Claim by claim

Are seed oils bad for you?

By GyroscopeUpdated November 14, 202612 sources

No. Swapped in for butter, seed oils such as canola, soybean and sunflower lower LDL, and in trials they didn’t raise inflammation. The fried food and packaged snacks they often come in are the better thing to eat less of.

Seed oils have become the villain of the moment. The claims are that they cause inflammation, that they’re toxic when heated, that they’re industrial, and that the omega-6 in them crowds out omega-3. Each claim starts from something true, which is why they’re so sticky, and then goes further than the evidence in people.

What’s in a seed oil?

“Seed oil” usually means oils pressed or extracted from seeds: canola, soybean, corn, sunflower, safflower, cottonseed, grapeseed and others. Most are high in polyunsaturated fat, mainly linoleic acid, an omega-6. Your body can’t make linoleic acid, and the US Adequate Intake is 12 g a day for women and 17 g for men aged 19 to 50.

Here’s a tablespoon of three seed oils next to olive oil and butter, from the USDA’s tables.

• Saturated
• Monounsaturated
• Polyunsaturated

Canola is a seed oil, and it’s mostly monounsaturated, like olive oil, with only 1.0 g of saturated fat a tablespoon.

“They cause inflammation”

This is the big one. The idea is that linoleic acid turns into arachidonic acid, which the body uses to make inflammatory signals, so more linoleic acid should mean more inflammation.

People have tested it directly. In 2012, Johnson and Fritsche reviewed 15 randomized trials in healthy people that raised linoleic acid in the diet. None found a significant rise in inflammation markers, including CRP, fibrinogen and cytokines. They concluded there was “virtually no evidence” from such trials that adding linoleic acid raises inflammatory markers.

The long studies point the same way. In 2019 the FORCE consortium pooled 30 cohort studies from 13 countries, with 68,659 people and 15,198 heart events. Instead of asking people what they ate, they measured linoleic acid in blood or fat tissue. People near the top of the range had about 7% less heart disease than people near the bottom, 22% fewer deaths from it and 12% fewer ischemic strokes. Arachidonic acid, the supposed culprit, wasn’t linked to more heart disease either.

The pathway is real chemistry. But in people, eating more linoleic acid didn’t add up to more inflammation.

“They raise heart disease”

The biggest study of oils and death followed 221,054 US adults in three Harvard cohorts for up to 33 years, with 50,932 deaths. People who used the most plant oil had 16% fewer deaths than those who used the least. Each extra 5 g a day of canola oil went with 15% fewer deaths, soybean oil 6% and olive oil 8%. Swapping 10 g of butter a day for plant oils went with 17% fewer deaths. That’s an observational study, so the people who ate more plant oil may have differed in other ways too, though the researchers adjusted for many of them.

The trials agree on the swap. The 2020 Cochrane review of 15 trials found that eating less saturated fat cut heart events by 17%, and that replacing it with polyunsaturated fat appeared to be a useful strategy. The 2026 US cholesterol guideline recommends “replacing saturated and trans fats with dietary monounsaturated and polyunsaturated fats” to lower LDL.

The two old trials that point the other way

Skeptics of seed oils have two strong studies, and they deserve the whole story.

The Minnesota data has its own story. The trial ended in 1973 and its main results weren’t published until 1989. In 2016 a team led by Christopher Ramsden at the US National Institutes of Health recovered the raw data from two old 9-track magnetic tapes and a large collection of paper files, with help from Robert Frantz, and analyzed it the way the original investigators had planned.

Here’s how we weigh these. They’re real randomized trials, and they’re why the outcome evidence on seed oils is mixed. They’re also more than 50 years old, one was run in hospitals, both used the margarines of the 1960s, and the larger set of 15 trials in the Cochrane review still found fewer heart events when saturated fat came down. We’re confident that swapping butter for plant oils lowers LDL. We’re less sure how much each kind of plant oil adds beyond that, and so are the 2026 Dietary Guidelines, which say “more high-quality research is needed to determine which types of dietary fats best support long-term health.”

“They oxidize when you heat them”

This one is true in a lab. Polyunsaturated fats have more kinks, and the kinks are where oxygen attacks. In one 2018 test, an olive oil company’s lab heated ten supermarket oils at 356 °F (180 °C), the top temperature for deep frying, for 6 hours with no food. After 6 hours the polar compounds, the breakdown products that build up in frying oil, had reached 21.8% in sunflower oil and 17.3% in canola, against 10.5% in extra virgin olive oil and 9.7% in coconut oil.

The practical advice is about long, hot cooking. Use extra virgin olive oil when you’ll hold oil hot for a long time, and don’t reuse frying oil for days. In that test every oil broke down more the longer it was heated. The authors point out that no food was cooked, and that water from real food changes the chemistry. Oils in the kitchen has the full test.

“They’re industrial and ultra-processed”

Refined seed oils are factory products, and some people dislike them for that alone. realfood.gov, the federal site for the 2026 guidelines, lists “industrial oils” among the things real food is made without. That’s the site’s own framing. The 2026 guidelines document itself names olive oil first, lists butter and beef tallow as other options, and doesn’t mention seed oils at all. The 2020 to 2025 edition named canola, corn, olive, peanut, safflower, soybean and sunflower oils as the oils to cook with “rather than butter, shortening, or coconut or palm oils”.

Processing is a real question about food, and processing.guide covers it. For fats, the trials measured what the oil does in your body, and the swap from butter to plant oil held up. The fried and packaged foods made with seed oils are a separate question, and eating less of them helps for reasons that go beyond the oil.

“Omega-6 crowds out your omega-3”

The argument is that omega-6 and omega-3 compete for the same enzymes, so a high ratio of omega-6 to omega-3 is harmful. The omega-3s that do the work in the heart studies are EPA and DHA, and those come from fish. Eating fish raises them directly, without cutting anything. And in the 30 cohort studies, people with more omega-6 in their blood had less heart disease. The omega-6 ratio myth goes through it.

What we don’t know

• Which unsaturated fat is best. The Cochrane review had very little data on monounsaturated fat as the replacement.
• Whether very high seed oil intakes behave differently. Most of the evidence covers ordinary diets.
• How much each person’s LDL moves. People respond differently, so your own test after a swap is the answer for you. How to check sets it up.