
When You Lose Weight, Where Does It Go? - Mitchhhs
https://mitchkirby.wordpress.com/
======
bobowzki
It doesn't surprise me at all...

I'm an MD. I don't remember this specific piece of information being mentioned
during my training. However, all the information need to reason about it was
included...

What really blows my mind is how many people get it wrong, including my
colleagues and especially dietitians ("they had one job").

~~~
JshWright
> including my colleagues and especially dietitians

That is fairly astounding... I'm a paramedic and while I don't recall talking
about this specifically, the answer was immediately obvious from the
(comparatively limited) amount of A&P training we receive.

~~~
bobowzki
It is. Read the BMJ article I posted and you'll see. They did interviews.

------
MarcScott
And the reverse biochemical process is equally misunderstood. I used to teach
high school Science and everyone thought a tree's mass came from the soil.

[http://youtu.be/2KZb2_vcNTg](http://youtu.be/2KZb2_vcNTg)

~~~
duaneb
This is not incorrect, a tree's mass does come partially from the soil, it's
just that the majority comes from the atmosphere. It's not like the tree just
puts out roots cause it likes the feeling. Where do you think all the water
mass in a tree comes from?

I would more chalk this up to too little biology, too late, for people to
learn well. And, frankly, it's not something everyone NEEDS to know;
misunderstandings are not the end of the world.

~~~
gayprogrammer
> Where do you think all the water mass in a tree comes from?

Still, we don't normally assume that water is the primary component of "soil".
The key realization is that a tree is not made of the dirt around it.

------
joshvm
The question is somewhat badly phrased.

I think many people interpret it as "Why do I lose weight?" or even "Where
does that fat go when I lose weight" and answer correctly: your body uses up
your fat stores as fuel when there's no easy sugar available. Calling that a
wrong answer is a bit unfair, there's nothing technically incorrect about it.

However, that's not what was asked - the question is where does the mass go?
After all, if you light a fire inside a box, the box doesn't get lighter.

The trap is that there are by-products from the chemical reactions which have
to go somewhere, and that somewhere is typically through respiration (and
excretion).

------
mikeash
I thought it was ironic to find this statement in an otherwise solid article:

"(So wrong. Mass cannot be converted to energy except through nuclear
reactions)"

This is, of course, wrong. All energy has mass. A charged battery has more
mass than an empty one. A chunk of fat weighs more than all of the byproducts
that come from metabolizing it.

It's _almost_ true, because the conversion factor between mass and energy is
_huge_ , so the change in mass for more mundane quantities of energy is far
below what's reasonably possible to measure. But there's nothing special about
nuclear reactions aside from the quantity involved.

~~~
Mitchhhs
Thanks for catching! Simplified it so hopefully i'm not saying anything
incorrect now. Thanks!

~~~
mikeash
Nice! I think that simplification works nicely and gets the point across well.

------
sytelus
Great calculation, wrong conclusion. Exercise does not cause any significant
weight loss, of course unless you are doing it 6+ hours a day. All major
weight loss typically comes from controlled diet. This means eliminating non-
complex carbs. This is a major misconception in general public and people just
don't seem to get it. Exercise helps build muscles, metabolism and endurance,
but for weight loss your only practice option is diet control. The major
energy expenditure for body is not walking or moving but just staying warm and
brain. The reason people gain weight is not lack of exercise but because they
eat all these fast burning carbs that rapidly gets deposited as fat instead of
getting used up for body's energy need. If you want to lose weight rapidly,
just make sure to add lot of fat and protein in your diet with as little carbs
as possible and watch the weight drop.

~~~
DanBC
Here's a meta analysis that says that exercise does help for weightloss. It
also agrees with you about the health benefts of exercise, and that people get
those health benefits even iftheir weight doesn't decrease.

But it clearly says that exercise combined with diet changes is better than
either alone.

[http://www.cochrane.org/CD003817/ENDOC_exercise-for-
overweig...](http://www.cochrane.org/CD003817/ENDOC_exercise-for-overweight-
or-obesity)

> The results of this review support the use of exercise as a weight loss
> intervention, particularly when combined with dietary change. Exercise is
> associated with improved cardiovascular disease risk factors even if no
> weight is lost.

------
riemannzeta
This appears to have been copied without attribution from
[https://www.youtube.com/watch?v=4pkSLKucVbM](https://www.youtube.com/watch?v=4pkSLKucVbM)

~~~
riemannzeta
on closer inspection, this version is not nearly as good (and hence probably
independent)

------
Jun8
Detailed discussion:

[http://physics.stackexchange.com/questions/2605/how-does-
mas...](http://physics.stackexchange.com/questions/2605/how-does-mass-leave-
the-body-when-you-lose-weight)

I was suprised this was not linked in the post.

------
stephengillie
6 billion people, each burning 2000 calories per day equals 12 trillion
calories.

6,000,000,000 * 2,000 = 12,000,000,000,000

But wait, those are Kcals.

6,000,000,000 * 2,000,000 = 12,000,000,000,000,000

1 calorie is the energy required to raise 1 gram of water 1 degree C. There
are about 4404.8838 grams in a gallon.

12,000,000,000,000,000 / 4404.8838 = 2,724,248,934,784.61

So this is enough to raise 2 trillion gallons 1 degree, or raise 1 gallon by 2
trillion degrees Celcius.

2,724,248,934,784.61 / 100 = 2,724,248,934.78461

Or boil away 2 million gallons of water. Per day.

~~~
Namrog84
any particular reason you chose 6 billion? (Just easier math?)

While I thought the current estimates were closer to 7.1 billion. Though I
doubt those 7+ billion people eat 2000 calories per day. When you consider how
many people are starving in the world, I am not sure if its offset by the
overeating overweight nations.

tl;dr; why 6 billion?

~~~
mikeash
I'd bet it's because the world population is a mesofact, and it's easy to
forget that your information is out of date:

[http://www.boston.com/bostonglobe/ideas/articles/2010/02/28/...](http://www.boston.com/bostonglobe/ideas/articles/2010/02/28/warning_your_reality_is_out_of_date/)

~~~
asavadatti
The author talking about the Boiling Frog myth as a fact is quite ironic in
this context.

~~~
mikeash
Well, when used as an analogy, it doesn't have to be correct. But still,
you're right.

While looking this up, I came across the craziest sentence in the Wikipedia
article:

"German physiologist Friedrich Goltz demonstrated that a frog that has had its
brain removed will remain in slowly heated water, but an intact frog attempted
to escape the water when it reached 25 °C."

Apparently this isn't quite as crazy as it sounds, as the brainless frogs
still exhibit some reflexes, to the extent that they'll jump out of hot water.
But still, it reads like a study in "WTF did you expect to happen?!"

~~~
pbhjpbhj
>"WTF did you expect to happen?!" //

Often we all assume something and so no one really investigates it properly.
Science must concern itself with things we think we know are _a priori_ truths
in order to provide a solid basis for building on.

That said Goltz experiment seems quite reasonable to me, ~"do frogs have non-
brain based reflexes that react to temperature?", ~"do frogs react to absolute
temperatures or just changes in temperature".

[https://archive.org/stream/studiesfrombiol00martgoog#page/n4...](https://archive.org/stream/studiesfrombiol00martgoog#page/n405/mode/2up)
at p.389 suggests that the frog with "no cerebrum" is in some way alive in
Goltz experiment - perhaps it is missing that point that makes you say WTF?

It seems the excitability of headless frogs in varying temperatures was quite
a point of interest, see eg p.393 _ibid_.

~~~
mikeash
It's just a knee-jerk reaction, "Duh, of course the frog with no brain doesn't
jump." The difference between intuition and reality is the humor here.

------
ffn
What about rubbing? I mean, when you rub against stuff (even wind), some of
your cells get taken off. I'm all for peeing, pooping, and breathing, but how
much weight do we lose just through simple friction? And if it's significant,
should we employ more rubbing in our daily exercise routines?

~~~
csours
No, because it is not scalable.

Your epidermis grows at a specific rate and will not really grow faster if you
remove it. If you remove it faster, you will develop scar tissue, which is not
desirable.

An interesting way to remove fat is lactation. It is the only biologically
"native" way to directly remove lipids. Unfortunately this is only available
for lactating females.

~~~
2mur
Sebaceous glands function in a similar manner. Holocrine secretion where you
basically expel the cell entirely. Not surprising really since breast tissue
is essentially modified sweat/salivary gland.

[https://en.wikipedia.org/wiki/Holocrine](https://en.wikipedia.org/wiki/Holocrine)

------
knivets
The url leads to the home page of the blog, this should be instead -
[https://mitchkirby.wordpress.com/2015/04/21/where-does-
weigh...](https://mitchkirby.wordpress.com/2015/04/21/where-does-weight-go/)

------
amitutk
tl;dr - "The surprising answer to this question is that we breathe it out."

O₂ in, CO₂ and H₂O out

~~~
whoopdedo
I had never thought about the question before, but now that I see it it's a
"Watson" moment. My original guess would have been urine & feces.

~~~
StavrosK
I'm proud to say that I got it right, after a brief "urine" moment, but I
think it's only because I had watched the Veritasium segment about where trees
come from, way back when.

------
madcaptenor
A quick proof that the answer isn't "you poop it out": when you work out more,
you don't poop more. (You do, however, breathe more, which points towards the
correct answer.)

~~~
fysac
Physical activity speeds up the digestive process, so you do often poop more
when you work out.

~~~
gknoy
... combined with the fact that many who work out heavily increase their
caloric intake (eat more). :D

------
noonespecial
I've had even worse luck trying to convince people that trees are mostly made
from air and water.

~~~
lkbm
There's a nice Veritasium episode wherein he asks people on the street where a
tree's mass comes from:
[https://www.youtube.com/watch?v=2KZb2_vcNTg](https://www.youtube.com/watch?v=2KZb2_vcNTg)
(This isn't just one of those "let's laugh at how dumb people on the street
are" videos; it's done explicitly as a learning exercise. His PhD looked at
how addressing misconceptions and going from there to correct explanations
works much better than simply providing explanations:
[https://www.youtube.com/watch?v=RQaW2bFieo8](https://www.youtube.com/watch?v=RQaW2bFieo8)
)

I thought of it upon seeing the first chart in the BMJ article.)

~~~
JoeAltmaier
Let me guess: mostly from water? And the air of course - carbon. Sorry, I
won't watch the video, its far too spare a use of time.

~~~
lkbm
He pretty much just says carbon. Certainly is some water involved too, but
yeah, breath in CO2 and breath out just O2.

------
drcube
>It Gets Converted to Energy (So wrong. Mass cannot be converted to energy
except through nuclear reactions)

This is not true. There is energy in chemical bonds, and that energy is
released when the bonds break. When you burn gasoline, the mass of the
resulting molecules adds up to less than the mass of the original gasoline. A
tiny, tiny fraction less, but still less. Because there was energy in those
bonds, and energy has mass.

It's wrong in another way, too, because even in nuclear reactions mass isn't
converted into energy. Mass is mass, but energy also has mass. No mass is lost
in a nuclear (or chemical) reaction, it just goes somewhere else. There is no
"conversion" going on.

------
pbhjpbhj
I was thinking about this a couple of weeks ago, expiration that is. This
question raised itself and I've not focussed on answering it yet - if you
breathe more then, like forced breathing, can you accelerate carbon loss
[beyond that expected by the increased exercise involved in such breathing];
like fanning flames I guess.

Thinking now I suppose an [close] equivalent is probably can you increase
carbon dioxide production by increasing oxygen saturation of the air inspired?

There's probably enough in that idea to sell oxygen masks and little tanks and
a book for a new fad 'diet' ...

~~~
mixmastamyk
Not sure that would be true... the body is producing a set amount of CO2.
Breathing faster would expel it more quickly, but not change the total amount,
correct?

I suppose the act of breathing itself would burn more calories, but it isn't
the most challenging exercise, not to mention the dizziness mentioned already.

~~~
pbhjpbhj
I'm going from a position of ignorance wrt how breathing works at the
molecular level – how does the carbon arrive on site (which specific location)
to combine with the oxygen, what membranes are involved, what vapour pressures
and saturations might be pertinent. Ergo, I hadn't addressed the question as
yet. Basically I'm asking is exertion the only limitation on the CO2
production.

[1] answers a lot of this.

Again with presumptions from biological ignorance, in low oxygen environments
it seems we can't "burn" so much food, so oxygen saturation is definitely a
limitation. Also we know that oxygen saturation varies, so even if STP oxygen
percentage of air is the optimum sometimes we'll be sub-optimal - how does the
body cope then?

[2] appears to answer that one.

What I take away from that, wrt my enquiry, is that under increased breathing
[hyperventilation] renal compensation can kick in to combat the alkalemia
caused by the respiratory alkalosis [reduction of available H+ ions in the
blood, ie alkalination or increasing of pH] by extracting extra-cellular fluid
bicarbonate [HCO3-, see [3]] and excreting it in urine.

Now I'm moving back up the pathways, is renal compensation deleterious (and at
what point is that true). Does low level alkalosis with renal compensation
cause an increase in removal of food products from the body?

At least it seems that we now have created an increased carbon removal by
excreting HCO3- in urine!? [how much?]

So is there an increase in breathing frequency that maintains normal function,
not "hyperventilating" (in the common usage of the term, that makes you
dizzy), that doesn't cause harm? The question still stands I feel.

Also can you do something else that causes acidosis and cancel it out by
increased breathing?

Apologies again, I'm clearly fumbling around here but I've not studied biology
since I was 13 (decades ago).

\- - -

[1]
[https://en.wikipedia.org/wiki/Respiratory_acidosis#Mechanism](https://en.wikipedia.org/wiki/Respiratory_acidosis#Mechanism)

[2]
[https://en.wikipedia.org/wiki/Metabolic_alkalosis#Compensati...](https://en.wikipedia.org/wiki/Metabolic_alkalosis#Compensation)

[3]
[http://web.archive.org/web/20070930080050/http://www.lib.mcg...](http://web.archive.org/web/20070930080050/http://www.lib.mcg.edu/edu/eshuphysio/program/section7/7ch12/7ch12p48.htm),
via Wikipedia
[https://en.wikipedia.org/wiki/Renal_compensation](https://en.wikipedia.org/wiki/Renal_compensation)

~~~
mixmastamyk
> oxygen percentage of air is the optimum sometimes we'll be sub-optimal - how
> does the body cope then?

I think altitude sickness would be the result, not enough oxygen.

------
tantalor
When your car loses weight (fuel), where does the weight go? The exhaust, of
course; your breath is your exhaust.

------
MarcScott
Just asked my wife and daughter what the answer was, and they both thought you
excreted the mass. Then I asked my son, who is home from Uni, studying a
Biology degree, he said it was released as energy. FFS.

------
MrJagil
Please embrace my willingness to look foolish, I really want to be able to
explain this properly; can someone explain to me why fat doesn't get changed
into energy, and disappear through heat?

~~~
spott
>can someone explain to me why fat doesn't get changed into energy, and
disappear through heat?

Fat is some complex molecule made up of a bunch of carbons, hydrogens, and
oxygens. Where do you think these atoms go?

The short of it is the fat molecules are turned into other molecules (which
gives you energy) with the addition of oxygen, and the byproducts (water and
co2) are expelled as a byproduct.

------
abandonliberty
What's most interesting to me is that in most cases we don't gain or lose
muscle or fat cells.

They just grow and shrink in size.

------
lurkinggrue
Movement, CO2 and urine.

------
loourr
It gets dumped into the universe as heat.

~~~
mizzao
The energy (molecular bonds) gets dumped into the universe as heat, but not
the mass (i.e. carbon atoms).

------
a8da6b0c91d
How would respiration be surprising to anyone with a high school level
education?

> This analysis makes it clear why exercise is so powerful in the weight loss
> equation.

There are many reasons to exercise, but it's quite clear weight loss isn't one
of them. I mean, you need to exercise to get your abs to pop, but not being a
lard-ass is totally about avoiding excess calorie intake and exercise really
doesn't enter into it. People don't realize what a trivial amount of calories
working out burns.

~~~
recursive
It's not clear to me. Exercise is a good way to burn calories. But perhaps
more significantly, if you do it regularly, you will burn more calories even
while at rest.

~~~
Zergy
Because the calories lost via exercise are insignificant, compatible to
skipping a snack unless its very intense.

~~~
TillE
It's really easy to burn an additional 500+ calories with exercise, which is
very significant. Of course it's not a substitute for a proper diet, but it
can be a huge factor in weight loss.

~~~
Someone1234
500 calories in one session? Or over a whole week? In one session it is
definitely not "easy." In a whole week it is.

If you walked at 2.5 MpH and weighed 160 pounds, it would take you two and a
half hours to burn your 500 calories. To me 2.5 hrs is not an "easy" workout.
Somewhere between medium to hard (depending on fitness level).

A single Big Mac is almost 500 calories, two snickerdoodles are more than 500
calories on average.

> Of course it's not a substitute for a proper diet, but it can be a huge
> factor in weight loss.

That's just not true.

A single combo from McDonalds could set you back an entire weeks worth of
exercise, literally. If you exercised for an hour a day every day, it only
takes a single meal to undo most of that weight loss.

PS - Exercise is, of course, very important for overall health and a long
life. Nobody is arguing otherwise. But for weight loss? Heck no. The maths
simply doesn't work at all.

~~~
sdml
It depends on whether you consider intensity or duration to determine how
"easy" a workout is. Sure, 2.5 hours is a lot of time to invest, but walking
at 2.5 MpH hardly seems strenuous enough to qualify as exercise.

A 160 pound person could burn 500 calories in ~40 minutes jogging at a 10
minute per mile pace, and could easily offset a Big Mac in a single day's
exercise (not saying that the person should use this to justify eating a Big
Mac).

------
Doji
> Mass cannot be converted to energy except through nuclear reactions

This is wrong enough to make me stop reading.

~~~
ihodes
Just curious, how else can mass be converted to energy?

EDIT/replying:

I'm inclined to continue to believe that it can't in any other way.

1\. Not through a chemical reaction; energy can be released in a chemical
reaction, but the total mass/atoms involved remains the same.

2\. Not through kinetics; energy can be produced, but mass remains the same as
well.

cf. some interesting caveats here
[http://en.wikipedia.org/wiki/Conservation_of_mass](http://en.wikipedia.org/wiki/Conservation_of_mass)

~~~
glomph
Any chemical reaction?

~~~
bobowzki
No mass is lost in a chemical reaction.

~~~
peterfirefly
2H₂ + O₂ --> 2H₂O + Q

Q is the released energy. If you weigh the molecules before and after very
carefully, you will notice that they lose weight (or mass if you are being
pedantic about it). That mass loss corresponds exactly to the Q-value (through
E=mc²).

~~~
MarcScott
No mass is lost in this reaction. The mass of the atoms remains the same. The
energy that is released is due to the difference in bond energies. To balance
the equation you would need to add a Q to the other side, corresponding to the
bond energies.

~~~
sp332
The energy Q is in the molecules. It is released during the chemical reaction.

~~~
MarcScott
The molecules consist of Hydrogen and Oxygen that have equal atomic masses,
either side of the equation, due to the identical numbers of protons, neutrons
and electrons.

Any mass gained or lost will be due to changes in bond energies, and the
corresponding emission or absorption of photons that change the energy states
of the electrons in the bonds.

------
bborud
Nice try. Everyone knows that there are nuclear reactions that take place in
the digestive tract and that we poop out small unobtanium lumps which are
gathered in the night by underpant gnomes. that's the unknown middle step
revealed.

you heard it here first.

~~~
bitwize
I think you're confusing humans with Nibblonians...

~~~
denzil_correa
I think he is confusing reddit/4chan with HN.

