system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When someone loses weight after exercising & dieting for several months, what is lost and how?
gpt
1) **What is lost?**

Your blood cells are built to connect to, and transport, Oxygen (O*_2_*) and Carbon Dioxide (CO*_2_*). 

They transport oxygen to from your lungs to your cells, and take carbon dioxide from your cells to your lungs. That carbon atom is what your body excretes when it burns energy. 

2) **How?**

 That's *much* more complicated. Basically, if you eat more calories than your body uses then they're stored as fat. If you eat fewer calories than your body uses, your body will start to digest itself in order to make up the difference in energy needed.

Which cells your body digests depends on *a lot* of factors. For overweight people fat will be lost if the muscles are continually exerted, whereas fat and muscle cells will be lost without some exercise; either way the scale will show a lower number.  
For obese people, only fat will be lost at first due to the extreme unnecessary fat reserves.