Seed Oils: A Fair and Evidence-Based Assessment

This is the single most consequential dietary change of the last century, and it happened with almost no informed consent from the people doing the eating.

What seed oils are

Seed oils are extracted from seeds: sunflower, soya, canola, rapeseed, safflower, cotton. They're high in polyunsaturated fats, low in saturated fat, and cheap to produce at scale. For the last fifty years, they've been marketed as the healthier alternative to butter and lard.

The argument was simple: saturated fat raises cholesterol, seed oils don't, therefore seed oils are better. But nutrition science doesn't work that way. A single nutrient doesn't tell the whole story.

Industrial seed oils are not food in any ancestral sense. Their composition, the way they're extracted, and how they behave under heat all matter, and all point in the same direction. Linoleic acid accumulates in tissue. It oxidises. It takes years to clear, with a half-life in adipose tissue measured in years rather than days.

The case that seed oils are healthier than traditional fats was never just oversimplified. It was wrong.

The omega-6 to omega-3 imbalance

Both omega-6 and omega-3 polyunsaturated fats are essential. The omega-6 to omega-3 ratio in modern Western diets is estimated at 15–20:1, far higher than the roughly 1:1 to 4:1 reconstructed for ancestral diets.¹

Seed oils are omega-6 dominant. Sunflower oil is about 65% linoleic acid (omega-6). Soya oil is about 50%. Meanwhile, foods rich in omega-3 (fish, grass-fed beef, pastured eggs, flax seeds) are either scarce in modern diets or actively replaced by processed alternatives.

The mechanistic concern is that linoleic acid is metabolised to arachidonic acid, which is the precursor for pro-inflammatory eicosanoids; advocates of the "oxidised linoleic acid hypothesis" argue this contributes to chronic disease, although mainstream randomised trials of inflammatory markers in healthy adults are mixed.²

Inflammation isn't the enemy. It's necessary for healing. But chronic inflammation, running silently in the background, damages your arteries, ages your skin, inflames your joints, and disrupts your hormones.

Seed oils didn't cause this alone. But replacing traditional fats with seed oils and removing fish and organ meat from the diet created a metabolic landscape that favours inflammation. The growing evidence on this is compelling and increasingly difficult to dismiss.

Your body needs omega-6, but the ratio to omega-3 is what determines whether it becomes inflammatory.

Oxidation and high-heat cooking

Polyunsaturated fats contain multiple carbon–carbon double bonds, which makes them more susceptible to oxidation than saturated or monounsaturated fats. This is established lipid chemistry.³

When you heat seed oil to high temperatures, those double bonds break. The molecules rearrange. New compounds form, including oxidised linoleic acid metabolites and trans fats. Your body doesn't recognise these as food. Your immune system sees them as foreign.

This matters because seed oils are often used for deep frying, roasting, and cooking at temperatures that maximise oxidation. The very uses the food industry champions are the ones that create the most damage. Industrial restaurants and food manufacturers rely on seed oils precisely because they're cheap and functional at high heat. But functional doesn't mean healthy.

Saturated fats like butter and coconut oil have only single bonds. They're stable at high heat. Your body recognises them. They don't oxidise easily. This isn't a coincidence that ancestral cultures used these fats for cooking. It's chemistry working in their favour.

Industrial processing concerns

Modern seed oils aren't simply pressed from seeds like olive oil. They're extracted using industrial solvents (often hexane), bleached, and deodorised at high temperatures to remove the smell of rancidity. This process itself creates oxidised compounds.

When you buy a bottle of sunflower oil, you're buying a product that's already been oxidised once during production, then oxidises again every time you heat it. The cumulative damage is non-trivial.

Olive oil is cold-pressed. Coconut oil is mechanical-pressed. Butter is churned. Seed oils are chemically extracted using industrial solvents. The difference in processing affects the final product's stability and how your body processes it. Residual solvent traces may be removed, but the oxidative damage from processing is permanent.

The food industry argues this process is safe because any solvent residues are removed. Maybe. But that doesn't make oxidised compounds disappear. Once they're there, they're there.

The production method matters as much as the source material.

What the research shows

The evidence on seed oils isn't unanimous, and honest science requires acknowledging that. Some studies show neutral effects on heart health. Others show increased inflammation markers. Some show both, depending on dosage and population.

What's become clearer over the last decade is that the original case for seed oils was oversimplified. The argument "cholesterol goes down, so they must be healthy" ignored oxidation, inflammation, and omega-6 dominance. It also ignored that raising cholesterol isn't actually bad if the cholesterol is protective (HDL) versus harmful (oxidised LDL).

The growing consensus among researchers studying inflammation and metabolic health is cautious. Seed oils probably aren't harmless. Small amounts occasionally are likely fine. But regular consumption as a primary cooking fat, combined with low omega-3 intake, creates an environment where inflammation thrives.

The metabolic effects over time

The effects of seed oil consumption aren't immediate or obvious. You don't eat a meal fried in seed oil and feel inflammation. You eat it regularly for weeks and months. Your body gradually adapts. Inflammatory markers rise incrementally. Your triglycerides creep up. Your HDL falls. Your waist circumference increases. Energy declines. Sleep suffers.

Many of these changes are attributed to aging or stress. In reality, they're partially a response to cumulative oxidised fat consumption and chronic omega-6 dominance. This isn't saying seed oils are solely responsible. It's saying they're a significant contributing factor in a system that's already pushing toward inflammation.

This isn't the same as saying seed oils cause disease. It's saying they're a contributing factor in a system that's already pushing toward chronic inflammation.

How seed oil dominance developed

The shift to seed oils wasn't driven by superior nutrition. It was driven by economics. In the 1950s, industrial agriculture produced enormous quantities of soya and sunflower seeds as byproducts of animal feed production. Oil extraction from these seeds was cheap and scalable.

The food industry needed to sell this oil profitably. So marketing campaigns began. Butter and lard were reframed as unhealthy. Seed oils were positioned as modern, scientific, heart-healthy. Universities received funding to research seed oils. Public health organisations recommended them. Doctors prescribed low-fat diets, which meant seed oil-based diets.

This wasn't based on solid evidence. It was based on economic incentive aligned with public health messaging. The irony is that the messaging succeeded so completely that we still believe it decades later, despite accumulating evidence suggesting it was oversimplified.

Your grandparents didn't eat seed oils. They ate butter, lard, and occasionally olive oil. They had lower rates of heart disease, lower rates of inflammation, healthier metabolic profiles. Not because they were younger or less stressed. Because their fat sources didn't create the inflammatory environment that seed oil dominance creates.

A practical, honest approach

Get them out completely where you can. Linoleic acid stored in tissue takes roughly two to four years to substantially clear, and every meal cooked in seed oil is another meal that delays the detox. Cook in butter, ghee, beef tallow, or coconut oil. These are stable at heat, your body recognises them, and they have been on human plates for as long as cooking has existed.

For cold uses like drizzling on salads, extra virgin olive oil is fine. The point is not to be perfect. The point is to stop volunteering for the inflammatory load every time you eat.

Restaurant food is the harder problem, because almost every kitchen now defaults to seed oils. You will not always control this. Reduce the frequency where you can, ask when it makes sense to ask, and accept that the home kitchen is where the real change happens.

The goal isn't purity. It's getting industrial seed oils out of the home kitchen and back to fats your body actually recognises as food.

The bottom line

Industrial seed oils are not the heart-healthy upgrade they were sold as. They are an industrial fat humans had no significant exposure to before the twentieth century, they oxidise inside you and into the tissue you live with for years, and they sit underneath a measurable share of the chronic inflammatory disease the modern world treats as normal ageing.

Swap them out at home for butter, ghee, beef tallow, or coconut oil. Reduce restaurant food where you can. Move toward fats your ancestors used for thousands of years. The body has no ancestral experience metabolising these oils in the quantities the modern food supply delivers, and there is no good reason to keep volunteering for the experiment.

References

1. 1. Simopoulos AP. The importance of the ratio of omega-6/omega-3 essential fatty acids. Biomed Pharmacother. 2002. https://pubmed.ncbi.nlm.nih.gov/12442909/
2. 2. DiNicolantonio JJ, O'Keefe JH. Omega-6 vegetable oils as a driver of coronary heart disease: the oxidized linoleic acid hypothesis. Open Heart. 2018. https://pmc.ncbi.nlm.nih.gov/articles/PMC6196963/
3. 3. National Institutes of Health, Office of Dietary Supplements. Omega-3 Fatty Acids — Health Professional Fact Sheet. https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/