
When somebody loses weight, where does the fat go? - non_sequitur
http://www.bmj.com/content/349/bmj.g7257
======
mikeash
For anyone coming straight to the comments looking for an answer to the
question: it turns into CO2 (and a bit of water) and you breathe it out.

When you realize that you have to breathe out all the weight you lose, I think
it gives you a bigger appreciation for how difficult weight loss can be.

~~~
adekok
2 pounds a day. That's how much CO2 you breath out, and the hard limit for
weight loss.

You can lose more than that, (see boxers before a weigh-in), but that's all
water loss, and is temporary. And, with a strong likelihood of death.

~~~
leshow
I'd suspect the more exercise you do the more that number moves up, you'd be
breathing harder. I'd hesitate to try and put a 'hard limit' on anything like
this.

~~~
adekok
It's pretty difficult to double that number. Can you really _double_ your
metabolism without ill effect? Or without massively changing your lifestyle?

The normal expectation is maybe 10-25% change. Any more than than would
require things likes 8 hours of exercise per day.

~~~
mtreis86
5-6000 Calories is normal for an 8 work hour shift at -10c.

[https://academic.oup.com/jcem/article/99/12/E2772/2833739](https://academic.oup.com/jcem/article/99/12/E2772/2833739)

~~~
fpoling
This is very interesting paper, but is it a right one? I do not see any
mentioning of working in such conditions nor 5-6000 calories per day.

~~~
mtreis86
That paper isn't the right one. I can't find it now. Here is another
interesting one though...

[http://www.dtic.mil/dtic/tr/fulltext/u2/a240839.pdf](http://www.dtic.mil/dtic/tr/fulltext/u2/a240839.pdf)

I will update this if I find it again.

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JshWright
An interesting corollary to this is "Where do trees get their mass?"

Veritasium has a fun video demonstrating how unintuitive this is:

[https://www.youtube.com/watch?v=2KZb2_vcNTg](https://www.youtube.com/watch?v=2KZb2_vcNTg)

~~~
StavrosK
I love that you used the word "unintuitive" rather than imply that people were
uneducated for thinking it. Good way of thinking about knowledge.

~~~
lostconfused
Because it's not that simple. Every moment of a person's life adds to their
sum total of knowledge but it's very limited. Simply being a living human
isn't going to give you a good understanding of quantum mechanics or general
relativity, but it will give you some basic idea of things falling towards the
ground. Something being unintuitive doesn't mean just that the individual
lacks knowledge but that there is very little chance that this knowledge would
be encountered in ones life time at all without making an effort to gather a
deeper understanding.

------
krallja
Reminds me of the question “what are trees made of?”

(The answer to both questions is “mostly atmospheric CO2.”)

~~~
k__
Hehe, true.

I once saw a documentary about "big animals" in the past.

Like, why did we have giant spiders and insects while they have a rather
simple respiratory system that doesn't scale very well.

Because: trees!

Somehow plants managed to create wood, which bound the C from CO2, but the
rest of the biosphere didn't know what to make of the wood. So there were more
and more trees and more and more C got removed from the atmosphere. A much
bigger part of the atmosphere was O2 and so the insects could grow bigger.

But in the end the fungi figured out how to make short work of wood and in
turn released much of the C again into the atmosphere as CO2, so the animals
shrunk again.

~~~
Cd00d
I thought ants were what made short work of the wood. Something about formic
acid? Do you know more - I need to reset my ignorance.

~~~
k__
No sorry.

Maybe it was both? The documentary just told about the fungi.

~~~
Cd00d
okay - thanks. Years ago someone told me a story of the ants saving us from a
wood death, and I never checked it out, but recounted it just the other day.
Fungi seems more reasonable on reflection.

------
DoodleBuggy
The TLDR:

> Physical activity as a weight loss strategy is, therefore, easily foiled by
> relatively small quantities of excess food.

> Our calculations show that the lungs are the primary excretory organ for
> fat. Losing weight requires unlocking the carbon stored in fat cells, thus
> reinforcing that often heard refrain of “eat less, move more.”

"Eat less, move more." If anyone isn't aware of that by now, I don't know what
to say.

~~~
DanBC
the eating less part is more important (for weight loss) than the moving more
part, especially because people doing exercise for weight loss often
overestimate how many calories the exercise is burning, and underestimate how
many calories are in food.

~~~
wccrawford
Plus, the difference between "not losing weight" and "losing weight" can be a
_lot_ of calories.

If you were gaining weight regularly and start exercising to lose it, you'd
have to do a _ton_ of exercise to compensate.

But if you're on the verge of losing weight and start exercising, it would be
possible to tip the scales, so to speak. So long as you don't start eating
more because of it.

I'm 205 lbs, and the charts say I need to eat between 1500 and 1800 calories
per day to lose weight. I can eat 2600 and maintain my weight.

So if I'm just barely maintaining my weight at 2600 calories and I want to
lose weight, I'd need 800 calories worth of exercise.

Moderate-pace walking burns 300 calories per hour for me. I'd need almost 3
hours of deliberate walking at a good pace to lose that extra weight, every
single day. And I couldn't reward myself with a treat afterwards.

I got the exercise numbers from MyFitnessPal. The food calories I got from a
site I found the other day, but I don't remember which.

------
pianom4n
There's a TEDx talk about this:
[https://www.youtube.com/watch?v=vuIlsN32WaE](https://www.youtube.com/watch?v=vuIlsN32WaE)

~~~
gebeeson
Thanks for posting that link - after reading the article, that video was the
first thing I thought of. It's a great talk that is simple to understand.

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s0rce
Why is CO2 the correct answer, the answer should be energy/heat + CO2 + H2O?
The "burning" of fat in oxygen to CO2 and H2O is exothermic.

~~~
Xophmeister
TFA _does_ say "energy/heat + CO2 + H2O", but most of the output in terms of
mass -- which is what one is interested in when it comes to losing weight --
is carbon dioxide.

~~~
colechristensen
The fat molecules burned usually have about twice as many hydrogen atoms as
carbon atoms so you end up with about as many CO2 molecules as H2O molecules.
Carbon weighs about 14 times as much as hydrogen and most of the oxygen in
both comes from the atmosphere. So when you lose weight, 7 parts go to CO2 and
1 part goes to water. A very very tiny proportion goes to energy with mass-
energy equivalence.

------
ZeWaren
It's pretty obvious when you do long fasts. Every time I fast for more than I
week, I lose several kilos doing nothing, not going to the bathroom even once
except to pee.

~~~
DoodleBuggy
What is your intake during the fast? Bone broth? Water? Electrolytes?

~~~
ZeWaren
Water only. No broth, no tea, just plain water.

~~~
byte1918
Why fast for so long? How is this healthy?

------
dugluak
a kid asked me a similar question years ago - when you delete a file from the
computer, where does it go?

~~~
gnu8
Most computers have a fan that blows the discarded bits out of the CPU. Some
of them are built fanless though so the bits just radiate away...

------
beefman
Related is Feynman's comment on where trees come from: Most people think they
grow out of the Earth, but they really grow out of the air.

------
somebee
This is one of my all-time favourite fun facts. First learned about it in Tell
Me Something I Don't Know: [http://freakonomics.com/podcast/tell-me-something-
i-dont-kno...](http://freakonomics.com/podcast/tell-me-something-i-dont-know-
a-new-freakonomics-radio-podcast-2/)

------
corpMaverick
I am confused by this...

> We suspect this misconception is caused by the “energy in/energy out” mantra
> and the focus on energy production in university biochemistry courses.

I understand from the article that chemistry (Which I can't understand) tells
you that it is converted on CO2.

But still. Clearly I can move because I have energy that I acquire from food.
How is this wrong ?

~~~
wyufro
Their point, which I agree is fairly unclear, is that the fat is not converted
to energy, it's converted to CO₂ and water. That conversion releases energy,
which you then use to move, but the total mass of the process is constant
(conserved).

~~~
gwright
Just to add a bit of clarification, the excess energy part of the equation is
coming from the energy stored in the chemical bonds of the original molecules.

The chemical energy of the original molecules is more than the chemical energy
of the resulting molecules leaving some energy to do work and to generate
heat.

------
sampo
Well, at 37.7 MJ/kg energy content in fat and E=mc², 4.2 micrograms of the
mass of that 10 kg does disappear.

------
dandare
> Most people believed that fat is converted to energy or heat, which violates
> the law of conservation of mass. We suspect this misconception is caused by
> the “energy in/energy out” mantra...

I would not call it a misconception but simplification. Otherwise it is a very
surprising article.

------
pwinnski
Observations from Figure 1, "Responses of a sample of doctors, dieticians, and
personal trainers to the question “When somebody loses weight, where does it
go?”"

0 doctors were willing to admit they didn't know.

0 dieticians thought sweat was involved.

Those results seem unsurprising.

Only dieticians correctly answered CO2.

~~~
arkades
Overstating the case about “0 dietitians ... only dietitians.” It looks as
though about 2 people thought sweat was involved, and only 1 dietician
answered co2. Pretty much everyone answered “heat.”

The question itself is shit, too, which is what happens when you dont have
survey epidemiologists eyeball your questions. In “losing weight,” you’re
doing a number of things - chucking the actual carbons (mass), and removing
the potential energy stored as carbon-carbon bonds (heat). Given the
widespread colloquialism of “losing weight,” people failing to interpret it as
regarding the removal of carbons specifically isn’t unreasonable.

Sweat and urine are also correct answers, for that matter. Some of the carbon
is used to create bicarbon and carbonic acids, via co2, which are lost in
urine and sweat. Whether you want to count that as co2 or not is an argument
about semantics - it goes through co2, but isn’t lost -as- co2. Of course,
from the perspective of “it’s all breathed out as the result of perfect
oxidation,” it’s wrong... but why let biology get in the way of undergraduate
chemistry, right?

This isn’t a reflection on docs, PTs, or dietitians. This is GIGO.

------
agumonkey
Someone on youtube made a perfect video explaining this with enough precision:

not sure it's this one
[https://www.youtube.com/watch?v=vuIlsN32WaE](https://www.youtube.com/watch?v=vuIlsN32WaE)
but close

~~~
perilunar
It's the same guy — the presenter in the video, Ruben Meerman, is one of the
authors of the paper .

~~~
agumonkey
i knew the diagrams reminded me something but was too lazy to check, thanks

------
stillbourne
[https://www.youtube.com/watch?v=vuIlsN32WaE](https://www.youtube.com/watch?v=vuIlsN32WaE)

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yodon
At the risk of crossing the streams here... “wait! HN must be listening to the
Microphone on my phone because I talked with my son about this exact topic 36
hours ago and now it’s in my HN feed!”

Yes, that’s a joke, but it’s also a completely factual statement (except for
the Microphone thing) and a good reminder that with enough page views even
extremely low probability events will happen with great frequency and
predictability to someone in your audience (you just can’t predict to whom
they will happen)

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scottmcdot
After the reaction between adipose triglyceride and oxygen, do we wee out the
water or sweat it out?

------
s0rce
It is "burned" basically the same as if you lit it on fire, it converts to CO2
and H2O.

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pessimizer
Isn't this the same question as "after you eat, where does the food go?" or
"after you put gas in your gas tank, where does the gas go?"

------
compsciphd
I thought it just walked away. :)

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heroku
why do you guys put a bait title to insult me with your technical junks /:

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agjacobson
Out your piss.

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revelation
You ask an ill-specified question and get an ill-specified answer. Of course
that fat "goes into" energy/heat, it says so right in the oxidation process.

It's an interesting reiteration of HS chemics, but of no practical concern.

