The Nonlinear Library LW - So What's Up With PUFAs Chemically? by J Bostock
Apr 27, 2024
11:32
Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: So What's Up With PUFAs Chemically?, published by J Bostock on April 27, 2024 on LessWrong.
This is exploratory investigation of a new-ish hypothesis, it is not intended to be a comprehensive review of the field or even a a full investigation of the hypothesis.
I've always been skeptical of the seed-oil theory of obesity. Perhaps this is bad rationality on my part, but I've tended to retreat to the sniff test on issues as charged and confusing as diet. My response to the general seed-oil theory was basically "Really? Seeds and nuts? The things you just find growing on plants, and that our ancestors surely ate loads of?"
But a twitter thread recently made me take another look at it, and since I have a lot of chemistry experience I thought I'd take a look.
The PUFA Breakdown Theory
It goes like this:
PUFAs from nuts and seeds are fine. Deep-frying using PUFAs causes them to break down in a way other fatty acids do not, and these breakdown products are the problem.
Most of a fatty acid is the "tail". This consists of hydrogen atoms decorating a backbone of carbon atoms. Each carbon atom can make up to four bonds, of which two have to be to other carbons (except the end carbon which only bonds to one carbon) leaving space for two hydrogens. When a chain has the maximum number of hydrogen atoms, we say it's "saturated". These tails have the general formula CnH2n+1:
For a carbon which is saturated (i.e. has four single bonds) the bonds are arranged like the corners of a tetrahedron, and rotation around single bonds is permitted, meaning the overall assembly is like a floppy chain.
Instead, we can have two adjacent carbons form a double bond, each forming one bond to hydrogen, two bonds to the adjacent carbon, and one to a different carbon:
Unlike single bonds, double bonds are rigid, and if a carbon atom has a double bond, the three remaining bonds fall in a plane. This means there are two ways in which the rest of the chain can be laid out. If the carbons form a zig-zag S shape, this is a trans double bond. If they form a curved C shape, we have a cis double bond.
The health dangers of trans-fatty acids have been known for a long while. They don't occur in nature (which is probably why they're so bad for us). Cis-fatty acids are very common though, especially in vegetable and, yes, seed oils.
Of course there's no reason why we should stop at one double bond, we can just as easily have multiple. This gets us to the name poly-unsaturated fatty acids (PUFAs). I'll compare stearic acid (SA) oleic acid (OA) and linoleic acid (LA) for clarity:
Linoleic acid is the one that seed oil enthusiasts are most interested in. We can go even further and look at α-linoleic acid, which has even more double bonds, but I think LA makes the point just fine.
Three fatty acids, usually identical ones, attach to one glycerol molecule to form a triglyceride.
Isomerization
As I mentioned earlier, double bonds are rigid, so if you have a cis double bond, it stays that way. Mostly. In chemistry a reaction is never impossible, the components are just insufficiently hot. If we heat up a cis-fatty acid to a sufficient temperature, the molecules will be able to access enough energy to flip. The rate of reactions generally scales with temperature according to the Arrhenius equation:
v=Aexp(EakBT)
Where A is a general constant determining the speed, Ea is the "activation energy" of the reaction, T is temperature, and kB is a Boltzmann's constant which makes the units work out. Graphing this gives the following shape:
Suffice to say this means that reaction speed can grow very rapidly with temperature at the "right" point on this graph.
Why is this important? Well, trans-fatty acids are slightly lower energy than cis ones, so at a high enough temperature, we can see cis to trans isomerization, turning OA o...
This is exploratory investigation of a new-ish hypothesis, it is not intended to be a comprehensive review of the field or even a a full investigation of the hypothesis.
I've always been skeptical of the seed-oil theory of obesity. Perhaps this is bad rationality on my part, but I've tended to retreat to the sniff test on issues as charged and confusing as diet. My response to the general seed-oil theory was basically "Really? Seeds and nuts? The things you just find growing on plants, and that our ancestors surely ate loads of?"
But a twitter thread recently made me take another look at it, and since I have a lot of chemistry experience I thought I'd take a look.
The PUFA Breakdown Theory
It goes like this:
PUFAs from nuts and seeds are fine. Deep-frying using PUFAs causes them to break down in a way other fatty acids do not, and these breakdown products are the problem.
Most of a fatty acid is the "tail". This consists of hydrogen atoms decorating a backbone of carbon atoms. Each carbon atom can make up to four bonds, of which two have to be to other carbons (except the end carbon which only bonds to one carbon) leaving space for two hydrogens. When a chain has the maximum number of hydrogen atoms, we say it's "saturated". These tails have the general formula CnH2n+1:
For a carbon which is saturated (i.e. has four single bonds) the bonds are arranged like the corners of a tetrahedron, and rotation around single bonds is permitted, meaning the overall assembly is like a floppy chain.
Instead, we can have two adjacent carbons form a double bond, each forming one bond to hydrogen, two bonds to the adjacent carbon, and one to a different carbon:
Unlike single bonds, double bonds are rigid, and if a carbon atom has a double bond, the three remaining bonds fall in a plane. This means there are two ways in which the rest of the chain can be laid out. If the carbons form a zig-zag S shape, this is a trans double bond. If they form a curved C shape, we have a cis double bond.
The health dangers of trans-fatty acids have been known for a long while. They don't occur in nature (which is probably why they're so bad for us). Cis-fatty acids are very common though, especially in vegetable and, yes, seed oils.
Of course there's no reason why we should stop at one double bond, we can just as easily have multiple. This gets us to the name poly-unsaturated fatty acids (PUFAs). I'll compare stearic acid (SA) oleic acid (OA) and linoleic acid (LA) for clarity:
Linoleic acid is the one that seed oil enthusiasts are most interested in. We can go even further and look at α-linoleic acid, which has even more double bonds, but I think LA makes the point just fine.
Three fatty acids, usually identical ones, attach to one glycerol molecule to form a triglyceride.
Isomerization
As I mentioned earlier, double bonds are rigid, so if you have a cis double bond, it stays that way. Mostly. In chemistry a reaction is never impossible, the components are just insufficiently hot. If we heat up a cis-fatty acid to a sufficient temperature, the molecules will be able to access enough energy to flip. The rate of reactions generally scales with temperature according to the Arrhenius equation:
v=Aexp(EakBT)
Where A is a general constant determining the speed, Ea is the "activation energy" of the reaction, T is temperature, and kB is a Boltzmann's constant which makes the units work out. Graphing this gives the following shape:
Suffice to say this means that reaction speed can grow very rapidly with temperature at the "right" point on this graph.
Why is this important? Well, trans-fatty acids are slightly lower energy than cis ones, so at a high enough temperature, we can see cis to trans isomerization, turning OA o...