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Comment by astrophysician | Hacker News Reader

> 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.

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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.

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

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

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.

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