Be Excellent To Each Other

From a fascinating thread on Quora, a question that took me some puzzling to answer: from a physics perspective, when you exercise and lose weight, where has that weight gone?

See if you can answer without reading the link

Edit -- I was wrong; that link is weak. The one I saw was on Stackexchange.

Conversion of mass into energy? Waaiiit a minute!

Poop
Edit: read it now... I R Surprised

Luckily, a fitness person appeared on my twitter feed, so I've asked him that, too.

It's a decent question, though.

Nope. That's just food you couldn't digest. Your intestines absorb, they don't emit.

OK, my "haven't-read-the-article-yet" guess would be that it gets oxidized into water and carbon dioxide ( and other stuff, possibly - I don't know if fat is purely hydrocarbon ) and then piddled and breathed out. I'd guess the "other stuff" would also get picked up by the kidneys and peed out. I guess there must be nitrogen coming from somewhere to make all that urea.

One carbon atom contains 4.2832644 × 10^-13 kcal. Or to put it another way, you could get 1500 kcal per day from 3.50 × 10^15 carbon atoms, which is just 69.79 nanograms of carbon. Not much weight to be lost that way

Citation for first calc: http://www.google.co.uk/search?q=+%281.994+*+%2810%29^%28-23%29+g+*+%28speed+of+light%29^2+in+kilocalories%29&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-GB:official&client=firefox-a

My 'haven't read the link' answer would be fats and sugars utilised within the cells as an energy source releasing water and waste products. The water you perspire/respire/piss out, the waste products you poop out.

I'm guessing from the fact this thread exists that I'm wildly wrong.

Oh. That appears to be correct. Thread is disappoint.

In A-level physics we once worked out the total worked out the total mass lost for all the electricity production for the UK for a year. Even though the energy was in the bumptojoules, the total mass was only about a kilo I think. c² is so huge you have to have vast amounts of energy for the mass to be noticeable.

No, it's not. You don't poop it out. Faeces is stuff you couldn't digest, not solid matter your body is disposing of.

Meh, mostly correct then.

No, because you don't know what the waste product is, so you don't know how you get rid of it, so you haven't mentioned the thing I found most interesting in the first place!

Hair, which gets shaved off.

On second thoughts, I think it goes to America.

There's only one way to be sure. Let's find someone on a diet, and place them inside an entirely sealed bubble. At the end of the week, we can analyse the contents of the bubble for a truly revolting answer.

Well, water and CO2 are the main waste products, because it's a pretty simple oxidation of a hydrocarbon. The problem here appears to be that the question is phrased "where does the weight go", not "what's the most interesting thing I read in this article"

Piss and gas. And to a minor degree shit colouring.

Squirt was right. What I found interesting is that the water is removed in three ways: urine, sweat (hardly any; sweat is about cooling you, not losing water), and exhalation (more than you'd think; exhaled air is close to 100% humidity and you get through tens of thousands of litres of air a day). The CO2 moves to the lungs through the blood stream and that's exhaled too. Tiny amounts of other waste products like lactic acid end up in your pee too, but they hardly count for anything in the grand scheme of things.

So the answer is: if you go on an exercise-and-healthy-eating bender and lose two pounds in a week, then you breathed most of that weight out. I found it surprising that it almost all goes out as a gas, is all.

So exercise freaks are causing Greenhouse gases?

BAN ALL GYMS!

Sugars -> water and CO2 [i think] via sweat, breath and I guess pee. As for the percentage split between them I've no idea but would assume most is expelled out your gob hole, as cells dump the excess co2 into your blood stream which take it to the lungs. Though it could also dissolve into the sweat so skin is probably plays a role. Sorta the inverse of photosynthesis.

Which reminds me, this is like the question of where the mass of a tree comes from.

I miss Dimrill.

Aw, thanks to too much knowledge of the respiratory system, I knew the answer to this, but I was too late

So in the time it took me to write that on. Y iPad the answer was posted. BUT STILL COUNTS!

Back of envelope calculations here ( number from wiki, on the whole )

Average resting respiratory volume = 8 litres / min
So a day's volume = 11520 litres
At body temp ( roughly ), fully humid air contains about 45g per 1000 litres, so you lose about half a litre of water a day through resting breathing rates. This will increase hugely if you exercise, as your respiratory volume increases loads then.

Note : The above calculations may be entirely wrong.

I was expecting Gaywood to complain about shit colouring so I could science his face.

He failed to take my bait

What happens when Mr Dave exercises and loses weight?

He can't lose weight - everyone else in the world just gains a little bit.

Oh..

Massive lolz.

What a silly con't

Maybe he thinks you asked the question "why do I get smaller?", which he's kinda answered? Especially as you asked where does the "weight" go, which I don't think is possible. Weight can't go anyway. It's mass etc

He's actually right, but answering the wrong question. The number of fat cells in your body is apparently genetically pre-determined and as you gain and lose weight they inflate and deflate like millions of tiny fat-saturated balloons. This is one reason why some people gain weight more easily than others. It also means that surgical treatments like liposuction are temporary; your body regenerates the same approximate number of fat cells over time.

Apparently, women have an extra layer of subcutaneous fat, too. If you were to immerse a man and a woman into very cold water and left them, the woman should survive longer.

Kate Winslet is an EVIL BITCH!

MIND. BLOWN.

Well then, have a citation: http://scienceblogs.com/notrocketscienc ... _in_ad.php

So, the guy who had a 6 stone chunk of his body removed on Embarrassing Bodies last week will just regrow new cells?

He might regrow the cells, but remember that they also took out the fat that was stored in those cells. New grown ones don't automatically have fat in them, I would assume?

No, but the fact he had a lot of fat cells in the first place, and one day will again, means he's always going to be predisposed to putting the weight back on.

Did he have a lot of fat cells, or did he have a lot of fat in his fat cells?

<pops out of hidden smuggling compartment>

I went for a walk at lunchtime and when I got back I breathed out double egg and chips. Thanks, Doc.

That would certainly explain your breath.

Your face explains my breath.

Where does your breath go when you exercise?

Into the trees!

I don't be-leaf it!

Instead of doing exercise, can I just breathe in-and-out very rapidly to lose the same amout of fat?

Yes, as long as you tie ankle weights to your lungs.