Eli5: When you go to sleep weighing a certain amount and wake up weighing less. Where did that weight go?
In: Biology
the people saying that you lose weight through breathing are absolutely not trolling. they are correct. in fact, the only way you can lose weight permanently is through breathing, by expelling CO2. but the reason permanent weight loss is difficult and slow is because it takes a while for all that CO2 that you expel to add up to a significant amount. but that is the mechanism by which weight loss is achieved.
other types of shedding body mass are significant, yes, as others have pointed out. you can easily lose kilograms worth of water weight in not more than a few hours through intense exercise. but all that weight will be gained back when you rehydrate. the same with urinating/defecating. you only pee what you drink (some of it), and you only poop what you eat (some of it), so to pee 200g worth of piss, you had to have drank 200g of water beforehand, the same with poop, so these forms of weight “loss” aren’t actually losses, they are just fluctuations around a constant level.
hair/skin shedding also follows a similar logic. if you shed some hair, your body will accumulate some nutrients and grow it back, the same with skin, so the net weight of those on your body remains roughly the same.
but it is correct that if you lose quite a bit of weight on only one night, most of that loss probably came through water loss. which will be recuperated as soon as you drink water.
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