Are Seed Oils Bad for You? What Is In the Bottle You Cook With Every Night (2026)

By Shannon Nicole, Founder, Switch to America · August 14, 2026 · Last updated October 13, 2026

What is a seed oil, and how is it made?

Quick answer: soybean, corn, canola, sunflower, cottonseed, safflower, grapeseed and rice bran. Olive oil and avocado oil are not seed oils, and I will come back to why that matters less than you would hope.

Nobody presses a soybean and gets oil the way you press an orange and get juice. There is not enough oil in the bean for that to work at scale.

So the industry does this instead. The seeds are cleaned, dehulled and rolled into flakes. Then hexane, a petroleum solvent, is flooded through the flake bed to dissolve the oil out of them. The hexane is boiled off and recaptured for reuse. What is left is called crude oil, and it is not something you would want on your dinner.

It then goes through four more steps to become the clear, odourless liquid you know: degumming, neutralising, bleaching with adsorbent clay, and deodorising by steam distillation under vacuum.

Read that list again and notice what it is for. Bleaching removes the colour. Deodorising removes the smell and the taste. Every one of those steps exists to take away something the oil was doing to tell you about itself. By the time it reaches the bottle it has no colour, no smell and no flavour, which means it also has no way to tell you when it has gone off.

That is the whole trick. Butter tells you. Bacon fat tells you. This does not.

What actually happens when you heat it?

Quick answer: the more polyunsaturated the oil is, the more toxic aldehydes it throws off when it gets hot. This has been measured, in a lab, with numbers.

Professor Martin Grootveld published a paper in Frontiers in Nutrition in 2022 that shallow fried in a range of culinary oils and then measured what came out of them by nuclear magnetic resonance.

After 90 minutes at 180°C, which is an ordinary frying temperature and a shorter session than a Sunday roast:

• Sunflower oil produced 17.22 mmol/kg of the alpha,beta-unsaturated aldehydes, which are the toxic ones
• Corn oil produced 15.67 mmol/kg
• A monounsaturated algae oil produced 4.78 mmol/kg
• Coconut oil, which has almost no polyunsaturated fat in it, produced "little or no aldehydes"

Those are not opinions about oil. That is the same pan, the same heat, the same clock, and a four hundred percent difference in what ends up in the food depending on which bottle you reached for.

And it does end up in the food. The same work found aldehydes in the fried potato itself, not just left behind in the pan.

Separate work has found that heating a linoleic acid rich oil for 24 hours at 180°C produces on the order of 207 mg of acrolein per kilogram of oil. Twenty four hours sounds absurd for a home kitchen. It is a completely normal week for the fryer behind the counter at a restaurant, and the same oil is used over and over until it is dark.

The oil that comes out of that fryer is the oil in most of what you eat when somebody else cooks it.

Do seed oils cause inflammation? The honest answer

Quick answer: the evidence for that specific claim is much weaker than the internet tells you, and I am not going to build a case on the shakiest brick in the pile.

Here is the other side, stated properly, because a post that only argues one way is not worth your time.

In 2017 the American Heart Association published a presidential advisory led by Frank Sacks. It reviewed the randomised controlled trials that lowered saturated fat and replaced it with polyunsaturated vegetable oil, and reported roughly a 30% reduction in cardiovascular disease. That is a real body of trials and it is a large effect. It is why doctors have been saying this for forty years.

There is a nuance in that advisory that nobody quotes, and it is the useful part. Sacks was explicit that what you replace the saturated fat with is the whole question. Swap it for refined carbohydrate and sugar and the benefit is not there. The trials that worked were trials that also fixed the rest of the diet.

So no, I am not going to tell you a bottle of sunflower oil is the reason your knees hurt. Anybody who tells you that with confidence is selling something.

What I will tell you is this. Those trials fed people fresh oil as a replacement for animal fat inside an otherwise decent diet. They are not a study of what happens when the same oil is heated to 180°C, cooled, reheated, and used again, which is the only way most of it reaches an American mouth. Grootveld's measurements are about that second thing, and the two bodies of evidence are not talking about the same substance.

The claim	What is actually true	Named authority, year	What it does NOT mean
"Seed oils cause inflammation"	Weakly supported. Large randomised trials replacing saturated fat with polyunsaturated oil cut cardiovascular disease by about 30%	Sacks et al., AHA Presidential Advisory, Circulation, 2017	Not that heated oil is the same as fresh oil. The trials tested fresh
Heating these oils is the same as heating any fat	Sunflower at 180°C for 90 min: 17.22 mmol/kg toxic aldehydes. Coconut: little to none	Grootveld, Frontiers in Nutrition, 2022	Not that one meal will hurt you. It is a dose question, repeated daily
The bottle contains what it says	20 arrested in Aug 2025 for dyeing sunflower oil and selling it as extra virgin olive oil	Europol and NAS Carabinieri, 2025	Not that all olive oil is fake. It means the label is not the proof

Is the oil in the bottle even the oil on the label?

Quick answer: often not, and this is the part of the story that has actual arrests attached to it.

In August 2025, Europol's intellectual property crime unit worked with the Italian NAS Carabinieri and the Tribunal of Darmstadt and arrested twenty people and seized 150,000 litres of fake extra virgin olive oil. The method was simple. Take low quality sunflower oil, add chlorophyll, beta-carotene and soya to fix the colour, and sell it into Italian and German restaurants as the good stuff. Europol put the gang's profit at up to eight million euros a year.

That was not an isolated raid. In Europol's Operation OPSON XIII, reported in 2024, the Carabinieri seized about 42 tonnes of adulterated oil being sold as Italian extra virgin, along with 623 litres of chlorophyll waiting to be used for the same purpose. In Spain, arrests included the owners of a canning company caught putting sunflower oil into olive oil containers.

Now sit with what that means for the advice you have been given. You were told to avoid the seed oil and buy the olive oil. The people running the fraud know you were told that. That instruction is precisely what makes the fraud profitable.

And it is not only olive. We went through this in detail on the avocado oil testing, where independent laboratory work found the overwhelming majority of samples were either rancid before they were opened or cut with something cheaper.

So the answer to "which bottle should I buy" is starting to look like a trick question.

Why does this land hardest on women over forty?

Quick answer: because you are the one who did what you were told, for thirty years, and got handed a prescription anyway.

You threw out the lard. Your mother cooked in it and you were told it was killing her, so you replaced it with the yellow bottle and you felt responsible doing it. You bought the tub with the heart on the front. You baked with the shortening because the recipe said to.

Thirty years of that, and now you are tired in a way sleep does not fix, your joints are loud in the morning, and someone in a ten minute appointment has told you it is just your age.

I am not going to hand you a single bottle and say that was the cause. That would be the same move that got you here, just with a different villain on the label.

What I will say is that you were not careless. You were obedient, to an industry that made a fortune selling you the replacement, and that industry is now on record in a European courtroom colouring one oil to look like another.

So what do I cook with instead?

Quick answer: a fat that is solid at room temperature and came from an animal, because it cannot be faked the way a clear liquid can.

Beef tallow. It sits in the jar as solid white fat. You can see it. You can smell it. If it goes off it tells you, immediately, and you will not need a laboratory to find out.

That single property is the answer to both halves of this post. It is barely polyunsaturated, so the aldehyde problem largely goes away at heat. And it cannot be quietly diluted with something cheaper and dyed back to the right colour, because it is not a colour. It is a texture.

Four things, in order:

• Buy one jar of tallow and use it for one thing. Eggs. Every morning. Do not overhaul the kitchen this week, because you will not finish.
• Find a local farm and ask the farmer for the fat. They usually have it and it is usually cheap, because most people no longer know to ask. You will also get to look the person in the eye, which no bottle allows.
• Notice where the oil is coming from that you did not buy. The restaurant fryer, the packaged crackers, the dressing. That is most of your exposure, and it is not in your cupboard.
• Stop hunting for the perfect bottle. That hunt is the product. Every time you upgrade to a more expensive clear liquid you are paying more for a thing you still cannot verify.

What else do people ask?

Is olive oil a seed oil? No. Olive oil is pressed from the fruit, not from a seed, and so is avocado oil. They are a different category entirely. The problem with those two is not how they behave in the pan, it is whether what you bought is what you think it is.

Do I need to throw out the bottle I have? No. Use it up cold if you want, in a dressing, where it is never heated. The measurements in this post are about heat.

Is not saturated fat supposed to be bad for me? The American Heart Association's 2017 advisory says replacing it with polyunsaturated oil lowered cardiovascular disease by about 30% in randomised trials. That is real and this post does not dispute it. It also says that what you replace it with decides everything, and that swapping it for refined carbohydrate gives you nothing. Those trials used fresh oil, not oil that has been through a fryer.

Who you become

You become the woman who stopped shopping for a better bottle and started asking a different question altogether. Not which oil is healthiest, but which fat can actually tell me the truth about itself.

Then you fry your eggs in something you can see, from a farm you can drive to, and you notice that nobody had to run a laboratory test to prove it was what it said it was.