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Comment by renewiltord | Hacker News Reader

I 'discovered' this as a teenager because my weight in the morning would be lower than my weight at night and I had done nothing but lie in bed comfy all night. There was only one place it could reasonably go - now that I wasn't wetting my bed that is.

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The amount of mass exhaled as CO2 at night is nowhere near as much as the loss through perspiration and evaporation. Water loss happens pretty much constantly and is why one dies of dehydration after a couple days.

I was curious about this. I'm on a diet now and I often weight like 0.5-0.8 kg less in the morning.

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.

It pretty much has to be. 1kg would be 1L of water, which is not that much to lose over 5 hours.

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. :)

Short-term energy consumption on a 5 hour hike would be from stored glucose, fat breakdown IMHO would happen slower. A 20km hike can consume 1000-2000kcal depending on how hard it is (climb, terrain, weight of hiker, weight of pack matter) so that would be 0.25-0.5kg of carbs - so potentially that loss could be 50/50 carbs and water.

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.

Depends on what you’ve eaten recently. It’s pretty easy to burn fat on a hike if you go while fasting.

Humans lose moisture for a variety of reasons normally, you don’t need to be sweating in order to lose it, although that obviously drives the loss rate up. You lose a ton of water through just breathing.

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.

Yes. A lot of fad diets and supplements work on this principle; dehydrating you appears to lose weight really quickly initially. It's not sustainable.

The human body is darn efficient.

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.

I suppose you could also lose weight via the moisture in your breath, or sweat of operation. But yes, it’s pretty cool.

You do lose weight via the moisture, typically around 200g if this article[1] is to be trusted (doesn't cite its sources). More if you are sweating heavily for one reason or another. Much bigger effect than calories burnt in the same duration.

[1] https://washingtoncitypaper.com/article/221338/straight-dope...

how did you weigh yourself so precisely?

Its pretty common to weigh 1lb less in the morning than night, don't need a fancy scale for that. But decimal level accuracy scales are also common.

This takes surprisingly little precision, you really exhale quite a lot of mass over 8 hours.

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