exercise, what is actually lost and how does it exit your body?

So I know you "lose fat" obviously, but what in particular is being ejected? Also, how is it being ejected? I just realized my assumption about small clumps of fat being defecated out is probably ridiculous.
gpt
You exhale most of it. A 140 lb person resting all day exhales about 1 kg of CO2, but that's exchanged for O2, so there's a net 270 grams or so of carbon lost.

Fatty acids are 75 to 80% carbon by weight, or around 95% CH2 by weight. Through cell respiration this gets converted to CO2 and H2O. These both enter your bloodstream, and the CO2 that ends up in your lungs gets exchanged for O2.

The body is always breaking down glycogen, fat, and protein, but in different proportions depending on what's already in your bloodstream. After you eat there's a lot of glucose, fat, and protein in your bloodstream, so that inhibits the retrieval of fat from adipose tissue. As you get further in time from the digestion of your meal, more fat is extracted from adipose tissue to be used for fuel.

So that's how you lose weight, by eating less fuel and letting your body feed on its own fat stores for energy. The energy is gotten by burning carbon-containing molecules, and the "smoke" from that burning exits through your lungs.