LW - Thoughts on seed oil by dynomight

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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: Thoughts on seed oil, published by dynomight on April 20, 2024 on LessWrong.
A friend has spent the last three years hounding me about seed oils. Every time I thought I was safe, he'd wait a couple months and renew his attack:
"When are you going to write about seed oils?"
"Did you know that seed oils are why there's so much {obesity, heart disease, diabetes, inflammation, cancer, dementia}?"
"Why did you write about {meth, the death penalty, consciousness, nukes, ethylene, abortion, AI, aliens, colonoscopies, Tunnel Man, Bourdieu, Assange} when you could have written about seed oils?"
"Isn't it time to quit your silly navel-gazing and use your weird obsessive personality to make a dent in the world - by writing about seed oils?"
He'd often send screenshots of people reminding each other that Corn Oil is Murder and that it's critical that we overturn our lives to eliminate soybean/canola/sunflower/peanut oil and replace them with butter/lard/coconut/avocado/palm oil.
This confused me, because on my internet, no one cares. Few have heard of these theories and those that have mostly think they're kooky. When I looked for evidence that seed oils were bad, I'd find people with long lists of papers. Those papers each seemed vaguely concerning, but I couldn't find any "reputable" sources that said seed oils were bad. This made it hard for me to take the idea seriously.
But my friend kept asking. He even brought up the idea of paying me, before recoiling in horror at my suggested rate. But now I appear to be writing about seed oils for free. So I guess that works?
On seed oil theory
There is no one seed oil theory.
I can't emphasize this enough: There is no clear "best" argument for why seed oils are supposed to be bad. This stuff is coming from internet randos () who differ both in what they think is true, and why they think it. But we can examine some common arguments.
We ate seed oil and we got fat.
One argument is that for most of human history, nobody dieted and everyone was lean. But some time after the industrial revolution, people in Western countries started gaining weight and things have accelerated ever since. Here's BMI at age 50 for white, high-school educated American men born in various years:
For the last few decades, obesity (BMI 30) has grown at around 0.6% per year. Clearly we are doing something wrong. We evolved to effortlessly stay at a healthy weight, but we've somehow broken our regulatory mechanisms. Anywhere people adopt a Western diet, the same thing happens.
Of course, the Western diet is many things. But if you start reading ingredients lists, you'll soon notice that everything has vegetable oil in it. Anything fried, obviously, but also instant noodles, chips, crackers, tortillas, cereal, energy bars, canned tuna, processed meats, plant-based meat, coffee creamer, broths, frozen dinners, salad dressing, and sauces. Also: Baby food, infant formula, and sometimes even ice cream or bread.
People eat a lot more vegetable oil than they used to (figure from Lee et al. (2022)):
Many vegetable oils (and particularly seed oils) are high in linoleic acid. And guess what's making up a rapidly increasing fraction of body fat? (figure from Stephan Guyunet):
Even many types of meat now have high linoleic acid levels, because the animals are now eating so much vegetable oil. It's plausible this is doing something to us.
And seed oils are highly processed.
Another common argument is that even if we can't identify exactly where the Western diet went wrong, we know that we spent almost our whole evolutionary history eating like hunter-gathers (and most of the rest eating like subsistence farmers). And hunter-gatherers are all thin. So maybe we should eat like they did?
That sounds kind of fanciful, but consider the most conventional dietary advice, the thing tha...