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

I know that mass and energy both have to be conserved in any system (if you take into account all the inputs and outputs).

So I am assuming that a (simplistic) way of looking at it is:

**If you are losing weight:**

    (Calories from food ingested) + (Calories removed from body fat stores) = (Calories burned)
    
    (Mass from food ingested) + (Mass removed from body fat stores) = (Mass lost)


**If you are gaining weight:**

    (Calories from food ingested)  = (Calories burned) + (Calories deposited in body fat stores)
    
    (Mass from food ingested)  = (Mass lost) + (Mass deposited in body fat stores)


When you are losing weight, the "Mass lost" will include what you pee, poop, breathe and sweat out.  But if you exercise more, what is that *extra* part of the mass lost?  

What are by-products of the chemical reactions in the cells when you use energy?  Is it H20 (you pee it out)?  Or is it CO2 (you exhale it)? Or something else?  I remember years ago learning about ATP, but I can't remember any details on that stuff any more.

My hunch is that you breathe heavier when you exercise and that mass is lost as carbon in your CO2.  But I'm sure there are some bio-chemists out there that can give me an explanation about that is going on.
gpt
The carbon in your body which is stored in fat you lose by breathing out (CO2). Its kind of a cool concept, the air you breath out is heavier than the air you breath in. As you debulk in size you will also lose a certain amount of water, although your body tries to keep a pretty consistent amount of water per cell volume to maintain homeostasis. Try looking up an oxidative metabolism wiki if you would like more chemical details. 

Cool thing to note though is that you don't exactly have CO2 cruising around in your blood stream. This is a gas, and would cause many problems. Your body transfers the CO2 around as carbonic acid, which is degenerated into CO2 in the blood -> air transition in your lungs.