Seed Oils: What the Evidence Actually Says

Few nutrition topics generate as much heat as seed oils — sunflower, soybean, rapeseed/canola, corn, safflower. Depending on which corner of the internet you land in, they are either a leading cause of modern disease or entirely fine. Both positions overstate what is known.

The claim

The argument against seed oils runs roughly like this: they are high in linoleic acid, an omega-6 polyunsaturated fat; omega-6 fats can be converted into inflammatory signalling molecules; our intake of them has risen sharply over the last century in step with rates of chronic disease; therefore they drive that disease.

The first three statements are broadly true. The fourth does not follow from them.

What the research shows

• Randomised trials feeding people more linoleic acid do not reliably show a rise in inflammatory markers. This is the load-bearing step in the argument, and it does not hold up well when tested directly.
• Observational studies generally find that people with higher linoleic acid levels in their blood have lower — not higher — rates of cardiovascular disease and type 2 diabetes. That is the opposite of what the theory predicts.
• Replacing saturated fat with polyunsaturated fat lowers LDL cholesterol, and trials of that swap have generally shown reduced cardiovascular events.
• The conversion of linoleic acid into inflammatory compounds in humans is modest and considerably less efficient than the theory assumes.

This is not a settled question in every detail, and honest researchers still argue about the optimal omega-6 to omega-3 ratio. But the strong claim — that seed oils are a primary driver of chronic disease — is not supported by the strongest available evidence.

The genuinely reasonable position

1. Olive oil is still the best default. It has the strongest outcome evidence of any culinary fat, by a distance. If you change one thing, make it this.
2. Seed oils are not poison. Rapeseed oil for high-heat cooking is a perfectly sensible choice, and cheaper than olive.
3. The real issue is what the oil is usually attached to. If most of your seed oil arrives via deep-fried and packaged food, the fix is the food, not a boycott of the bottle.
4. Eat more omega-3s. If the ratio worries you, the productive move is to raise the omega-3 side — oily fish twice a week, walnuts, flax — rather than waging war on the other one.
5. Don't reheat oil repeatedly. Repeatedly reheated frying oil produces degradation products that are worth avoiding, whatever the source.

What to cook with

• Everyday, most things: extra virgin olive oil. Yes, including for frying — its smoke point is higher than the internet claims.
• High-heat, neutral flavour: rapeseed, or a high-oleic sunflower oil.
• Flavour and occasional use: butter, sesame oil, avocado oil.
• Skip: partially hydrogenated anything, which contains trans fats and is banned or restricted in most places for good reason.