system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - When people lose weight, where does it go?

So, you go out, you exercise a lot, you eat less food and what you do eat is healthier.  At the end of the week, you are 5 lbs lighter (just a guess).  What happened to that 5 lbs of fat?  Is it converted directly to energy?  Do you have to wait until you next defecate to actually "lose" the weight?
gpt
The way your body uses food is that it burns it with oxygen, much like wood in a fireplace burns.  The reaction in your body is a lot more controlled, which is why you don't go up in flames.  But it does produce some of the same byproducts, namely heat, carbon dioxide, and water.

The same is true for your fat stores.  They get burned up with oxygen as well, just like if you ate a piece of fatty meat.  So where does the fat go?  You breathe it out as carbon dioxide, and urinate it out as water.  So imagine that next time you're in the gym.  Every breath you exhale is some of your fat leaving your body.