Seed oils: are sunflower, rapeseed, and soybean oils harmful?
For the past couple of years, sunflower, rapeseed, and soybean oils have been labeled as sources of inflammation and the primary cause of everything. The mechanism cited does exist. However, human data suggest the opposite, and this is a case where the difference between a mechanism and an outcome is everything.
Which oils are we talking about
"Seed oils" typically refer to eight types: sunflower, rapeseed (canola), soybean, corn, cottonseed, safflower, rice bran, and grapeseed oil. They share one common feature: a high proportion of linoleic acid, the primary omega-6 fatty acid in the diet.
Olive and avocado oils are usually not included in this list: they are derived from the fruit pulp and consist primarily of monounsaturated oleic acid.
What is the accusation
The chain of reasoning seems logical:
- Linoleic acid is converted into arachidonic acid in the body.
- Arachidonic acid produces eicosanoids—signaling molecules involved in inflammation.
- Therefore, the more omega-6 in the diet, the more inflammation occurs.
- Therefore, seed oils are the cause of diseases rooted in inflammation.
The first two points are correct; they are standard biochemistry. The third and fourth are assumptions, and they must be verified by measurement, not just reasoning.
What the measurements showed
The key study here is a pooled analysis of 30 cohort studies published in Circulation in 2019. It was designed to address the main problem with such topics: linoleic acid was not self-reported but measured in blood and tissues as a biomarker.
Sample size: 68,659 people, 15,198 cases of cardiovascular events. Results for the top of the linoleic acid distribution versus the bottom:
| Outcome | Hazard ratio | How to read |
|---|---|---|
| Cardiovascular disease overall | 0.93 | 7% lower risk |
| Death from cardiovascular causes | 0.78 | 22% lower |
| Ischemic stroke | 0.88 | 12% lower |
The direction is the opposite of what the mechanism would suggest. Study in Circulation.
Why a biomarker is more reliable than a questionnaire: a person does not remember how much sunflower oil they consumed over a year, and blood tests do not depend on memory. This also addresses the counter-argument that people with a "healthy" diet are simply healthier overall: blood levels reflect intake, not intentions.
What about inflammation
The premise itself was also tested separately. Studies that measured inflammatory markers alongside levels of linoleic and arachidonic acids find a correlation in the opposite direction to what the mechanism predicts: higher levels are associated with lower markers, not higher.
The explanation is simple and long-known: the conversion of linoleic acid to arachidonic acid in humans is inefficient and tightly regulated. Increasing linoleic acid in the diet barely raises arachidonic acid in tissues. In other words, the step between the first and second points of the accusation does not work in practice.
Where the debate is still open
To be fair, there is no consensus. There are systematic reviews that find a positive correlation between linoleic acid content in adipose tissue and a number of conditions. Observational data generally struggle to distinguish between "oil" and the "food that the oil came in."
And this seems to be the main point. In a typical diet, seed oils are not consumed by the spoonful from a bottle, but rather in chips, pastries, sauces, and deep-fried foods. Such food has calories, salt, sugar, and everything else covered by the NOVA classification. Attributing its effects to a single component is the same mistake that has, at various times, made eggs or fat as a whole the culprit.
How such studies are generally structured and why it is so easy to cite them in both directions—a separate analysis.
What is actually worth paying attention to
Overheating. Polyunsaturated acids oxidize faster than monounsaturated ones when heated, and this is a real, measurable phenomenon. It is about temperature and reuse, not the mere fact of sunflower oil's existence: an analysis in the article on smoke point and oxidative stability.
Quantity. Any oil provides 9 kcal per gram and about 900 per 100 ml. A tablespoon in a salad is unnoticeable in taste but noticeable in a food diary.
Ratio, not prohibition. A typical diet is high in omega-6 and low in omega-3, and a reasonable step is to add the latter rather than discard the former. There is material on the omega-3 index regarding the measurable side of this.
In short
Data on humans do not confirm that seed oils increase the risk of cardiovascular disease; in the largest analysis, the correlation is the opposite. There is no reason to change your diet because of this debate in either direction. What matters is the same as before: how much oil, at what temperature, and as part of what food.
For more on how types of fats differ from each other in essence, see which fats are healthy and which are not. For palm oil, which gets singled out separately, see here.
Frequently asked questions
- Is it true that seed oils cause inflammation?
- The mechanism for converting linoleic acid into arachidonic acid exists, but in humans, it is weak and tightly regulated. Studies that have measured both levels and markers of inflammation simultaneously find an inverse relationship.
- What did the largest study show?
- A pooled analysis of 30 cohorts (68,659 people, 15,198 events) in Circulation found that higher levels of linoleic acid were associated with a 7% lower risk of cardiovascular disease, a 22% lower risk of cardiovascular mortality, and a 12% lower risk of ischemic stroke.
- Should I stop using sunflower oil?
- The data does not support such a step. What matters is the quantity, the cooking temperature, and the type of food the oil is consumed with: in a typical diet, it comes along with baked goods and deep-fried foods, which have their own effects.
Read next
- Which fats are healthy and which are not — Dividing fats into "healthy" and "unhealthy" is convenient for a headline but almost useless at the grocery shelf: a single product usually contains all types at once, and the difference between them is not about general health benefits, but about specific effects.
- Cooking oil: smoke point and what matters more — The smoke point is usually cited as the main criterion for choosing a cooking oil.
- Palm oil: is it actually harmful? — Palm oil has become the universal explanation for everything bad in food.
This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.
Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.