Between 1968 and 1973, more than 9,000 patients in six Minnesota mental hospitals and a nursing home ate whatever the kitchen served them. Half got a diet where butter, lard and ordinary margarine were swapped for corn oil. Their cholesterol fell by 14 percent, exactly as planned. Their lives stayed just as long, and for every 30 points their cholesterol dropped, the risk of dying went up by about a fifth.
Ancel Keys, the man most associated with the case against saturated fat, co-led the project. The full results stayed in the files for over forty years.
Seed oils went from lamp oil and soap vats to nearly every frying pan in about a century. Along the way, cottonseed oil was smuggled into lard, McDonald's fries swapped beef tallow for trans fat, and chicken fat turned out to carry more omega-6 than canola. Today seed oils are the fight that never ends online: poison in a bottle to one side, heart-healthy to the other. Both sides leave things out. Some of the case against these oils is solid chemistry, some of it is a stretch, and the case for them has holes of its own. Sorting one from the other changes what goes in your pan.
What seed oils are
The name covers the oils pressed and extracted from soybeans, corn, sunflower seeds, safflower, cottonseed, rapeseed (sold as canola) and grape seeds. Making them is an industrial job. The seeds are crushed, then washed in hexane, a petroleum solvent that pulls out the oil pressing leaves behind, and the solvent is boiled off. The crude oil is then degummed, treated with lye, bleached with clay and deodorised with steam at high heat, which turns a dark, smelly liquid into the clear, flavourless oil on the shelf.
Compare that with the older fats. Butter is churned cream. Tallow and lard are melted out of beef and pork fat. Olive oil is pressed from the fruit.
The chemistry matters too. Seed oils are rich in linoleic acid, an omega-6 fat with two weak spots in its structure where oxygen can attack. Soybean and corn oil are about half linoleic acid. Canola is under a fifth. Olive oil sits around 8 percent, lard at 10, and butter and tallow at 2 to 3. The more of those weak spots a fat carries, the faster it breaks down under heat and air, which is why tallow and ghee can sit in a hot pan for a long time and a bottle of corn oil goes rancid on the shelf.
There is no canola plant. Canola is a name registered in Canada in 1978 for rapeseed bred to be low in erucic acid, a fat that food rules still cap at 2 percent of the oil. Before it was food, rapeseed oil lit European homes, railway coaches and lighthouses, and greased machinery. Canola gets sold as the healthy one, and it is the least extreme of the group: about 63 percent monounsaturated fat, 19 percent linoleic acid and 9 percent omega-3. That omega-3 is the catch in a hot pan. It has three weak spots, so it breaks down even faster than linoleic acid, which makes canola better suited to cold uses than to frying.
Watch for "high oleic" on the label. Ordinary sunflower oil is about 66 percent linoleic acid. High-oleic sunflower oil is bred to be 84 percent monounsaturated with under 4 percent linoleic acid, which puts its chemistry closer to olive oil. It fixes the linoleic acid problem, though it still goes through the same industrial refining.
Cold-pressing is no shield for a fragile oil. Flaxseed oil is 53 percent omega-3, and it starts reacting with air the moment the seed is crushed. Even kept in the fridge, cold-pressed flaxseed oils carried up to 37 percent more oxidation products by the end of their three-month shelf life.
How they took over the kitchen
Cottonseed was once waste. Cotton growers left it to rot or dumped it in rivers, and it lit lamps for a few years in the 1850s until petroleum took over. Its first trip into food was in disguise. Meatpackers secretly mixed cottonseed oil into lard, which came out in 1884, when Armour, trying to corner the lard market, found it had bought more lard than the country's pigs could have produced. A law then forced the label "lard compound". It went into olive oil too, and Italy banned American olive oil in 1883.
The seed also carries its own poison, gossypol. In 1929, low fertility among men in a province of China was linked to cooking with crude cottonseed oil, and when China tested gossypol as a male pill on more than 8,000 men in the 1970s, about 1 in 10 became permanently infertile. That's why cottonseed oil sold as food has to be refined.
Once the scandals sank its price, Procter & Gamble, then a candle and soap maker, bought cottonseed oil as a cheap stand-in for animal fat. The hardening process behind Crisco was meant for soap, until it turned out to make a fat that looked and cooked like lard. In 1911 the company put it on sale as Crisco, short for "crystallised cottonseed oil". It turned down "Krispo", and dropped "Cryst" because it sounded too much like Christ. To sell it, the company gave away free cookbooks in which every recipe called for Crisco.
Soybean oil followed and outgrew everything else. Across the 20th century, the amount of soybean oil Americans ate rose more than a thousandfold. Linoleic acid went from under 3 percent of everyone's calories to over 7 percent. The body keeps a record of that. Linoleic acid in Americans' body fat rose by 136 percent in half a century, and body fat turns over slowly, so it reflects years of eating.
Babies show how directly it happens. In the 1970s, many Dutch babies were fed formula in which all the milk fat had been replaced with corn oil. Linoleic acid made up 1 to 3 percent of their body fat at birth and about a third of it by four months, while British babies on cow's-milk formula never went above 3 percent.
Hardened seed oils brought trans fats with them, which turned out to be some of the worst fats ever put in food. The US moved to remove artificial trans fats from the food supply after 2015. The liquid oils stayed.
How they got the heart-healthy label
In 1953, Ancel Keys drew a graph of six countries that showed more fat in the diet going with more heart disease. Figures were available for 22 countries at the time, and when all of them were plotted, the neat line turned into a much messier cloud. The idea stuck anyway. By 1961, the American Heart Association was telling Americans to eat less saturated fat and more polyunsaturated fat, the kind seed oils are full of.
The association's money has its own story. Its big break came in the late 1940s, when listeners to a radio contest on the show Truth or Consequences sent in $1.5 million. The show was sponsored by a Procter & Gamble product, from the company that made Crisco. The journalist Nina Teicholz traces how the whole case against saturated fat was built in .
Fast-food fryers felt the advice too. McDonald's cooked its fries in 93 percent beef tallow until 1990, when campaigns against animal fat, led by a millionaire who spent his own fortune on the fight and by a consumer group, pushed the big chains to vegetable oil, with beef flavouring added to keep the taste. Fries became one of the largest sources of trans fat in the American diet. By 1994, trans fats were estimated to cause at least 20,000 heart deaths a year in the US, and the big chains only got the hardened oils out of their fryers by 2009.
Part of the advice holds up. Swapping saturated fat for seed oils lowers LDL cholesterol, reliably. The question was always whether that lower number means fewer heart attacks and deaths, and that's where it gets interesting.
The tests that went missing
The Minnesota project from the opening was one of two big attempts to put seed oils on the plate in place of animal fat. Beyond the deaths, its cholesterol drop came with no fewer heart attacks and no less hardening of the arteries at autopsy. Only part of it was published, in 1989, and the full picture came out in 2016, from the original records.
Sydney ran the second, in 458 men who had already had a heart attack. Half were told to replace animal fats with safflower oil and safflower margarine. After several years, 17.6 percent of them had died, against 11.8 percent of the men who carried on as before. The full figures for heart deaths were only recovered and published in 2013.
The diet changes that did show a benefit hide a detail. When the ones that raised omega-6 alone were separated from those that raised omega-3 at the same time, with fish or other sources, the mixed diets cut heart attacks and heart deaths by 22 percent. The omega-6-only diets went the other way, with 13 percent more, although that figure was too uncertain to be sure of. The good results seem to have come with the omega-3.
The case for seed oils
Seed oils have their defenders, and their evidence deserves a fair hearing.
When people's blood and body fat were tested for linoleic acid and they were followed for years, those with the most had slightly fewer heart and stroke events and fewer deaths from them, across 30 groups and nearly 69,000 people. People who ate the most linoleic acid had 15 percent fewer heart events than those who ate the least. And when healthy adults were given more linoleic acid, the usual blood markers of inflammation stayed flat.
There are limits on both sides. Figures that follow people over time struggle to separate the oil from everything else about the people who eat it, and some of the work on linoleic acid in the blood was part-funded by Unilever, which made Flora margarine until it sold its spreads business for $8 billion in 2018. The inflammation markers are only a handful of signals measured in healthy people. And when every long-term attempt to cut saturated fat was pooled, the cut reduced heart and stroke events by 17 percent but made no clear difference to deaths.
So the honest answer on seed oils and the heart is that it's still open. Anyone who tells you it's settled, either way, is selling something.
Where the real damage happens
The firmest case against seed oils is about heat. Heat a polyunsaturated oil and those weak spots react with oxygen, breaking the fat into aldehydes, reactive compounds that can damage proteins and DNA. The more linoleic acid in the oil, the more aldehydes it throws off. Saturated and monounsaturated fats hold up far better.
Reuse makes it worse. When sunflower oil was used again and again for deep-frying chips, the aldehydes in the chips rose with every round, and chips bought from fast-food outlets carried them in amounts worth taking seriously. A restaurant fryer full of seed oil, topped up and reused all week, is about as bad as it gets.
The fumes count too. Among Chinese women who never smoked, regular exposure to cooking-oil fumes went with about 1.7 times the risk of lung cancer, and cooking without an extractor fan with about twice the risk. When you fry, switch the fan on or open a window.
Then there's the company seed oils keep. They are the cheap fat inside crisps, biscuits, fried food, sauces, dressings and ready meals. Ultra-processed food brings sugar, refined starch and overeating along with the oil. Cut the processed food and a big share of the seed oil goes with it.
Omega-6 and omega-3
That omega-3 detail is the part of the missing results you can act on. Omega-6 and omega-3 fats compete for the same enzymes in the body, so the balance between them matters. As seed oils rose through the 20th century, the American ratio of omega-6 to omega-3 from plants climbed from about 6 to 1 up to 10 to 1, and the estimated omega-3 level in people's tissues fell by almost half.
You can fix that from both ends. Eat less seed oil and more oily fish, like sardines, mackerel and salmon. Grass-fed beef carries a little more omega-3 as well.
What to cook with
Butter, clarified butter and ghee. Low in linoleic acid and stable in the pan. Plain butter suits gentle cooking, because its milk solids burn. Clarified butter and ghee have those solids removed, so they take the heat of frying.
Beef tallow and lard. The traditional frying fats. Tallow is especially stable, and lard a step behind it. Rendering your own takes an afternoon, or comes ready in a jar. With lard, read the label: some supermarket tubs are hydrogenated, the same hardening process behind Crisco. is rendered pork fat and nothing else.
Extra virgin olive oil. Cold-pressed and mostly monounsaturated, it belongs in dressings, drizzled over finished food, or over gentle heat. Frying at 180°C quickly strips out the antioxidants and vitamin E that make extra virgin worth its price, so leave the frying to tallow, lard, ghee and clarified butter. If the pan does get too hot, olive oil copes better than seed oils: in repeated home deep-frying, it lasted 24 to 27 hours before it had to be thrown out, against 15 hours for a vegetable oil blend. Avocado oil is its closest relative: it comes from the fruit's flesh, sits at about 71 percent monounsaturated fat and 12.5 percent linoleic acid, and suits the same jobs.
Olive oil also has the best heart record of any fat here. People at high heart risk who were given free extra virgin olive oil and told to use it generously had about 30 percent fewer heart attacks, strokes and heart deaths over five years than those told to cut back on fat. The comparison was against low-fat advice, so it speaks for olive oil and against the low-fat plan, and says nothing against butter.
Coconut oil. Almost all saturated fat, and among the most stable fats in the kitchen.
A plate built on meat, fish, eggs and butter is naturally low in seed oils, which is one more reason keto and carnivore eating cuts them out almost by default. The animal matters, though. Chicken fat is 19.5 percent linoleic acid, more than canola, and lard is 10, because chickens and pigs store the fat from their grain and soy feed. Beef tallow sits at 3 percent and butter at 2, so beef, lamb and dairy fats are the low-linoleic choices.
Eating out is the hard part, because restaurants fry in seed oil. Choose grilled over deep-fried, ask what the kitchen cooks in, and treat the chip shop as an occasional stop.
The verdict on seed oils
Seed oils reached the frying pan as a waste product with a sales team: cheap to make, sold with free cookbooks, and backed by heart advice that ran ahead of its evidence. The heart question is still open, and the results that should have settled it were left half-published for decades. What is settled is the chemistry: they break down under heat, they break down faster when reused, and they ride into the diet on ultra-processed food.
The Minnesota patients had no say in what went into their pan. You do. Fry in tallow, lard, ghee or clarified butter, save the olive oil for dressings and gentle heat, and cut back on the fried and packaged food that carries so much of the seed oil.
Cutting seed oils out of the shopping basket? Where Seed Oils Are Hiding in Your Food — and How to Read Labels Around Them — every name they go by on a label, and the foods they turn up in.
Wondering what a plate without them looks like? Why Animal Foods Deliver What Plant-Based Diets Promise — and Can't — what meat, fish, eggs and dairy carry that plants struggle to match.
Know someone who still fries in vegetable oil because the label says heart-healthy? Send them this. A jar of tallow by the stove is one of the easiest swaps in the kitchen.
Disclaimer: This article is general information. For advice about your own health, talk to a qualified health professional, especially if you have a heart condition or take medicines.
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