Water is urinated out!
What we call fat is often more than 40% water.
When your body burns fat, the residue is water and CO2.
You breath out CO2, you breath out water, and you urinate and sweat water.
Sure, urine and sweat also contains many other things, but you can breath out a lot of water and a lot of CO2.
It is very easy to breath out or to sweat out 2 lbs (1kg) of water per day, if you live in a dry climate.
If you live in an extremely dry and hot place, and if you are a big overweight person, you can sweat and breath easily 6lbs (3 kg) of water, and if you are very physically active, even double as much.
How much water we breath out, you can easily see during a cold winter day, when every single breath is visible as a cloud of smoke coming out from you.
WS