Golden cooking oil being poured into a bowl of oil

Are Seed Oils Bad for You? What the Evidence Shows

10 min read

You reach for the same bottle of sunflower or rapeseed oil you have used for years, and a warning you saw online plays in your head: these oils are toxic, they drive inflammation and heart disease, and you should bin them today. The main claims have been tested in trials and large cohort studies, which follow large groups of people for years.

This guide covers what seed oils are, what controlled trials found on inflammation, what large studies and trials show for heart disease and diabetes, the trials critics point to, and where the real concern with cooking oil sits.

68,659
Adults with linoleic acid measured in blood or body fat, across 30 cohort studies in 13 countries
Marklund et al. 2019, Circulation
22%
Lower risk of cardiovascular death with higher linoleic acid levels, people near the top versus near the bottom of the range, the largest of the results
Marklund et al. 2019, Circulation
15
Randomised trials in healthy people, none of which found a significant rise in inflammation markers with more linoleic acid
Johnson and Fritsche 2012
35%
Lower risk of type 2 diabetes with higher linoleic acid levels, people near the top versus near the bottom of the range, in 39,740 adults
Wu et al. 2017, Lancet Diabetes and Endocrinology
Sources: Marklund et al. 2019, Circulation. Johnson and Fritsche 2012, Journal of the Academy of Nutrition and Dietetics. Wu et al. 2017, Lancet Diabetes and Endocrinology. The cohort results are associations, not proof of cause.

What seed oils are

Seed oils are cooking oils pressed from seeds, such as sunflower, rapeseed, soybean, corn, grapeseed and cottonseed oil. Most are rich in linoleic acid, an omega-6 polyunsaturated fat. Rapeseed oil is the exception: the 2025 Bazina review lists it as mostly oleic acid, the monounsaturated fat in olive oil, with 16% to 25% linoleic acid. A 2018 Cochrane review describes omega-6 fats as vital for many functions in the body. The NHS (National Health Service) says omega-6 fats are found in vegetable oils such as rapeseed, corn and sunflower, and that most people get enough omega-6 in their diet.

The online worry is that people now eat far more linoleic acid than before, and that this turns it harmful. That concern has been tested in trials.

The inflammation claim, put to the test

The main charge is that omega-6 fat fuels chronic inflammation. The idea is that the body turns linoleic acid into arachidonic acid, which it uses to make some signalling molecules involved in inflammation.

A 2012 systematic review by Guy Johnson and Kevin Fritsche gathered 15 randomised controlled trials in healthy people. None of the trials found a significant rise in a wide range of inflammation markers, including C-reactive protein, fibrinogen and cytokines, when people ate more linoleic acid. The authors concluded there was virtually no evidence from these trials that adding linoleic acid to the diet raises inflammatory markers. These were trials in healthy people, so they do not cover people with inflammatory conditions.

That does not make fried food healthy. It means the narrower claim, that the linoleic acid in seed oils inflames the body, is not supported by these controlled trials.

What studies show for heart disease and diabetes

Linoleic acid levels and cardiovascular risk Percent lower risk, people near the top versus near the bottom of linoleic acid levels Cardiovascular death 22% Ischaemic stroke 12% Total cardiovascular disease 7%
Source: Marklund et al. 2019, Circulation, 30 cohort studies, 68,659 adults. Each figure compares people at the middle of the bottom fifth of linoleic acid levels with people at the middle of the top fifth. The 6% lower risk of coronary heart disease was not significant and is not shown. Observational data.

In 2019, Matti Marklund and colleagues ran the same analysis in each of 30 cohort studies in 13 countries and pooled the results, covering 68,659 adults and 15,198 cardiovascular events. They measured linoleic acid in blood and body fat rather than relying on food questionnaires. Comparing the middle of the top fifth of linoleic acid levels with the middle of the bottom fifth, higher levels were linked to a 7% lower risk of cardiovascular disease, a 12% lower risk of ischaemic stroke and a 22% lower risk of cardiovascular death. The 6% lower risk of coronary heart disease was not statistically significant. Blood linoleic acid reflects all sources, including nuts and seeds, not seed oil alone.

Randomised trials have not confirmed a heart benefit. The 2018 Cochrane review by Lee Hooper and colleagues included 19 randomised trials with 6,461 people. For death from any cause (10 trials, 4,506 people) and for cardiovascular events, it found low-quality evidence that more omega-6 fat made little or no difference. It was uncertain about cardiovascular deaths. It found high-quality evidence that more omega-6 fat lowered total cholesterol a little over the long term. In most of these trials, omega-6 fat replaced saturated or monounsaturated fat.

For diabetes, Jason Wu and colleagues pooled 20 cohort studies with 39,740 adults in 2017. Comparing people near the top and near the bottom of linoleic acid levels, higher levels were linked to a 35% lower risk of type 2 diabetes. These cohort results are associations and do not prove cause.

  • The claim online: seed oils are toxic, drive inflammation, and cause heart disease and diabetes.
  • What the studies show: controlled trials found no rise in inflammation markers, large cohorts link higher linoleic acid to lower cardiovascular disease, stroke and diabetes risk, and the Cochrane trials found little or no difference in deaths.

The trials critics point to

Two older trials are often cited against seed oils, and both were re-analysed by Christopher Ramsden and colleagues in the BMJ. In the Sydney Diet Heart Study, run from 1966 to 1973 in 458 men who had recently had a coronary event, replacing saturated fat with linoleic acid from safflower oil and margarine was linked to higher death rates: 17.6% died in the linoleic acid group compared with 11.8% of controls. An updated meta-analysis that added these data showed non-significant trends toward higher risk of death from heart disease.

In recovered data from the Minnesota Coronary Experiment, run from 1968 to 1973 in a nursing home and six mental hospitals with 9,423 people, a diet using corn oil lowered cholesterol by 13.8% but showed no benefit for death. A meta-analysis of 5 similar trials found no benefit for death from heart disease or from any cause. The authors concluded that replacing saturated fat with linoleic acid lowers cholesterol, but that the trials do not support a lower risk of death.

In our view, these trials are a fair reason to doubt that adding seed oils protects your heart. They were run decades ago in specific groups. The Sydney trial was in men who had recently had a heart event. Both trials looked at deaths, and in Minnesota heart damage found at autopsy, so they do not speak to inflammation markers.

Where the real concern sits

There is a version of the worry that does hold up, and it is about oil that is heated hard and reused, not the oil in the bottle. A 2025 review in Food Science and Nutrition, led by Naser Bazina, describes how heating oil to frying temperatures oxidises its fats into substances such as aldehydes. It reports that prolonged and repeated use of frying oil produces trans fats and toxic by-products, and that repeated heating and cooling cycles speed up the damage. The type of oil matters too: the review reports that oils higher in oleic acid, such as olive oil, tend to be more stable under heat than oils high in linoleic acid, such as corn oil, which are more prone to rapid oxidation.

In our view, a takeaway fryer that runs the same oil all day is a different thing from the splash you use to roast vegetables at home. Cook with fresh oil, avoid reusing frying oil many times, and do not heat it until it smokes and darkens.

What to do

These steps are ours, built from the studies above and NHS guidance.

1
Use liquid oils in place of hard fats
The NHS says: "There's good evidence that replacing saturated fats with some unsaturated fats can help to lower your cholesterol level." It names olive and rapeseed oil as sources of monounsaturated fat, and sunflower oil as a source of omega-6.
2
Use fresh oil for frying
The Bazina review links the damage to prolonged and repeated use. Our advice: fry with fresh oil, avoid running it through many batches, keep it below the point where it smokes, and pick an oil higher in oleic acid, such as olive or rapeseed, for high-heat frying.
3
Eat oily fish
The NHS advises eating at least 2 portions of fish a week, with 1 of them oily. In our view, getting omega-3 this way is a more useful step than cutting omega-6.
4
Focus on the whole meal
In our view, the problem with chips and fried snacks is the whole package of processed food, salt and energy. Build meals around vegetables, protein, whole grains and fruit.

Frequently asked questions

Do seed oils cause inflammation?
In a review of 15 randomised trials in healthy people, none found a significant rise in inflammation markers such as C-reactive protein when people ate more linoleic acid.
Is olive oil a better choice?
The NHS names olive oil and rapeseed oil as sources of monounsaturated fat, and the Bazina review reports that oils higher in oleic acid, such as olive oil, tend to be more stable under heat than oils high in linoleic acid. The NHS also says men should not eat more than 30g of saturated fat a day and women no more than 20g.
Is the omega-6 to omega-3 ratio a problem?
None of the studies in this guide tested the ratio directly. The NHS says most people get enough omega-6. In our view, eating oily fish for omega-3 is a better step than cutting omega-6.
Should I throw out my sunflower oil?
The inflammation trials and cohorts here found no sign that linoleic acid, the omega-6 fat the NHS links to sunflower oil, harms healthy people, though trials have not shown a heart benefit either. In our view, you do not need to throw it out. Avoid reusing frying oil many times and avoid heating it until it smokes.
Why do people say seed oils are toxic?
In our view, the claim mixes real chemistry about heated, reused oil with the different question of fresh oil in a balanced diet.

The bottom line

Controlled trials did not find the inflammation the seed oil claim predicts, and cohorts of tens of thousands of adults link higher linoleic acid levels to lower cardiovascular disease, stroke and diabetes risk. Randomised trials have not shown that seed oils lower the risk of death, and one older trial in men with heart disease found higher death rates. The clearest concern is oil that has been fried hard and reused. Our advice: cook with liquid oils, keep frying oil fresh, eat oily fish, and focus on the whole plate.

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Related reading

Sources

  • Marklund M, Wu JHY, Imamura F, et al. Biomarkers of Dietary Omega-6 Fatty Acids and Incident Cardiovascular Disease and Mortality. Circulation 2019. pubmed.ncbi.nlm.nih.gov/30971107
  • Hooper L, Al-Khudairy L, Abdelhamid AS, et al. Omega-6 fats for the primary and secondary prevention of cardiovascular disease. Cochrane Database of Systematic Reviews 2018. pubmed.ncbi.nlm.nih.gov/30488422
  • Johnson GH, Fritsche K. Effect of dietary linoleic acid on markers of inflammation in healthy persons: a systematic review of randomized controlled trials. Journal of the Academy of Nutrition and Dietetics 2012. pubmed.ncbi.nlm.nih.gov/22889633
  • Wu JHY, Marklund M, Imamura F, et al. Omega-6 fatty acid biomarkers and incident type 2 diabetes: pooled analysis of individual-level data for 39,740 adults from 20 prospective cohort studies. Lancet Diabetes and Endocrinology 2017. pubmed.ncbi.nlm.nih.gov/29032079
  • 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. pubmed.ncbi.nlm.nih.gov/23386268
  • 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. pubmed.ncbi.nlm.nih.gov/27071971
  • Bazina N, Ahmed T, Almdaaf M, et al. Chemical Changes in Deep-Fat Frying: Reaction Mechanisms, Oil Degradation, and Health Implications. Food Science and Nutrition 2025. pubmed.ncbi.nlm.nih.gov/41089756
  • NHS. Fat: the facts. nhs.uk/live-well/eat-well/food-types/different-fats-nutrition
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