Are Seed Oils Bad For You? Both Sides Are Quoting Half The Evidence

There is a fight about cooking oil, and the strange thing about it is that both sides are winning.

Not because the question is unanswerable. Because each side is arguing from a different half of the same filing cabinet, and neither one opens the other drawer.

We are going to open both.

First, take the concern seriously — because something really did change

It would be easy to open by calling this a moral panic about salad dressing. That would be unfair, because the underlying observation is true and it is large.

Linoleic acid is the main fat in soybean, corn, sunflower, safflower and cottonseed oil. It is what people mean by “omega-6”. And the amount of it stored in American bodies has gone up enormously.

A systematic review of studies measuring linoleic acid in the fat under Americans’ skin found it has risen 136% over the last half century, closely tracking the rise in how much people eat [8]. Body fat composition is a slow, honest record of a long-term diet — it is much harder to fool than a food questionnaire.

So the exposure is real, it is recent, and it is big. Anyone who answers “nothing has changed, relax” is not being straight with you. The question is what the change did.

The mechanism everyone names, and what happened when it was tested

Ask why seed oils are supposed to be harmful and you will get a specific biochemical story. It goes like this.

Linoleic acid is the raw material your body converts into arachidonic acid. Arachidonic acid is the raw material for a family of signalling molecules that drive inflammation. Eat more of the first, get more of the second, get more inflammation. Textbook biochemistry, and every step of it is real.

It is also, as a chain, testable. You can feed people more linoleic acid and measure whether arachidonic acid in their tissue actually rises. People did.

A systematic review gathered every human trial that changed dietary linoleic acid and measured the arachidonic acid in blood [1]. The results are not subtle.

Cutting dietary linoleic acid by up to 90% was not significantly correlated with any change in arachidonic acid (p = 0.39). Raising it by up to six-fold was not either (p = 0.72) [1]. In red blood cells, same answer. Of seven red-cell studies, the one that found a significant change found it going the wrong way: cutting linoleic acid by 29% produced a 4% rise in arachidonic acid [1].

Now, is that because the measurement is insensitive? No — and this is the detail that closes the door. In the same review, two other dietary fats did raise arachidonic acid: gamma-linolenic acid, and arachidonic acid itself [1]. The assay works fine. It is linoleic acid specifically that does not do this.

If you prefer your evidence prospective rather than pooled, there is a controlled feeding trial too. Sixty-two women were randomised to three twelve-week diets where the food was provided. The low-linoleic-acid diets successfully lowered linoleic acid in the blood — so the intervention worked — and produced no difference in arachidonic acid between groups [2]. The authors’ own summary: lowering dietary linoleic acid “did not result in the expected changes in fatty acids associated with the LA-ARA inflammatory axis.”

And downstream, where the inflammation itself would show up? Fifteen randomised trials, reviewed: none found a significant effect on C-reactive protein, fibrinogen, cytokines, adhesion molecules or TNF-alpha [3]. A later meta-analysis pooled thirty trials in 1,377 people and found nothing either [4].

One honest asterisk on that last one, because it is the only crack in this wall: the same meta-analysis reported that among people whose linoleic acid intake went up by an especially large amount, CRP might rise [4]. That is a subgroup signal, not a main finding, and it deserves to be followed up rather than either buried or brandished.

So: the named mechanism, examined at the step where it would have to happen, does not happen. That is what carries the Unsupported rating on this page, and we want to be precise that it is that claim we are rating — not the whole subject.

Now the drawer the reassurance articles never open

Here is where the confident “the science is settled, seed oils are fine” version of this article stops being honest.

Because there were randomised controlled trials. Real ones, large ones, with the food actually provided. And they did not go well.

In Sydney between 1966 and 1973, 458 men who had recently had a heart attack were randomised to replace animal fats with safflower oil and safflower oil margarine [5]. Most of the results sat unpublished for four decades until a team recovered them.

The men on safflower oil died more. All causes: 17.6% versus 11.8%. Cardiovascular death: 17.2% versus 11.0%. Coronary death: 16.3% versus 10.1% [5]. Every one of those differences was at or inside the conventional threshold for statistical significance.

Then the bigger one. The Minnesota Coronary Experiment ran from 1968 to 1973 across one nursing home and six state mental hospitals, randomising 9,423 people aged 20 to 97, double-blind, with every meal provided [6]. It replaced the kitchens’ usual fats with corn oil.

It worked, mechanically. Serum cholesterol fell 13.8% in the intervention group against 1.0% in the control [6]. The diet did precisely what it was designed to do.

And it saved nobody. No mortality benefit in the full cohort or in any subgroup the original investigators had specified in advance. No benefit in coronary atherosclerosis or heart attacks found at autopsy [6].

Worse than nothing, actually. In the adjusted analysis there was a 22% higher risk of death for every 30 mg/dL drop in cholesterol — hazard ratio 1.22, and a tight interval running 1.14 to 1.32 [6]. The people whose cholesterol responded best to the corn oil died soonest.

The same paper pooled five such trials covering 10,808 people and found no benefit on coronary mortality (1.13, 0.83 to 1.54) or death from any cause (1.07, 0.90 to 1.27) [6].

If you have only ever read that seed oil worries are internet nonsense, that is the material you were not shown. It is in the BMJ. It is publicly funded, and the authors declare no financial interest in the outcome [6] — which, on this topic, is worth saying out loud.

And now the problems with those trials, which the alarm side never mentions

We read both papers in full rather than their abstracts, and three things turned up that change how much weight those results can carry.

One: the control group was eating trans fat. In the Minnesota trial the comparison diet was built from the era’s ordinary margarines and shortenings, and the paper says plainly that these “were rich sources of industrially produced trans fatty acids” so the control diet “contained substantial quantities of trans fat” [6]. This was never corn oil versus butter. It was corn oil versus saturated fat plus a fat we now know is genuinely dangerous.

Two: the control group’s linoleic acid went up too. Compared with what the hospitals served before the trial, the control diet raised linoleic acid by about 38%, from 3.4% to 4.7% of calories [6]. It was not a seed-oil-free comparison.

Three: the dose was enormous. The intervention took linoleic acid from 3.4% of calories to 13.2% — a rise of more than 280% — while halving saturated fat [6]. That is not what happens when you fry an egg in canola oil. It is an extreme substitution, engineered to move cholesterol as far as possible.

Add that only 2,355 of the 9,423 stayed on the diets a year or more, and that just 149 autopsy files were ever completed [6], and you have a trial that is genuinely important and genuinely limited at the same time. Both of those, together.

Then there is the largest dataset nobody in the argument cites

If the trials are messy, is there anything cleaner? There is one thing, and it is very large.

In 2019, thirty long-running studies from thirteen countries pooled their raw participant-level data: 68,659 people, 15,198 cardiovascular events, followed for anywhere from two and a half to nearly thirty-two years [7].

Crucially, they did not ask anyone what they ate. They measured the linoleic acid in people’s blood and fat tissue, which sidesteps the well-known unreliability of food questionnaires.

People with more linoleic acid in them had less cardiovascular disease. Total cardiovascular disease: hazard ratio 0.93. Ischaemic stroke: 0.88. And cardiovascular death: 0.78 [7] — a 22% lower rate, with the whole plausible range sitting below 1.

Arachidonic acid, the supposed villain of the mechanism, was not associated with higher risk at all. The people with the most of it had slightly less cardiovascular disease [7].

This is observational, so it cannot prove causation, and healthy-user effects are a real problem in this kind of data. But it is the largest and best-measured thing anyone has, and it points firmly the opposite way from the trials.

So how can the trials and the cohorts both be right?

This is the part neither camp gets to, and it is where the answer actually lives.

They are not answering the same question.

The trials asked: if we take institutionalised people in 1968 and swap their animal fat for a great deal of corn oil, do fewer of them die? The answer was no.

The cohorts asked: do people walking around with more linoleic acid in their tissue have fewer heart attacks? The answer was yes.

Those can both be true, because a substitution trial and a cross-sectional exposure are different things. And there is a third question, which is the one you are actually asking, and which nobody has run a trial on at all.

Nobody has tested the modern exposure. The trials tested replacing saturated fat with seed oil. Nobody’s diet in 2026 works like that. The realistic modern exposure is seed oil added — to fried food, packaged snacks, sauces, restaurant cooking — on top of everything else, usually inside foods that are also energy-dense and heavily processed.

No randomised trial has tested that, because it would be enormous, expensive and probably unethical to run for long enough to matter. Which means the honest answer to “what does the seed oil in my diet do to me” is that the specific experiment has not been done.

What we can say is that the reason offered for worrying — the inflammation pathway — has been checked and does not hold up.

What we are rating, exactly

Unsupported: the claim that seed oils harm you by raising arachidonic acid and driving inflammation. Tested by systematic review, by controlled feeding trial, and by thirty randomised trials measuring inflammation itself. It fails at the first step and never recovers.

Not rated Unsupported, and genuinely open: whether replacing saturated fat with large amounts of linoleic acid lowers mortality. Two recovered trials say no. A meta-analysis of five says no. The largest biomarker dataset in the world says the opposite. That is a live disagreement and we are not going to pretend otherwise.

Untested: what seed oils do when added to an already ultra-processed diet, which is how almost everyone actually eats them.

What to actually do

Stop treating the oil as the villain in the meal. Almost all the seed oil most people eat arrives inside fried and packaged food. If you cut those, your linoleic acid drops as a side effect — and you will have changed a dozen other things at the same time, most of which have better evidence against them.

Cooking at home with canola or sunflower oil is not the problem worth solving. The mechanism cited for fearing it has been tested and does not hold, and the people with the most linoleic acid in their bodies have less cardiovascular disease, not more [7].

Be suspicious of the price of the alternative. “Seed-oil-free” has become a badge you can charge for, on snacks and restaurant menus and skincare. The badge is currently better evidenced as a marketing position than as a health intervention.

And if you switched to tallow and you feel better, we are not going to argue with you about your own body. Just know which part of that is measured and which part is not. Nobody has run the trial on your kitchen.

This is journalism, not medical advice. If you have heart disease or you are managing cholesterol with a clinician, that conversation should involve them rather than a website. Our other pages on contested nutrition claims: fibre, artificial sweeteners and erythritol, where a similar marker-versus-cause problem runs through the whole literature.