Seed Oils: A Fair and Evidence-Based Assessment

Every kitchen cupboard in Britain probably has a bottle of vegetable oil. It sits so quietly you forget it's there. But there's growing evidence that this staple might not be as neutral as we've been told.

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 15:1 to 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.