Short answer: The evidence is insufficient to call seed oils harmful or helpful for the brain, because no trial has tested them on cognition. In trials, linoleic acid doesn’t reliably raise inflammation markers. Repeatedly heated frying oil is a plausible concern (mostly lab and animal data). Cook with extra-virgin olive oil, avoid reused deep-frying oil, and eat less ultra-processed food.

There is not enough evidence to say that seed oils harm the brain, or that they help it. No trial has tested seed oils, or avoiding them, on memory, cognitive decline or dementia. The claims on both sides rest on biochemistry, animal studies and heart research.

What you can do is simple. Use extra-virgin olive oil as your main fat and avoid food fried in oil that has been reheated many times. Cut back on ultra-processed food, which is where most seed oil comes from. A splash of canola oil in a salad dressing is not a threat to your brain.

What are seed oils?

Seed oils, often sold as “vegetable oils”, are pressed or extracted from plant seeds. Industrial processing typically uses high heat, a chemical solvent (usually hexane) and deodorization. The main ones are:

  • Soybean oil: the main source of the rise in linoleic acid intake in the US during the twentieth century. Consumption per person rose more than 1,000-fold between 1909 and 1999 (Blasbalg 2011).
  • Corn oil: widely used for frying and in processed food.
  • Canola (rapeseed) oil: lower in omega-6 and higher in omega-3 than other seed oils.
  • Sunflower oil: common in snack foods; high-oleic versions are increasingly available.
  • Safflower oil: one of the highest in linoleic acid among common cooking oils.

All are rich in linoleic acid, an omega-6 polyunsaturated fat. Soybean and corn oil are each about half linoleic acid. Sunflower oil ranges from a few percent (high-oleic types) to about two-thirds (traditional types).

Linoleic acid made up about 2.8 percent of calories in the US food supply in 1909 and about 7.2 percent by 1999 (Blasbalg 2011). These are food-supply figures, not measured intake. That is a two- to three-fold rise within a few generations.

Does the omega-6 to omega-3 ratio matter?

The main argument against seed oils is about balance. Omega-6 and omega-3 fats compete for the same enzymes that turn them into longer, active forms. When omega-6 intake is very high compared with omega-3, the balance may tip toward inflammatory signaling molecules.

Ancestral human diets are thought to have provided omega-6 and omega-3 at about 1:1. Modern Western diets deliver about 15:1 to 17:1, because linoleic acid from seed oils rose while omega-3-rich foods such as wild fish and leafy greens fell. In a widely cited review, Simopoulos (2002) argued that this imbalance promotes heart disease, cancer, and inflammatory and autoimmune diseases.

Critics, including Fritsche (2008), argue that the total amount of omega-3 matters more than the ratio. In their view, Western diets are not too high in omega-6 but too low in omega-3, so the fix is more fatty fish rather than less seed oil. Eating preformed DHA is the most reliable way to raise blood DHA (Brenna 2009). See our article on omega-3s and the brain.

The ratio still matters for some people. The body converts very little plant omega-3 (ALA) into DHA: ALA supplements raise blood EPA but barely move DHA (Brenna 2009). Linoleic acid competes with ALA for the same enzymes. Blasbalg and colleagues (2011) estimated that the twentieth-century rise in linoleic acid has likely lowered tissue EPA and DHA. If you rely on plant ALA rather than fish, this competition may matter more. Raising omega-3 and moderating omega-6 together is a reasonable approach.

Does linoleic acid cause inflammation?

Linoleic acid is essential: the body can’t make it and needs small amounts. It is also the starting material for arachidonic acid, which the body turns into inflammatory signaling molecules. In the brain, long-term overproduction of these molecules contributes to the activation of immune cells and to ongoing inflammation. That process is implicated in Alzheimer’s disease, Parkinson’s disease and age-related cognitive decline (Heneka 2015). This is the basis for the worry about seed oils.

But controlled trials don’t show that eating more linoleic acid raises inflammation. A systematic review of 15 randomized trials (Johnson and Fritsche 2012) found that none reported a significant rise in inflammatory markers with higher linoleic acid intake in healthy adults.

A later review (Innes and Calder 2018) agreed. In healthy adults, more linoleic or arachidonic acid did not raise the levels of many inflammatory markers. The same review noted evidence that a high omega-6 diet can blunt the anti-inflammatory effects of omega-3 fats, and that this interaction is not well understood.

These trials have limits. Most were short (weeks to months), were done in healthy young adults, and measured inflammation in the blood rather than in the brain. Whether decades of high linoleic acid intake on a poor, ultra-processed diet is as harmless as a few weeks of controlled feeding is an open question.

Heated oils and oxidation

Polyunsaturated fats are less stable than monounsaturated or saturated fats. When seed oils are exposed to high heat, light or oxygen, especially during repeated deep frying, they oxidize and form reactive compounds called aldehydes.

The most studied is 4-HNE, which forms when omega-6 fats oxidize. It binds to proteins, DNA and cell membranes and disrupts how they work. Higher 4-HNE levels have been found in the brains of people with Alzheimer’s disease (Markesbery and Lovell 1998).

Cooking conditions make a large difference. A single moderate-heat sauté in sunflower or soybean oil produces far less oxidation than reusing the same oil for many rounds of deep frying, which is common in commercial kitchens. A review by Ganesan and colleagues (2018) concluded that repeated heating increases oxidation in vegetable oils and argued that eating these oils may worsen cardiovascular risk. Much of the evidence they reviewed comes from animal studies.

Extra-virgin olive oil resists oxidation better. It is about 70-80 percent monounsaturated oleic acid and is rich in polyphenols. In a 2021 study by an Australian olive-oil testing laboratory (De Alzaa 2021), chips, chicken nuggets and broccoli were deep-fried for four cycles at 180°C. Food fried in extra-virgin olive oil had fewer polar compounds (breakdown products) than food fried in canola or grapeseed oil.

Seed oils and ultra-processed food

Most seed oil in Western diets doesn’t come from a bottle used at home. It comes inside ultra-processed foods: packaged snacks, fast food, frozen meals, commercial baked goods, dressings and sauces.

That makes seed oils hard to study on their own. They travel with refined carbohydrates, added sugar and additives, and with a lack of fiber, polyphenols and micronutrients. Large cohorts consistently link ultra-processed food to cognitive decline and dementia risk, though the evidence is observational (Gomes Gonçalves 2023; Li 2022). It is impossible to separate the role of seed oils from the rest of the package.

Say someone cuts seed oils by dropping fast food and packaged snacks. Any improvement may owe as much to less refined carbohydrate, fewer additives and fewer calories as to less linoleic acid. Cutting seed oils through a whole-food diet may well be beneficial, but the benefit may not come mainly from the oils.

What the reviews and trials conclude

Heart health

The best trial evidence on vegetable oils is about the heart, not the brain. A 2020 Cochrane review (Hooper 2020) pooled 15 randomized trials with about 59,000 people. Cutting saturated fat for at least two years reduced combined cardiovascular events by about 21 percent. Replacing saturated fat with polyunsaturated fat or with carbohydrate both appeared useful. There was little or no effect on cardiovascular deaths or deaths from any cause.

Not all trials agree. Ramsden and colleagues (2016) analyzed recovered data from the Minnesota Coronary Experiment (1968–73), a randomized trial in 9,423 residents of a nursing home and six state mental hospitals. Replacing saturated fat with corn oil lowered cholesterol by about 14 percent but did not reduce deaths. In their meta-analysis of five similar trials, these diets showed no benefit on coronary or all-cause deaths. Neither the Cochrane review nor the Minnesota data measured cognition.

This fits the American Heart Association’s view that polyunsaturated vegetable oils are preferable to saturated fat for the heart. Still, the benefit may come partly or largely from less saturated fat rather than from linoleic acid itself. For monounsaturated fat, the main fat in olive oil, Hooper found too little trial data to judge the effect of replacing saturated fat.

Brain health

For the brain, the evidence is much thinner. No large randomized trial has tested seed oil intake, or avoidance, on cognitive decline or dementia. What exists is:

  • Observational studies: diets high in omega-6 and low in omega-3 are linked to worse cognition, but these are confounded by ultra-processed food.
  • Lab and animal studies: arachidonic acid products and oxidized fats can promote brain inflammation in cell cultures and animals.
  • Indirect trial evidence: in a PREDIMED sub-study, Mediterranean diets with olive oil or nuts led to better cognitive scores than a low-fat diet. This compared whole diets, not specific oils.

A 2017 systematic review of observational studies (Solfrizzi 2017) found that saturated fat intake was linked to worse cognitive function. Higher intake of fish, monounsaturated fat and polyunsaturated fat (especially omega-3) was linked to less cognitive decline. Its summary does not report separate findings for omega-6 fats or seed oils.

In short, the direct evidence that seed oils are uniquely harmful to the brain is not strong, and the evidence that they help it is also weak. The evidence for diets rich in olive oil and fish is more consistent, though mostly observational.

Olive oil as the benchmark: the PREDIMED trial

The PREDIMED trial assigned over 7,400 people at high cardiovascular risk to one of three diets. These were a Mediterranean diet with extra-virgin olive oil (about one liter per week), a Mediterranean diet with mixed nuts, or a low-fat control diet. The cognitive sub-study (Valls-Pedret 2015) included 447 people in Barcelona, 334 of whom were retested after a median of 4.1 years.

The olive oil group did better than the control group on combined scores for attention and executive function and for overall cognition. The nuts group did better on memory.

This doesn’t prove that seed oils are harmful. It sets a benchmark: a diet rich in extra-virgin olive oil led to better cognitive scores than a low-fat diet in a randomized trial. No comparable trial has tested a seed oil. When choosing your main fat, the evidence favors olive oil.

What to do

  1. Use extra-virgin olive oil as your main cooking and finishing fat. It was the fat used in PREDIMED, holds up well at normal cooking heat, and contains polyphenols. Our Mediterranean brain salad is an easy place to start.
  2. Don’t worry about moderate seed oil use. A meal cooked in canola oil or a dressing made with sunflower oil is not a meaningful threat to your brain.
  3. Avoid food fried in reused oil. The oxidation products of reheated seed oils are a plausible concern, based mostly on lab and animal studies. Eat less commercially deep-fried food, especially from places that reuse frying oil heavily.
  4. Eat more omega-3. The best way to improve your omega-6 to omega-3 balance is to eat more EPA and DHA from fatty fish (two or more servings a week), such as salmon. If you consider a fish oil supplement, note that it can increase bleeding risk with anticoagulant or antiplatelet drugs. Doses above about 1 g a day have been linked to a higher risk of atrial fibrillation in trials. Ask a doctor or pharmacist first.
  5. Eat less ultra-processed food. Cooking more meals from whole ingredients cuts seed oils, refined carbohydrates and additives at the same time. This is where the evidence, though observational, points most clearly.
  6. For high-heat cooking, choose stable oils. High-oleic sunflower oil resists oxidation better than traditional sunflower or corn oil. For most home cooking, extra-virgin olive oil or avocado oil are better choices.

For the brain, the evidence against excess sugar, ultra-processed food and low omega-3 intake is more consistent than the evidence against seed oils themselves. Deal with those first.

Frequently asked questions

Are seed oils toxic?

No. Seed oils eaten in moderate amounts and cooked at reasonable temperatures are not toxic. The concerns are about chronic overconsumption, oxidation during repeated high-heat cooking, and their role as a marker of ultra-processed food. However, the oxidation products formed when high-PUFA oils are heated for a long time are toxic to cells at levels that can be reached in degraded frying oil.

Is canola oil healthier than other seed oils?

Canola oil has less omega-6 linoleic acid than soybean or corn oil (about 20 percent vs. 50-60 percent). It has more monounsaturated oleic acid (about 60 percent) and some plant omega-3. It is a reasonable cooking oil when extra-virgin olive oil isn’t practical, but it lacks olive oil’s polyphenols.

Should I switch to butter or coconut oil?

The evidence doesn’t support this for brain health. In a meta-analysis of 16 trials, coconut oil raised LDL cholesterol by about 10 mg/dL compared with non-tropical vegetable oils, while also raising HDL cholesterol (Neelakantan 2020). Butter is also high in saturated fat. No trial has compared these fats on cognition.

What about high-oleic seed oils?

High-oleic sunflower and soybean oils have been developed to contain mostly monounsaturated oleic acid instead of linoleic acid. This makes them more stable during cooking and lowers the omega-6 load. They are an improvement over traditional seed oils for high-heat cooking but still lack the polyphenols of extra-virgin olive oil.

Sources

  • Blasbalg, T. L., Hibbeln, J. R., Ramsden, C. E., Majchrzak, S. F., & Rawlings, R. R. (2011). Changes in consumption of omega-3 and omega-6 fatty acids in the United States during the 20th century. American Journal of Clinical Nutrition, 93(5), 950-962. doi:10.3945/ajcn.110.006643
  • Brenna, J. T., Salem, N., Jr., Sinclair, A. J., & Cunnane, S. C. (2009). alpha-Linolenic acid supplementation and conversion to n-3 long-chain polyunsaturated fatty acids in humans. Prostaglandins, Leukotrienes and Essential Fatty Acids, 80(2-3), 85-91. doi:10.1016/j.plefa.2009.01.004
  • Simopoulos, A. P. (2002). The importance of the ratio of omega-6/omega-3 essential fatty acids. Biomedicine & Pharmacotherapy, 56(8), 365-379. doi:10.1016/s0753-3322(02)00253-6
  • Johnson, G. H., & Fritsche, K. (2012). 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, 112(7), 1029-1041. doi:10.1016/j.jand.2012.03.029
  • Innes, J. K., & Calder, P. C. (2018). Omega-6 fatty acids and inflammation. Prostaglandins, Leukotrienes and Essential Fatty Acids, 132, 41-48. doi:10.1016/j.plefa.2018.03.004
  • Markesbery, W. R., & Lovell, M. A. (1998). Four-hydroxynonenal, a product of lipid peroxidation, is increased in the brain in Alzheimer’s disease. Neurobiology of Aging, 19(1), 33-36. doi:10.1016/s0197-4580(98)00009-8
  • Ganesan, K., Sukalingam, K., & Xu, B. (2018). Impact of consumption and cooking manners of vegetable oils on cardiovascular diseases: a critical review. Trends in Food Science & Technology, 71, 132-154. doi:10.1016/j.tifs.2017.11.003
  • De Alzaa, F., Guillaume, C., & Ravetti, L. (2021). Evaluation of chemical and nutritional changes in chips, chicken nuggets, and broccoli after deep-frying with extra virgin olive oil, canola, and grapeseed oils. Journal of Food Quality, 2021, 7319013. doi:10.1155/2021/7319013
  • Hooper, L., Martin, N., Jimoh, O. F., Kirk, C., Foster, E., & Abdelhamid, A. S. (2020). Reduction in saturated fat intake for cardiovascular disease. Cochrane Database of Systematic Reviews, 5, CD011737. doi:10.1002/14651858.CD011737.pub2
  • Ramsden, C. E., Zamora, D., Majchrzak-Hong, S., Faurot, K. R., Broste, S. K., Frantz, R. P., et al. (2016). Re-evaluation of the traditional diet-heart hypothesis: analysis of recovered data from Minnesota Coronary Experiment (1968-73). BMJ, 353, i1246. doi:10.1136/bmj.i1246
  • Valls-Pedret, C., Sala-Vila, A., Serra-Mir, M., Corella, D., de la Torre, R., Martinez-Gonzalez, M. A., et al. (2015). Mediterranean diet and age-related cognitive decline: a randomized clinical trial. JAMA Internal Medicine, 175(7), 1094-1103. doi:10.1001/jamainternmed.2015.1668
  • Gomes Gonçalves, N., et al. (2023). Association between consumption of ultraprocessed foods and cognitive decline. JAMA Neurology, 80(2), 142-150. doi:10.1001/jamaneurol.2022.4397
  • Li, H., et al. (2022). Association of ultraprocessed food consumption with risk of dementia: a prospective cohort study. Neurology, 99(10), e1056-e1066. doi:10.1212/WNL.0000000000200871
  • Solfrizzi, V., et al. (2017). Relationships of dietary patterns, foods, and micro- and macronutrients with Alzheimer’s disease and late-life cognitive disorders: a systematic review. Journal of Alzheimer’s Disease, 59(3), 815-849. doi:10.3233/JAD-170248
  • Heneka, M. T., et al. (2015). Neuroinflammation in Alzheimer’s disease. The Lancet Neurology, 14(4), 388-405. doi:10.1016/S1474-4422(15)70016-5
  • Fritsche, K. L. (2008). Too much linoleic acid promotes inflammation — doesn’t it? Prostaglandins, Leukotrienes and Essential Fatty Acids, 79(3-5), 173-175. doi:10.1016/j.plefa.2008.09.019
  • Neelakantan, N., Seah, J. Y. H., & van Dam, R. M. (2020). The effect of coconut oil consumption on cardiovascular risk factors: a systematic review and meta-analysis of clinical trials. Circulation, 141(10), 803-814. doi:10.1161/CIRCULATIONAHA.119.043052