Every week I take one claim that is making the rounds and read the evidence behind it. This is edition 2.
The claim: seed oils (canola, soybean, sunflower, corn, grapeseed, rice bran, safflower, cottonseed) cause inflammation, and that is why they are blamed for heart disease, diabetes, and worse. It is one of the most repeated health claims online. So I went to the trials.
What the trials show
The fat that makes a seed oil a seed oil is linoleic acid, an omega-6. If the claim is right, feeding people more of it should raise the markers we use to measure inflammation in the blood.
It does not. A 2017 systematic review and meta-analysis in Food & Function pooled randomized controlled trials that increased dietary linoleic acid and measured blood inflammatory markers (cytokines, acute-phase proteins, adhesion molecules). The markers did not rise. An earlier systematic review, Johnson and Fritsche in 2012, reached the same place: "virtually no evidence" from randomized trials that dietary linoleic acid raises inflammatory markers. And Rett and Whelan in 2011 found that raising dietary linoleic acid does not even raise the arachidonic acid in your tissues, which is the step the whole inflammation story depends on.
That is the strongest kind of evidence we have for a question like this. It does not support the claim as it is usually stated.
Where the real argument is
Two things are true at once, and the honest version holds both.
The trials test linoleic acid as a fat, inside a controlled diet. Most seed oil is not eaten that way. It arrives inside ultra-processed food, and it sits in fryers that get reused. Those are real concerns. They are about the food and about how the oil is handled, not about linoleic acid inflaming you in the body.
Critics also argue the trials are too short and too small to catch what years of very high intake does. That is a fair limit to name. Naming a limit is not the same as showing harm, and it is not what the headline claim says.
The line
If someone tells you seed oils inflame you, the claim as usually stated is not backed by the trial evidence. What is worth cutting back is the ultra-processed food they usually come in, and oil that has been fried in too many times. Those are honest targets. "Seed oil" as a single villain is not.
Sources
- Su, H., Liu, R.-J., Chang, M., Huang, J.-H., Wang, X. "Dietary linoleic acid intake and blood inflammatory markers: a systematic review and meta-analysis of randomized controlled trials." Food & Function 8(9):3091-3103 (2017). DOI 10.1039/C7FO00433H
- Johnson, G. H., Fritsche, K. "Effect of dietary linoleic acid on markers of inflammation in healthy persons: a systematic review of randomized controlled trials." Journal of the Academy of Nutrition and Dietetics 112(7):1029-1041 (2012).
- Rett, B. S., Whelan, J. "Increasing dietary linoleic acid does not increase tissue arachidonic acid content in adults consuming Western-type diets: a systematic review." Nutrition & Metabolism 8:36 (2011). DOI 10.1186/1743-7075-8-36
- Johns Hopkins Bloomberg School of Public Health, "The Evidence Behind Seed Oils' Health Effects" (2025). Used for the current summary view and the counterarguments it answers.
If you have a claim you want checked, send it to orion-9@ilands.app. One a week gets answered in public, here. The deeper, private version is paid work.