Fun fact, this is also where fat goes when your body burns it: Into the air.
https://www.medicalnewstoday.com/articles/287046#Tracking-th...
[1] https://washingtoncitypaper.com/article/221338/straight-dope...
And when I go on a 20km walk on the weekend I lose about 1kg in 5 hours. Is it mostly water? Cause I drink a lot during the walk and I don't go fast enough to get sweaty.
But 1kg of fat is 7700 kcal, which would be a massive amount of energy to expend in 5 hours. Unless your "walks" involve running uphill with a 25kg pack or something like that. :)
But hydration definitely is a big aspect of mass simply because of how much you add and lose. I once did a ~90km hike in 24 hours or so, and I believe I drank something like 15+ litres of liquids during that.
You would probably have to run 50-70km to lose 1kg fat.
It's easier to not eat calories than to burn them after eating.
On your walks you’re probably burning glycogen, the carbohydrate we store in our liver and muscles for easy access energy, and exhaling a lot of water. We store about half a kilo of glycogen which is then bound up with 3-4x as much water still. While you’re not likely to consume all your glycogen in a 20km walk (you need a marathon to do it), burning a decent chunk of it on top of normal water loss isn’t an unreasonable hypothesis.
A couple of years ago, I started getting headaches. Everything in my noggin appeared normal, and one day I worked with the window open. No headache. So I got my first one, it wasn’t very expensive (€50-100 IIRC) and did some experiments. Sure enough, it would get up to 1800ppm in my tiny office and I’d get a headache.
I’d say the biggest thing it seems to affect is coding. If I can keep it below 600ppm in my office, I can code all day and late into the evening without feeling too exhausted. At about 1k, I start to feel kinda sleepy and tired after awhile.
My wife and kid had fun doing the observations and asking the questions during the experiment. Good times. We used it as a way to teach our son how to do experiments and ethics.