When we lose weight, where does it go? | Hacker News Reader

When we lose weight, where does it go?

theconversation.com

144 pointsallthebest124 comments

> The most common misconception by far, was that fat is converted to energy. The problem with this theory is that it violates the law of conservation of matter, which all chemical reactions obey.

I mean what they're getting at must be true, we are obviously not converting 1kg of rest mass into energy but that sentence is very much not true.

Agreed. It's the most pedantic claim I've seen on the internet all of 2020. By that logic sugar and gasoline don't get converted to energy either.

Edit: actually, this is the worst article I've read on the internet in all of 2020.

Well, they don't.

I think it's the opposite of pedantic. They're asking "where" the weight goes. It does not transmutate into motion. The weight has to go somewhere.

Pee, poo, breath, sweat, snot, earwax, hair, nails, spit?

I assumed the CO2 we exhale would be the largest portion, and the article backs that up.

But mass can be converted into energy. Obviously that's not the primary weight loss driver (what a metabolism that would be!), but it certainly doesn't break the laws of physics.

EDIT: Because I was curious, I did the math.

Assuming diet of 2000kCal per day.

2000kCal / 3500kcal/lb fat = 0.5714 lb fat = 0.259kg fat

0.259kg = 2.329×10^16 joules = 5.556megatons of TNT

5.556megatons/day / 24 hours / 60 minutes / 60 seconds = 0.000064megatons/second = 64 tons of TNT per second

Less than I thought actually, but not someone you'd want to hang out with.

Mass cannot be changed into energy. Maybe it can down the hall in Mr. Einstein's physics class, but here in my nutrition class we will not violate the classical laws of the universe.

Mass is converted to energy by every chemical reaction! Say you have a mixture of rust and aluminium powder, which you ignite (the thermite reaction). It results in iron, aluminium oxide and heat. The resulting matter will weigh a tiny bit less, by exactly the energy produced divided by c^2

The real pedantry is in the comments.

On the patented Special Relativity Diet, by eating nothing and converting your rest mass to energy, you can lose... a whopping 93 nanograms a day!

> By that logic sugar and gasoline don't get converted to energy either.

They do not!

Some of the energy that was trapped in chemical bonds get released. Sugar and gasoline 'don't get converted to energy'. They CAN be used to create work (including electricity).

Where does a ball go when you drop it off a cliff? It turns in to energy, of course! No, it goes to the bottom of the cliff. The process releases energy, but the ball did not turn into energy.

In what way is it not true? When a reaction does convert mass to energy we call it a nuclear reaction

Chemical reactions also lose mass according to E=mc^2, but m is so small that it's hard to notice.

Yup. For example (which I calculated because I was curious), the direct mass conversion equivalent for 2000 food calories of energy is about 93 nanograms. Compare that to the typical value that 1lb of fat is about 3500 calories of usable energy.

In much the same way that no electrical device actually "consumes energy" (conservation of energy).

Your power company still won't accept that perfectly correct explanation in lieu of payment.

This is kind of pedantic, but to my understanding, even normal exothermic chemical reactions do convert mass to energy. It’s just that the amount of mass lost is extremely negligible. Conservation of mass is an oversimplification that is close enough to match any real world measurement you’d care about.

Nuclear reactions are when you start getting into doing mass-energy conversion at scale.

Thank you. I didn't know that.

Yes, conservation of mass is to conservation of energy what Newtonian mechanics is to general relativity. The former work pretty well in everyday life

Chemical bonds are on the scale of eV---the cost to liberate an electron from a hydrogen atom is 13.6eV, for example. So the best you can hope for, in a chemical reaction, is a change in mass of the scale of 10s of eV / c^2.

In comparison, a single proton weighs 938 MeV = 9.38e+8 eV. So the fraction of the mass that is converted into energy is on the scale of 1e-7.

This is the correct answer.

There's no such thing as conservation of mass, mass and energy are convertible into each other. But in the real of chemistry that conversion happens on a ratio of at most 1e-7, so when it comes to the human body we might as well say that the weight that goes in must equal what comes out, and that's close enough.

Typo. *in the realm of chemistry

There's no such thing as "conservation of matter." There's conservation of mass in basic chemistry, but it's really just a partial understanding of conservation of energy anyway.

It's almost as if the author has never heard of exothermic chemical reactions.

In biology, there is conservation of matter. Geology, too. Everywhere, really, except in atom-smashers, stellar cores, and nukes.

No, this isn't true. There's no conservation of matter anywhere. If you take 2 hydrogen atoms and weight them, then bond them together in a hydrogen molecule and weight that, the molecule weights less than twice one hydrogen atom.

But you still have the two hydrogen atoms, before and after. They are matter. Tiny (really tiny!) differences in weight don't change that.

I guess when you said "conservation of matter" I interpreted that as meaning "conservation of mass", seeing as we're talking about weight loss.

Conservation of matter is as exact a law as they come in chemistry, let alone biology. The part of mass lost as heat doesn't even match up one part in a billionth. As soon as it becomes relevant, you're not doing chemistry anymore, you're doing physics.

I know it's not "technically" correct, but when doing science, you use the laws applicable to whatever field you're working in.

Water and CO2? (And, of course, some other byproducts, but by and large.) I mean, I think this is a well-known answer, no?

Sorry I don't mean to sound facetious, I haven't opened the article, so I may be surprised, but I thought this was standard knowledge (at least, it was taught in middle school and high school, even though that's been a while ago for me).

Not that well known in some circles. I decided this year to lose some weight and only worked out the mechanics of it after researching throughly. It never factored into my education or general knowledge. It was demotivating to be honest knowing how much of me I had to breathe out.

I will say that it was one of the best changes I ever made to my life.

> I will say that it was one of the best changes I ever made to my life.

Wohoo! That's awesome, congrats! :) It is rather surprising just how much air needs to be exchanged, which is crazy and definitely not intuitive. (And I could definitely see how it could be demotivating... but, hey, we do breathe out quite a bit!)

Wow interesting. So I feel like I don’t breathe correctly. Could it be the reason why I can’t lose weight :D?

Just have to breathe a lot :)

Our school did not taught that.

Was this in the US? I went to a very large public middle school and high school in Texas, so our experiences may differ, but it was certainly standard there. (This, of course, varies even district to district, but perhaps my experience isn't as general as I imagined.)

I can't image that the mechanism isn't taught. I mean, this is basic biology that we probably all learned in middle school, and certainly had to learn in chemistry class. But I think we just fail to put 2-and-2 together, even the teachers. I think I was in my late 20s when it dawned on me that I was exhaling the food I ate. But I came to that realization in a matter of seconds based on first principles--principles learned in middle and high school.

Economics is similar. Almost every undergraduate students take economics. And Economics 101 is a pretty simple topic. But very few people learn to "think like an economist", even many economists. I guess alot of subjects are like this--real life isn't like the examples in textbooks, but that doesn't mean the rules and principles don't apply just the same. They do apply. Things can quickly get more complicated than in the textbooks. Perhaps we translate "complicated" to "it doesn't apply" rather than what we should be thinking: it does apply just the same, but so do other dynamics, and the interactions can produce counter-intuitive results.

> I think I was in my late 20s when it dawned on me that I was exhaling the food I ate. But I came to that realization in a matter of seconds based on first principles--principles learned in middle and high school.

But as noted elsewhere in the thread, you mostly aren't exhaling your food. Respiration is a reaction that takes in O2 and produces CO2. The only thing you can get rid of by breathing is carbon. But you need to excrete a mixture of carbon, hydrogen, oxygen, and nitrogen. (Plus trace elements.)

Sure, "it's more complicated". But the most basic, predominate mechanism is that you're literally burning carbon and exhaling CO2. Only from that realization can you start to identify and answer more sophisticated questions, including the exceptions.

My point is that you can get an incredible amount of mileage from the most basic principles taught in school. Which is literally the point of a basic education. But people tend to emphasize "don't believe everything you learn in school" rather than "most of what you're taught in school are foundational truths to your environment that can have immense utility in your day-to-day life." If you focus on the former rather than the latter, you won't get into the habit of learning how to apply your knowledge. And if you're not well practiced at application--which is a life-long endeavor for even the most basic rules--then superficially that knowledge tends to seem useless.

I never studied computer science in college--I was already writing Perl and JavaScript when I entered a 101 course which was teaching Pascal, and I dropped it the next week. But I did end up learning--and still learn--the theory and its application. I often hear computer science graduates lament that they never actually need to apply what they learned in school in their professional lives. Well, "need" and "can" are two different things and it's a choice you make. Failure to rigorously apply even basic principles is a big reason, IME, why so much software is utter crap. Do you need to consider algorithmic complexity for every little script you write? Yes! Not because it necessarily matters to the performance of the script, per se, but because the script will be part of a larger data processing system, and the way it fits into and shapes that system can and often will effect algorithmic efficiency down the line.

It's a useful skill to apply first principles to everything you encounter in life. Not just scientific principles, but moral and emotional principles, too.

> But the most basic, predominate mechanism is that you're literally burning carbon and exhaling CO2.

Well, again, you're burning oil (or if you're starving, meat), not carbon. It's mostly carbon by mass, but not by molarity.

That's not a more accurate representation than "burning carbon". The specifics are irrelevant and can sometimes even obscure basic relationships. The point is recognizing the relationship to more familiar forms of combustion and oxidation in day-to-day life; that's how you orient yourself when you come to the question for the first time.

My middle school biology class would have been in the early 1990s when many of the finer details of cellular respiration weren't in any textbooks (certainly not middle school). It wasn't until 1996 that a critical step in ATP synthase was confirmed. But you don't need to know anything about ATP to understand the basic role of carbon in energy extraction and respiration. Actually, now that I think about it biology classes tended to focus more on photosynthesis, which sort of makes sense historically--I suppose breaking CO2 photosynthetically was for hundreds of years the far more scientifically intriguing question, and perhaps that's why it was emphasized so heavily in primary and secondary school biology.

> The specifics are irrelevant and can sometimes even obscure basic relationships. The point is recognizing the relationship to more familiar forms of combustion and oxidation in day-to-day life; that's how you orient yourself when you come to the question for the first time.

I would guess that there are two familiar forms of combustion in day-to-day life: burning wood, and burning gasoline. Only the second is referred to as "combustion" in the vernacular. (Since it happens in an "internal combustion engine".)

Neither involves burning pure carbon; one burns carbohydrates and one burns petroleum. The definition of "combustion" that I was taught in chemistry class was "any reaction that produces carbon dioxide and water".

But you might have a very different day-to-day life and chemistry education, I guess.

Most of the energy is from re-binding the carbon from protons to oxygen. Binding two protons to an oxygen yields a lot less.

This is a fair point, but, prompted by the headline, I was thinking of this as a waste disposal problem, not as a "how much energy can I get out of a pound of fat?" problem.

UK guy here. When I was at School (in the 1980s) this was definitely not taught.

Mind you, a lot of things were not taught in UK schools in the 80s... education was pretty rubbish back then.

Hopefully by now you have opened the article and read it. Did it dazzle you?

Shocked me to my core; flabbergasted me to no end. I never could have imagined! :)

I recall from Chemistry: When hydrocarbons combust, the outputs generally are CO2 and Water.

Driven home by living with a gearhead after college. At least some of the liquid dripping from tailpipes is water.

Also relevant if you've ever looked into what a modern "condensing" boiler/furnace is:

https://www.ernstheating.com/blog/condensing-furnaces/

A consequence of the secondary heat exchanger is that a bunch of the steam from the exhaust condenses back into water, and unfortunately it's water containing a bunch of heavy metals and other nasty stuff, which usually ends up flushed into the municipal sanitary sewer. Probably still better than it going up the chimney and into the atmosphere, but it would be nice if there was a way to capture and isolate those pollutants in that relatively concentrated state, before they end up mixed in with everyone's bathtub water.

My hope is that this is highly dependent on fuel source. Really bad for coal, medium for kerosene and not a huge issue for natural gas.

Me too, but the reason I have it on my mind is because I was reading the technical manual [1] for the fancy new NG boiler I'm having installed next week, and they specifically list the max concentrations of lead, chromium, cadmium, and other scary things in the condensate water after the neutralizing unit, with a recommendation to check with your my regulations to ensure that it's legal to dispose of this water down the drain.

Possibly it's just an abundance-of-caution thing, but I assume the reason for the warning is that in some places it isn't, perhaps because of stricter environmental standards, or maybe a sewage treatment system which isn't set up for this.

1: https://www.viessmann.ca/content/dam/vi-brands/CA/pdfs/wall-...

> unfortunately it's water containing a bunch of heavy metals and other nasty stuff, which usually ends up flushed into the municipal sanitary sewer. Probably still better than it going up the chimney and into the atmosphere, but it would be nice if there was a way to capture and isolate those pollutants in that relatively concentrated state, before they end up mixed in with everyone's bathtub water.

You can't be claiming that your bathtub draws from untreated sewage? The treatment plant is a logical place to extract heavy metals and other nasty things; that is its entire purpose.

It would be nice if treatment plants did that.

Treatment plants in general obviously do do that.

If they did, there would not be a huge industry in removing heavy metals from industrial wastewater before flushing to municipal sewer systems.

You make it sound like a conspiracy.

It's only a conspiracy if it is against the law.

No, the point I'm making is that most of what sewage treatment plants do is deal with organics— soapy water, human waste, probably a bunch of clothing fibers, whatever.

Without knowing much about waste treatment processes, I could imagine that it would be relatively easier to extract and isolate heavy metals in a state where they're otherwise just in water, vs once they're extremely diffuse and mixed in with all the rest of it.

> I could imagine that it would be relatively easier to extract and isolate heavy metals in a state where they're otherwise just in water, vs once they're extremely diffuse

This is definitely true; the more concentrated something is, the more of it you can extract.

But there are significant logistical differences between having 100,000 filters in 100,000 households doing their filtering at the point of emission, and having one filter in a treatment plant which all the water is guaranteed to pass through. What happens if 20% of the individual filters break?

I'd say water and CO2 and urea. Protein is roughly 16% Nitrogen by weight.

I was about to say, excreting water and CO2 leaves you no way at all to get rid of nitrogen.

Body fat is just a mix of HCO https://en.wikipedia.org/wiki/Fat

With extreme weight loss blood, protean, and even bone mass is included. So it’s really Oxygen, Carbon Hydrogen, Nitrogen, Calcium, Phosphorus, Potassium, Sulfur, Sodium, Chlorine, and Magnesium plus some trace elements. However, the body heavily favors removing water and fat with mild weight loss.

Yes, something like (CH2O)n + n O2 → n H2O + n CO2 for sugars and analogous for fats and proteins.

This is a fantastic video that breaks down the process, including great visual aids: https://m.youtube.com/watch?v=vuIlsN32WaE

It's actually like 85% to 90% CO2 by weight IIRC, can't remember the chemistry off the top of my head, but I remember being shocked by that.

You literally breathe most of the weight off.

Edit: Found a source, memory was about right. Per this info-graphic [0] and this paper [1], approximately 84% of fat loss by mass is exhaled as CO2.

[0]https://www.bmj.com/content/bmj/349/bmj.g7257/F2.large.jpg [1]https://www.bmj.com/content/bmj/349/bmj.g7257.full.pdf

I learned this from a Ted talk, and surprised me quite a lot. The audience seemed surprised too.

> I haven't opened the article

Well done...

https://www.bmj.com/content/349/bmj.g7257

Meerman, R., & Brown, A. J. (2014). When somebody loses weight, where does the fat go? BMJ, 349(dec16 13), g7257–g7257. https://doi.org/10.1136/bmj.g7257

>Our calculations show that the lungs are the primary excretory organ for fat.

Thanks for backing it all up with a published article.

The survey in that study always left me with some complicated feelings. How is it possible that 100% of family doctors get this wrong, and what does this mean for their core competencies?

Don't get me wrong, I'm humble enough to understand they know infinitely more than I do, and I respect that their education and experience makes them completely capable of doing their job. But then how can they miss such a trivial understanding of how the human body works. It just doesn't match up.

“well, they had really heavy boots!”

> completely capable of doing their job [...] But then how can they miss such a trivial understanding [...] It just doesn't match up.

People badly underestimate how very rapidly understanding degrades as you move away from someone's focus of expertise.

You might laugh if you heard a conversation "You're a Doctor? Yes, of medieval french literature. Good, what do you think of my blood pressure medication?". Or people looking to their local TV meteorologist for climate-change-isn't-real expertise. Or "he's a Scientist!".

The press gets excited when say first-tier business school students don't know what causes Earth's seasons. But if the last time they touched a topic was middle school, it shouldn't shock to find a middle-school-ish understanding. Asking a protein chemist a quantum chemistry question is perhaps like asking a years-ago "I hate studying for quals" graduate student. If something isn't a focus, there's little selection pressure to prune misconceptions.

But perhaps one expects a 5-year old asking "What color is that Sun ball thing?" of first-tier astronomy graduate students to go well? Doesn't. And many of the few who get it right, learned it discussing common misconceptions in astronomy education, rather than from their own.

As you move away from people's active focus, understanding can become ramshackle startlingly fast.

One place this underestimate hurts, is judging the expertise adequate to create excellent insightful and accessible science education content.

Say you want an introduction to atoms for kindergarten. Surely a first-tier professor of physics is sufficient expertise for this, no? And yet, not so much. For example, it's possible to see an atomic nucleus with your naked eye. But only because their are a couple of oddballs, that can be made to fluoresce visibly. The very good rule of thumb is "high energy, can't see". And that's the confident answer you'll get from many a first-tier chemist and physicist. To reliably learn of the exception, you need someone whose focus is nucleus dynamics. A tiny community. But unless you engage them, you won't know you can include a "this is real! see the glowing dot!" photo. I don't know of any non-visitors at MIT with that focus. So my not-quite joke is that MIT has insufficient domain expertise to write a truly excellent kindergarten intro to atoms. One would need to draw on expertise more broadly for that. (A more common failure mode is being able to field cross-domain questions like "does this story convey the right insights? what might a better one look like?")

So what might it take to pull together a massive breath of expertise to create content? The scale is daunting. A cell bio tome textbook publisher commented on their hundred+ authors, and I not-quite joked "great, and how many for the second page?".

Incentives for researchers are a challenge. But a VR intro cell biology project, pulling in researchers with direct expertise for interviews, reported a recurring problem... of getting them to stop and leave. So intrinsic motivations are significant.

Even recognition there's a need, let alone one worth funding, isn't well established.

And the collaborative tech infrastructure to make this possible... that's not a small endeavor.

Can you give more detail about the fluorescing nucleus? I'm honestly amazed by this (and I have a degree in physics, which I guess is your point).

A couple of nuclei have spin-isomer (IIRC) decays that emit both the expected high-energy photon, but also a second visible one. I no longer remember which. :/

The challenge with seeing a single atom naked-eye, is getting visible photons fast enough.

For some value of see - it's just a point source. I had a professor object "that's not seeing the nucleus - it's just a diffraction-limited dot". Funny thing was, they were about to travel to a big star party, to I guess "not see" stars. Sigh. Admittedly the argument for pedagogical value is limited. But at least the years-later long-exposure photo of a single atom was interesting enough for popular press.

With an atom's electrons, the bottleneck is electron transition cycle time. So your photon budget is small and isotropic. And the retina requires localized hit(s) on deadline. With an pumped atom outside the eye, even with optics, my impression is you at best have a limits-of-perception experiment: "ok, I've a 50% confidence (my dark-adapted eyes) just saw a flash there".

Nuclear transitions are plenty fast. But they're also higher energy, and you can't see X and gamma rays. Well, except for the flash of retinal cell death, as with cosmic rays in astronaut eyes.

So with a nucleus that emits visible photons, you can tweeze, trap, strip and bombard an atom to fluorescence in a vacuum chamber, have a window that passes visible, and get a little dot, naked-eye visible with ambient room illumination. It's a cover photo somewhere IIRC, but I years back burned out on trying to re-find it.

But it's a fun concept, isn't it? And makes for a compact example of needing expertise. More compact than say a marine bio professor, writing a children's picture book on photosynthesis, burning lab time to figuring out what bottlenecks world phytoplankton mass. But they're sort of toy examples. Real need is more like being able to ask "Instead of an atoms-up primary school learning progression, might we do nucleons-up to materials? What might that look like? What stories might we use? What cross-cutting ideas might tie it together?". I wish I knew how to make progress on this.

It looks like the question surveyed was just "When somebody loses weight, where does it go?" The reason why FM physicians got this wrong is most likely just a framing issue. It's not a wrong answer; fats are broken down by beta-oxidation and the products are used in oxidative phosphorylation to generate ATP - which is then used for energy.

Honestly the question itself seems opaque and intentionally vague to generate surprising results. It's like asking a chemist "What happens to bonds during IR spectroscopy?" and then when they start talking about induced dipoles you say,"WRONG! The bonds stretch!!"

Likewise, where does that tree in your backyard come from? It is the product of carbon dioxide and water.

edit: The significance of this thought is when a child is asked where a tree or plant comes from, at least in the US, they inevitably answer the ground when the answer should be rain that falls from the sky and air. With the naive notion a tree comes from the ground we miss the obvious which is the ground doesn't sink when a tree grows from it.

It’s really cool to observe, e.g. placing white stems of scallions in water and seeing green leaves materialize within days. The underlying biochemistry is fascinating too. [1]

[1]: https://en.wikipedia.org/wiki/RuBisCO

I never though about a tree that way, thank you for mentioning this.

I found this page explaining the process: https://serc.carleton.edu/eslabs/carbon/1a.html

Neither did until I saw the clip in this article

>"People look at a tree and think it comes out of the ground, that plants grow out of the ground, " he says, but "if you ask, where does the substance [of the tree] come from? You find out ... trees come out of the air!"

https://www.npr.org/sections/krulwich/2012/09/25/161753383/t...

How did the farmer get his donkey out of the deep ditch? He filled it in with dirt. This is just Aesop's fable. But it falls into the same abstract thinking as where does fat go, C55H104O6+78O2 --> 55CO2+52H2O+energy[0], or what happens to a tree when it burns, CH4+4O2→CO2+H2O (plus heat!)[0]. We have a hard time seeing air (sic) as something tangible. Also, this inability to visualize abstract concepts leads to political points of view such as whether or not aerosol of breath contains disease or invisible gases causes global warming.

[0] Grabbed from first search results.

One fun variant of this story is: A tree is made of CO2 from air, water and minerals from the ground, and sunlight. If that wood is burned in a campfire, you get back CO2 in air, and water as vapor, and minerals as ash, and (some of) the sunlight as redder light and heat.

A related common misconception is to forget that plants, like us and most life, burn carbohydrates to CO2. Rainforest trees for instance, are only net consumers of CO2 for a couple of hours around noon.

I'd like to see science education content that weaves stories like this into a coherent tapestry. A rough-quantitative tapestry. Enabling transferable understanding. But we're a long long way from that. If anyone knows of a community pushing in that direction, I'd love to hear of it.

Yet another angle on the same story: The oxygen that makes this cycle possible wasn’t always available on Earth. It was the cycle itself (specifically photosynthesis in microbes) that created our oxygen rich atmosphere. [1]

I’ve heard of Big History [2] as one effort to weave lessons into a coherent tapestry. Agreed that this is a great way to learn.

[1]: https://en.wikipedia.org/wiki/Great_Oxidation_Event

[2]: https://www.bighistoryproject.com/home

> Yet another angle [...] oxygen [...] wasn’t always available

Yes! For example, flammability scales with oxygen concentration, so at peak, wildfires could be continent-scale. And oxygen as toxic bio waste, which our own cells still struggle to handle safely, ties a bunch of stories together.

> Big History as one effort to weave lessons into a coherent tapestry

Just looking at its solar system intro[1], sigh. Creating content that is correct, accessible, and insightful, is hard. Really hard. And since it's not incentivized, it rarely happens. Introductions to the solar system are often so misleading, engendering so many misconceptions you would need to fight with later if you actually cared, that viewing them is arguably net-negative learning.

Coherence regrettably requires correctness. Else instead of a tapestry of understanding, one has a tangled mess of misconceptions. Having tapestry, connections, helps prune misconceptions, by making it easier to see that something isn't fitting. And by making it easier to explore and thereby spot them. But tapestry also seems more vulnerable to misconceptions. Mangle a bit of tangle and you still have tangle, but not so with tapestry. And misconceptions are pervasive in science education. So it seems both broader scope and better quality are needed. A daunting challenge.

[1] https://www.bighistoryproject.com/chapters/2#our-solar-syste...

Veritasium has an entertaining video on this [1]. Trees get weirder and weirder every time I think about them.

[1]: https://www.youtube.com/watch?v=-PWMQR59M68

one hint is that rocks at the top of mountains have moss on them.

Trees are made of air.

also... 1 gallon of gasoline creates 20 pounds of CO2.

> Even the 150 doctors, dietitians and personal trainers we surveyed shared this surprising gap in their health literacy. The most common misconception by far, was that fat is converted to energy. The problem with this theory is that it violates the law of conservation of matter, which all chemical reactions obey.

For reference, if losing 10 pounds of body weight corresponded meant it was converted to energy completely, that would produce about 4 * 10^17 joules of energy. That’s equivalent to 95 megatons of TNT (or two Tsar Bombas) or 111 million megawatt hours.

> The only thing in food that makes it to your colon undigested and intact is dietary fibre (think corn). Everything else you swallow is absorbed into your bloodstream and organs and, after that, it’s not going anywhere until you’ve vaporised it.

This is an oversimplification; if digestion was 100% efficient, feces would be sterile. I think there are even bariatric surgeries that work by making the digestive system less efficient.

> This is an oversimplification

It's perhaps the most pernicious misunderstanding of calories on the internet.

All calories are not created equal. Even just taking the same ingredient (say, an apple) and preparing it (say, by sticking the apple in a Blendtec Blender vs eating it whole) changes the amount of calories available for absorption.

Right. Fiber is a debatable source of calories since their precise metabolism can change based on gut flora. As someone who lost 100lbs through calorie counting and exercise, just that subject alone was a long and interesting study. Some nutrition labels will assign them 0 calories, a seemingly benign 10 calories, or the same calories as other carbohydrates.

https://www.verywellfit.com/why-are-there-calories-in-solubl...

https://www.bodybuilding.com/fun/ask-the-macro-manager-does-...

Is it safe to say that taking an apple and manipulating it without adding anything to it could never INCREASE the calories available?

So is the reason that the calorie content in the apple changes in blending due to how difficult (or not difficult) it is to digest?

> could never INCREASE the calories available?

It wouldn't add calories, but it would make more calories available in the sense that your body is able to extract them.

For instance, cooking food makes more calories bio-available.

No, blending it increases the calories available to easily absorb. This is partly why juicers are unhealthy.

Basically. It's similar to how we typically process meat without adding anything to it before eating it, to make it easier to digest and to increase the available calories.

The way we process it is by cooking it.

What do you mean by "available"?

Fire was an important technology because it allows us to predigest food, getting more out of eating the same amount of stuff. This certainly does increase the calories available in the ordinary usage of the phrase -- but so does the blender example.

A blender is a bunch of knives. Just something else enabled by fire.

Do we approximately know by what percentage the calorie availability increases for blended vs regular fruit, for the average person?

> This is an oversimplification; if digestion was 100% efficient, feces would be sterile.

Generally 10% of energy is conserved when moving from one trophic level to the next. [1]

[1] https://en.wikipedia.org/wiki/Ecological_efficiency

Relatedly, this is the reverse of how trees get their mass:

https://youtu.be/2KZb2_vcNTg

This was my first thought as well. Embarrassing to admit that I didn't realize this until I was told it in my twenties and I never looked at trees the same way again!

My understanding was that a small portion went to energy, a portion eliminated from the body as solid waste, and the majority was broken down into fluids and eliminated along with urine. I did not realize anything about the CO2 until I read the article.

If you remember back to taking chemistry in school, you might recall Avogadro's Law, which states that equal volumes of gases have the same number of molecules.

So if we're constantly breathing in a certain volume of gas and then breathing out that same volume of gas, then it would stand to reason that we have to be losing weight continuously, because CO2 has a higher molar mass than O2.

I've been trying to exercise more (now that my foot is healed and it's easy to sit at home for way too long) ... now I'm sad to hear that I'm causing global warming.

Breathing out CO2 is not a factor contributing to global warming because it's carbon that's already in the carbon cycle.

The story is (likely purposefully) ignoring some other ways you can "lose weight/energy" from your body. But I am curious what would the complete list be...

* heat (the most obvious one, not explicitly mentioned I guess?)

* even in healthy humans, some fatty acids are excreted with bile into the gut and subsequently removed via feces

* if you are fasting (or in ketosis, such as via keto diet) you will exhale / sweat ketone bodies, which is a very ineffective way (energy balance wise) to use up fatty acids

* if you are a diabetic, you can excrete glucose in your urine; we have subsequently develop drugs that can help mimic this process as a diabetes treatment

* skin, hair and nails (very insignificant amounts)

* sperm & menses

Anybody got anything else I am not currently thinking about?

> Anybody got anything else I am not currently thinking about?

tears, mucus, dead skin cells, skin oils.

Earwax

Heat does not carry away a measurable amount of weight.

You are right. While the energy of the chemical bonds is converted into heat, it's not like the bonds itself have any weight. It's a pretty obvious statement.

Now I am embarrassed! :)

This is misleading in some ways. Even people who are not overweight "lose weight" through exhaling and sweat/urination. It's not really an "interesting" concept. I think the more interesting discussion is around how many people still believe that exercise is a better way to lose weight than simply eating less. Basically, the perspective is entirely wrong. It's not about "losing weight", it's about not putting into your body in the first place.

A simplification of a tangential - did you ever hear that when you first start your lifestyle change (diet/exercise) that it's very easy to lose weight at the start because you're "losing water"? Well, now you know where that water comes from.

edit: I should just also say that another interesting discussion is around the best ways to get people to eat less. And part of that involves lots of interesting (at least to me) biochemistry.

> Well, now you know where that water comes from

Glycogen in your muscles, not fat. That's what the early rapid weight loss comes from.

For the science geeks, here's a great TEDx demonstration with lab experiments: "The mathematics of weight loss " https://www.youtube.com/watch?v=vuIlsN32WaE

The presenter in the video is one of the authors of the article.

You exhale it. The opposite happens with trees. They get their mass from inhaling, not the soil.

There is a nice Ted talk about the same topic. Something I often link to people saying "I can eat everything I want in this diet..." The mathematics of weight loss | Ruben Meerman | TEDxQUT: https://www.youtube.com/watch?v=vuIlsN32WaE

My personal weight-loss (and now sports) is largely based on understanding how the human body works with metabolism.. carbs, fat, protein.. nice and geeky

This article basically states that we cannot convert fat to energy. This seems wrong. IIRC the body can use different sources for producing energy, from sugar over carbs to fat, however it will use first what is the easiest to convert (sugar) and last what is hardest (fat). All of which is done by enzymes, I believe.

They are ignoring body heat completely. When I exercise I do not only breath out carbons, I am also generating a much higher body heat.

The article does not mention gut bacteria. Does anyone know roughly how much of our food and water intake goes into supporting our gut biome and excreted?

Tapeworms were an early weight loss miracle cure (no idea how effective though), and I'm sure someone will be working on a modern version without the side effects.

I was taught that it was expelled as CO2 via breathing.

It looks like I wasn’t told about the water component.

This reminded me of the book: "Big Fat Myths: When You Lose Weight, where Does the Fat Go?" by Ruben Meerman.

Then I noticed he was one of the authors.

The Man Will Never Fly Society ("Birds fly, men drink") has always said it goes into the air. Funny to realize that they were right.

I always figured it was heat. Of course it depends on size & build, but it takes around 1,000 calories/day to maintain body heat.

This just makes me think that one of the main source of global warming is human population ( as co2 producers).

Exhaled CO2 is in equilibrium with plants inhaling CO2 to provide us food, so exhaling isn't considered a factor in global warming.

But you need large number of plants to make it in equilibrium. Currently rate if deforestation is in par with global population growth

It has to be either in equilibrium, or in excess on the side of plants. If it's in excess on the side of humans, people would be dying out due to lack of food, which would then put it back into equilibrium. Deforestation isn't relevant to food because we don't eat trees.

The TED talk that is linked in other comments debunks this too. All the CO2 exhaled by humans is short-term carbon that was recently taken out of the atmosphere by a plant. All plants and animals are short-term carbon sinks. The damage is caused by burning fossil fuels.

I suppose there might be a few artificial ingredients that are made from petroleum feedstock, and thus human metabolism cause it's first release as CO2, but it's tiny compared to the sugars and fats we consume from plants and animals.

When we turn dumb, where does all the intelligence and knowledge go?

When we lose fat, we breath it away CO2.

Reply on news.ycombinator.com