Seed Oils: Are We Blaming the Wrong Thing?

Most of the argument centres on omega-6 fat.

Many seed oils contain substantial amounts of an omega-6 polyunsaturated fatty acid called linoleic acid.

Linoleic acid is not a toxin. It is an essential fatty acid, meaning the human body cannot make enough of it and it must come from the diet. Irish healthy-eating guidance recognises linoleic acid as one of the essential fatty acids.

The controversy comes from what happens after linoleic acid enters the body. Some of it can ultimately contribute to pathways involving arachidonic acid and signalling molecules associated with inflammation.

That biochemical pathway is real. The leap is assuming that eating more linoleic acid therefore causes chronic inflammation in humans. That question has to be tested in people.

Parts of the pathway can be. But human metabolism is not a single-arrow diagram.

The body tightly regulates fatty-acid metabolism, and consuming more linoleic acid does not simply translate into a proportional increase in chronic systemic inflammation.

This is a recurring problem in online nutrition arguments: a plausible biochemical mechanism is treated as if it already proves a real-world health outcome.

Mechanisms matter. Human outcomes matter more.

This is one of the most important pieces of context missing from seed-oil debates. Foods do not exist in a nutritional vacuum. If you remove one fat, something usually replaces it.

WHO's current guidance recommends replacing saturated fat with polyunsaturated fats from plant sources, monounsaturated fats from plant sources, or suitable high-fibre carbohydrate foods rather than treating all fats as equal.

The evidence informing WHO's recommendation found that replacing saturated fat with polyunsaturated fat reduces LDL cholesterol, with WHO describing the recommendation to replace saturated fat with plant-source PUFA as strong. Its evidence review also reports that replacing 5% of energy from saturated fat with linoleic acid was associated with lower coronary heart disease risk in large cohort analyses.

So asking “is sunflower oil healthy?” is often less informative than asking “what is it replacing?” Replacing some butter or lard with an unsaturated plant oil is a different nutritional comparison from adding more oil to an already energy-dense diet.

Again, context matters.

WHO recommends replacing saturated fats with unsaturated plant fats in part because of cardiovascular evidence. The American Heart Association likewise supports omega-6 polyunsaturated fats and rejects the blanket idea that seed oils should be avoided because they are inherently inflammatory.

That does not turn a packet of deep-fried crisps into a heart-health food because sunflower oil appears in the ingredients. It means the oil itself should not automatically receive the blame for everything about the food containing it.

This is where the criticism has more legitimate chemistry behind it.

Unsaturated fats can oxidise. Heat accelerates oxidation. During prolonged or repeated high-temperature frying, oils can undergo oxidation, polymerisation, loss of some unsaturated fatty acids and formation of various degradation products.

A recent review of vegetable oils used for frying describes thermoxidation and polymerisation as major degradation pathways and notes that oil composition influences how an oil behaves during frying. A 2026 laboratory study also found substantial compositional changes and oxidation products when seed oils were subjected to prolonged heating at very high temperatures.

That is real. But there is an important distinction between using a fresh oil once to cook dinner and repeatedly heating the same oil at frying temperatures for long periods. Those should not be presented as the same exposure.

Yes. Repeated frying is a different question from ordinary cooking.

Research shows that repeated or prolonged high-temperature heating changes vegetable-oil composition and increases oxidation and degradation.

This is one reason a takeaway fryer that has been running oil through repeated heating cycles should not be used as evidence for what happens when you add a teaspoon of rapeseed oil to a pan at home.

Many commercial oils are refined. Typical industrial production can involve steps such as crushing, extraction, refining and deodorisation. Some oils are instead cold-pressed or mechanically extracted.

Processing is worth understanding. But processed does not automatically mean harmful.

The relevant questions are what the process changes, what residues remain and what the final evidence says about exposure.

Do not confuse refined seed oil with the old process of partial hydrogenation.

Industrially produced trans fats were historically associated heavily with partially hydrogenated oils. They are a separate issue.

Very high-temperature processing and frying can cause some fatty-acid isomerisation, but that does not mean a modern bottle of sunflower or rapeseed oil is equivalent to an old partially hydrogenated shortening.

WHO strongly recommends avoiding industrially produced trans fat and supports replacing saturated and trans fat with healthier unsaturated fats.

This is a useful Sláinte example.

If a packet of crisps contains potatoes, sunflower oil, salt, the ingredients list is relatively short. That does not automatically make the crisps nutritionally equivalent to boiled potatoes.

The frying process changes energy density, fat content and water content. Salt also matters.

The useful analysis is therefore not “sunflower oil = bad” but “what has the complete product become?”

How to Read an Ingredients List.

Olive oil has a strong evidence base and can be an excellent choice. That does not mean every other plant oil needs to be banned from the kitchen.

Taste matters. Cooking method matters. Price matters. Availability matters. The food you are cooking matters.

A person using a teaspoon of rapeseed oil to cook vegetables is not creating an obvious health problem that needs to be “fixed” by internet nutrition rules.

Seed oils are exactly the kind of topic where one ingredient can dominate the conversation.

Sláinte should not flag sunflower oil and stop. A useful product explanation asks:

• How much saturated fat does the food contain?
• Is it high in salt?
• Is there useful fibre or protein?
• Is this an everyday cooking ingredient or a deep-fried snack?
• What else is in the formulation?

That gives you context instead of another ingredient blacklist.