Are seed oils bad for you? What the debate actually says

A debate worth taking seriously

Few topics in nutrition have generated as much argument in recent years as seed oils. On one side, mainstream dietary guidelines — including those from the NHS and the British Heart Foundation — have recommended replacing saturated fats with polyunsaturated vegetable oils for decades. On the other, a growing number of researchers, clinicians, and well-read laypeople are asking whether that advice was built on solid ground.

This isn't a fringe argument confined to social media. It draws on peer-reviewed studies, reanalysed trial data, and a genuine disagreement within nutrition science about what the evidence actually shows. It deserves a straight look.

What seed oils are — and aren't

The term "seed oils" typically refers to a group of industrially produced vegetable oils: rapeseed (canola), sunflower, soybean, corn, cottonseed, safflower, rice bran, and grapeseed. What they share is that they're extracted from seeds (rather than fruit flesh, like olive oil or coconut oil), are high in polyunsaturated fatty acids (PUFAs) — particularly omega-6 linoleic acid — and are produced at industrial scale using a combination of mechanical pressing and chemical solvent extraction, typically with hexane.

Olive oil, coconut oil, and butter are not seed oils in this sense, which is why the debate tends to focus specifically on the industrially refined group above. It's a meaningful distinction — both chemically and in terms of how they're processed.

Where the mainstream position came from

The case for replacing saturated fat with polyunsaturated vegetable oils was built largely on the work of American physiologist Ancel Keys. His Seven Countries Study, published in 1970 and expanded in a major analysis in 1980, reported a strong correlation between saturated fat intake and coronary heart disease across different populations.¹ This became the scientific foundation for decades of dietary guidelines that recommended reducing butter, lard, and red meat in favour of vegetable oils and margarines.

That foundation has since been questioned — not by people hostile to mainstream science, but by researchers working within it.

The Minnesota Coronary Experiment — and why it matters

In 2016, Christopher Ramsden and colleagues at the National Institutes of Health published a reanalysis of data from the Minnesota Coronary Experiment, a large randomised controlled trial conducted between 1968 and 1973. The trial had been led by Ivan Frantz, a colleague of Ancel Keys. Its data had never been fully published — a fact that only came to light when Frantz's son found the original records decades later.

When Ramsden's team analysed the recovered data, the results were striking. Participants who replaced saturated fat with linoleic-acid-rich vegetable oil did lower their blood cholesterol. But they did not live longer. In fact, for participants over 65, increased linoleic acid consumption was associated with a significantly higher risk of death, despite the cholesterol reduction.²

This directly contradicted what the diet-heart hypothesis predicted. Lower cholesterol should have meant fewer deaths. It didn't.

A similar pattern had emerged from the reanalysis of the Sydney Diet Heart Study, published by Ramsden and co-authors in 2013. Again: lower cholesterol, but no survival benefit — and a higher rate of cardiovascular events in the group replacing saturated fat with vegetable oil (specifically a margarine high in linoleic acid).³

These are not small, obscure studies. They are among the largest controlled dietary trials ever conducted. The fact that their full results were not published at the time is, in itself, a meaningful part of the story.

The omega-6 linoleic acid hypothesis

James DiNicolantonio and James O'Keefe set out a detailed version of the concern in a 2018 paper in Open Heart, the journal of the British Cardiovascular Society. Their argument centres on oxidised linoleic acid — what happens to the omega-6 fat in seed oils when it's exposed to heat, light, or processing.

Linoleic acid is a polyunsaturated fat, which means it has multiple double bonds in its carbon chain. Those double bonds make it chemically reactive. When exposed to heat during cooking — particularly repeated high-heat frying — polyunsaturated fats oxidise and produce a range of breakdown products, including 4-hydroxynonenal (4-HNE) and malondialdehyde (MDA), both of which are cytotoxic in laboratory studies.⁴

DiNicolantonio and O'Keefe also point to changes in dietary patterns over the twentieth century. US consumption of soybean oil alone increased approximately 1,000-fold between 1909 and 1999, and per capita omega-6 intake rose sharply while omega-3 intake remained relatively stable — shifting the dietary omega-6 to omega-3 ratio from something close to 4:1 historically to estimates of 15:1 or higher in modern Western diets.⁵

The significance of that ratio is itself contested — but the direction of change is not.

What mainstream science currently says

It would be misleading to suggest the scientific consensus has collapsed. It hasn't. The American Heart Association published a Presidential Advisory in 2017 reaffirming that replacing saturated fat with polyunsaturated fat reduces cardiovascular risk, citing multiple meta-analyses and randomised trials.⁶ The UK's Scientific Advisory Committee on Nutrition (SACN) published a report in 2019 reaching broadly similar conclusions.⁷

The NHS continues to recommend choosing unsaturated fats over saturated fats for heart health.

But "the consensus holds" and "the questions are settled" are different claims. Nutrition science has a well-documented reproducibility problem. Many of its landmark findings rest on observational cohort studies that cannot establish causation. The randomised controlled trials that could establish it — like the Minnesota Coronary Experiment — have in some cases produced results that cut against the received view.

The honest position is that the evidence base for replacing saturated fat with polyunsaturated vegetable oil is less solid than official guidelines have tended to suggest, and that the questions raised by the recovered trial data deserve more scrutiny than they've received in public health messaging.

The processing question

Separate from the omega-6 argument is a more straightforward concern about how refined seed oils are made.

Cold-pressed oils — including extra virgin olive oil and cold-pressed rapeseed oil — are extracted mechanically, with no chemical solvents, at temperatures low enough to preserve their natural composition. Most supermarket vegetable oils are not produced this way. Industrially refined oils go through solvent extraction (typically hexane), followed by degumming, refining, bleaching, and deodorising — a series of high-heat processes that can reach 200–230°C. The finished oil is chemically consistent and shelf-stable, but it bears little resemblance to the raw seed it came from.

The question of whether residual oxidation products from this process matter to human health at the quantities consumed is genuinely unanswered. But it's a reasonable thing to think about, particularly for oils used frequently at high heat.

What this means practically

None of this means seed oils will harm you if you use them occasionally. The concern is more specific: refined seed oils used repeatedly at high heat, or as a near-daily cooking fat in large quantities, in the context of a diet already high in processed food — which is where most seed oil consumption actually occurs.

A few practical observations that most people on both sides of the debate would broadly agree with:

• Cold-pressed or extra virgin oils — olive, rapeseed, coconut, avocado — are preferable to refined equivalents, particularly for dressings and lower-heat cooking.
• Oils with a higher saturated or monounsaturated fat content (butter, ghee, coconut oil, extra virgin olive oil) are more stable at high heat than polyunsaturated-rich seed oils.
• The omega-6 to omega-3 ratio in the diet is worth thinking about, independently of the seed oil question. Most people eating a modern Western diet consume far more omega-6 than omega-3.
• Processed food — which is the primary vehicle for refined seed oil in most diets — has its own body of evidence linking it to poor health outcomes, separate from any debate about the oils it contains.

A note on omega-3 and omega-6

Both omega-3 and omega-6 are essential fatty acids — meaning the body cannot make them and must obtain them from food. The question isn't whether you need omega-6; you do. The question is one of balance, and most people eating a typical Western diet are getting far more omega-6 than omega-3.

Omega-3 fatty acids are found most abundantly in oily fish (as EPA and DHA) and in certain plant sources including flaxseed and chia seeds (as ALA, which the body converts to EPA and DHA at limited efficiency). For people who eat little oily fish, an omega-3 supplement is one of the most straightforward ways to shift the ratio.

We stock a small range of omega oils — including cod liver oil and omega-3 fish oil — selected for purity and sourced to standards we'd hold ourselves to. If you're thinking about the balance in your own diet, they're worth a look.

The practical takeaway

The seed oils debate is real, the science is genuinely unsettled in places, and the concerns raised by serious researchers are worth knowing about. But it doesn't require a dramatic overhaul of how you cook.

Use cold-pressed oils where you can. Use butter or extra virgin olive oil for cooking at higher temperatures. Eat less processed food — where the majority of refined seed oil exposure actually comes from. And think about whether your diet has enough omega-3 to balance the omega-6 you're already getting.

That's a more useful set of actions than either dismissing the debate entirely or concluding that vegetable oil is poisoning you. The truth, as is often the case in nutrition, sits somewhere more complicated than either camp tends to admit.

References

1. Keys A. Seven Countries: A Multivariate Analysis of Death and Coronary Heart Disease. Harvard University Press; 1980.
2. Ramsden CE, Zamora D, Majchrzak-Hong S, et al. Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968–73). BMJ. 2016;353:i1246. Read the paper
3. Ramsden CE, Zamora D, Leelarthaepin B, et al. Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis. BMJ. 2013;346:e8707. Read the paper
4. DiNicolantonio JJ, O'Keefe JH. Omega-6 vegetable oils as a driver of coronary heart disease: the oxidized linoleic acid hypothesis. Open Heart. 2018;5(2):e000898. Read the paper
5. Blasbalg TL, Hibbeln JR, Ramsden CE, et al. Changes in consumption of omega-3 and omega-6 fatty acids in the United States during the 20th century. Am J Clin Nutr. 2011;93(5):950–962. Read the paper
6. Sacks FM, Lichtenstein AH, Wu JHY, et al. Dietary Fats and Cardiovascular Disease: A Presidential Advisory From the American Heart Association. Circulation. 2017;136(3):e1–e23. Read the paper
7. Scientific Advisory Committee on Nutrition (SACN). Saturated Fats and Health. Public Health England; 2019. Read the report