The internet is awash with videos and articles warning about the dangers of seed oils — that they're toxic, inflammatory, and terrible for human health.
This article examines where these concerns came from, including worries about heating seed oils and the omega-3 to omega-6 ratio, why the scientific evidence contradicts the popular opinion, and what the research actually shows about reducing heart disease risk.
Table of Contents
- What are seed oils, and where did the concerns come from?
- The omega-6 inflammation debate
- What the evidence actually shows
- What about cooking with seed oils?
- The real problem with seed oils
- A closer look at cholesterol and CRP
- References
What are seed oils, and where did the concerns come from?
Seed oils are extracted from seeds like soybeans, corn, and sunflowers, and they've become a staple in modern cooking as well as many processed foods.
It's important to note that olive oil isn't a seed oil. Unlike seed oils, olive oil comes from the fruit of the olive tree, not the seeds. Olive oil, especially extra virgin olive oil, is mostly a type of fat called monounsaturated fat.
Some seed oils, like canola oil, also have a lot of monounsaturated fats. But in general, most seed oils are higher in polyunsaturated fats, especially omega-6 fats like linoleic acid.
Just like omega-3 fats, omega-6 fats are essential — the human body can't produce them on its own, so they must come from food, including nuts, seeds, meat, and eggs.
But over the years, concerns have grown about whether the massive increase in seed oil consumption is actually harming health. Some claim that seed oils are causing widespread inflammation, leading to weight gain and heart disease. Are these fears justified, or is the truth being twisted?
The fear surrounding seed oils didn't appear overnight. It has a long history, going back more than a hundred years. Seed oils were originally used for industrial purposes — machine lubrication, making candles, even producing soap. It wasn't until the early 1900s that people figured out how to process these oils to make them edible and fit for human consumption.
It sounds counterintuitive: why would oils originally designed for industrial use end up in the food supply? The answer is largely economic — seed oils could be produced cheaply and at scale, making them attractive as a food ingredient once the refining processes were developed.
In the 1960s and 1970s, the conversation around fat and heart health really started to heat up. Many health experts began advising people to cut back on saturated fats — butter, lard, and other animal fats — because studies suggested these fats raised cholesterol levels and therefore increased the risk of heart disease.
People began looking for alternatives, and that's where seed oils came in. Seed oils are high in polyunsaturated fats, which were considered better for heart health than saturated fats. People swapped butter for margarine, which was made from seed oils, and seed oils were increasingly used in processed foods, from salad dressings to chips and cookies.
This massive shift in diet meant people were consuming far more seed oils than ever before. And this raised an important question: could eating so much of these seed oils and omega-6 fatty acids actually be harmful? Especially if those oils aren't manufactured properly and contain impurities like trans-fats or oxidized fats? It's a legitimate question — and one that researchers have studied extensively over the past several decades.
The omega-6 inflammation debate
First, let's address the concerns about the balance between omega-6 and omega-3 fatty acids in the diet.
Omega-3 fatty acids, which are found in foods like fish, flaxseeds, and walnuts, are known for their anti-inflammatory effects. They help reduce inflammation in the body and are important for heart health, brain function, and overall wellbeing. That's why researchers and clinicians often highlight the benefits of eating fatty fish like salmon or supplementing with omega-3.
On the other hand, omega-6 fatty acids like linoleic acid, which seed oils are rich in, can worsen inflammation under certain conditions when tested in the lab.
Here's why: when omega-6 fats are consumed, they can be broken down into molecules that might cause inflammation in the body. This process is part of the normal immune response, but if omega-6 intake is very high while omega-3 intake is low, the concern is that the body could shift toward a more inflammatory state.
Chronic inflammation has been linked to a range of health problems, including heart disease, diabetes, arthritis, and cancer.
The ratio argument goes like this: hunter-gatherer diets are estimated to have contained omega-6 to omega-3 ratios of roughly 4:1, while modern Western diets may run as high as 20:1 or more. This dramatic shift, the theory goes, is a major driver of modern inflammatory disease.
The idea is that the modern diet, full of seed oils and processed foods, has far more omega-6 than omega-3, which throws off the balance between these fats and promotes inflammation.
But does eating more omega-6 really lead to more inflammation, or is this concern overblown? The answer, it turns out, is nuanced.
What the evidence actually shows
The theory that omega-6 fatty acids cause inflammation sounds logical at first if one extrapolates from what happens in a lab when testing single cells in a petri dish. But what matters is what happens in the real world — and a rigorous source of evidence comes from human randomized controlled trials.
For example, one randomized controlled trial of obese individuals fed one group a diet high in omega-6 seed oils, and the other group a diet high in saturated fat, mainly from butter. Both groups ate the same number of calories, and the macronutrient ratios — the amount of protein, carbs, and fat — were the same [1].
The seed oil group, after the 10-week study period, had less liver fat, reduced inflammation, and lower insulin levels compared to the saturated fat group.
This study was notable because it measured the amount of linoleic acid (the omega-6 fatty acid) in the blood, allowing researchers to confirm that the seed oil group were actually following the protocol and consuming more seed oils.
But that's just one study. Cherry-picking data can easily lead research in the wrong direction. It's far better to look at a meta-analysis — where multiple randomized controlled trials are pooled together, combined, and analysed. This means looking at the entirety of the evidence, rather than any single finding.
In a 2017 meta-analysis that combined 30 separate randomized controlled trials, researchers found that eating more linoleic acid — meaning more seed oils — didn't cause any changes in inflammation [2].
The separate trials were all pointing in the same direction, and no publication bias was detected. The authors of this analysis had no conflicts of interest to declare.
Randomized controlled trials are expensive to run and usually don't last a long time. Observational data from populations adds further context: high dietary intakes of linoleic acid are associated with reduced risks of diabetes [3].
High linoleic acid intake is also associated with reduced risks of heart disease — consistent with what the randomized controlled trial results would predict [4].
Again, the authors of these analyses had no conflicts of interest to declare.
Now, some studies comparing the ratio of omega-6 to omega-3 fats show that a higher omega-6 to omega-3 ratio can be linked to worse health outcomes. But — and this is important — that's usually because people aren't eating enough omega-3 fats, not because they're eating too much omega-6 [5].
So what does all this mean? It means that the idea that omega-6 fatty acids from seed oils are causing widespread inflammation in the body doesn't hold up when looking at the totality of the scientific evidence.
Instead, when saturated fats — like those in butter — are replaced with polyunsaturated fats from seed oils, there are real benefits for cardiovascular health. A large Cochrane review pooled 15 studies with over 56,000 participants and found that when people cut down on saturated fats and used polyunsaturated fats instead, there was a 17% drop in cardiovascular disease [6].
There's an important piece of history to address here. In the past, many people switched from butter to margarine, believing it was a healthier choice. The problem — and this is significant — early versions of margarine were made using a process called hydrogenation, which turned liquid oils into solid fats and created trans fats. Trans fats are a type of fat that has since been shown to be extremely harmful, increasing the risk of heart disease much more than saturated fats.
These early margarines were high in trans fats, and for years, people thought they were making a heart-healthy switch when, in reality, they were consuming fats that were even worse for their health.
Thankfully, trans fats have been phased out, and modern versions of margarine are much safer. But it's important to understand how trans fats contributed to the confusion around seed oils and heart health.
What about cooking with seed oils?
One of the major concerns people have about seed oils is whether cooking with them — especially at high temperatures — can make them more harmful. There are claims that heating seed oils creates dangerous compounds like trans fats or oxidized oils, which could then lead to inflammation or other health problems.
When seed oils are exposed to heat, light, or air, they can start to break down. This process is called oxidation, and it's accelerated during cooking.
Why does this happen to seed oils more than other oils? It's because seed oils have a lot of polyunsaturated fats. These fats have a special structure with what scientists call double bonds. These double bonds make the fats less stable, so they can break down more easily. When the oil breaks down, it can create free radicals and oxidized fats, which have the potential to damage cells in the body.
The concern is that when seed oils are heated — for example, for frying — these polyunsaturated fats with their double bonds can break down and form harmful compounds. What does the evidence show?
In 2022, a meta-analysis combining 33 studies found that heating seed oils to temperatures less than 200°C did not create trans-fatty acids. Above that temperature, however, and during prolonged heating, trans-fats were created [7].
The analysis concluded: "This provides further evidence in favour of public health advice that heating oils to very high temperatures and prolonged heating of oils should be avoided."
To explore concerns about oxidation specifically, one study heated sunflower oil for 1 hour at 180°C, and compared it to unheated oil. The study did find a small increase in oxidation, but not enough to increase blood levels of oxidized LDL or affect the inner lining of the blood vessels [8].
Another study took it a step further and heated the same seed oil 20 times, to mimic what may happen in fast food fryers — and that study found an increase in inflammatory markers [9].
The real problem with seed oils
So should people worry about consuming seed oils that have been heated? This brings the discussion to the real problem with seed oils.
Someone consuming seed oils that have been exposed to very high temperatures over a prolonged period of time is likely consuming takeaways, fries, and other processed foods.
So when people report that they've given up seed oils and feel much better, what they've actually done is stopped eating ultra-processed and fast food. That's a genuinely positive dietary change.
But that's entirely different from drizzling fresh seed oils over a home-made salad, cooking at home using seed oils, or replacing butter with modern margarines.
The robust evidence from human randomized controlled trials shows that when people switch from butter to unsaturated fats, there are measurable reductions in inflammation [2], improvements in blood cholesterol, and reduced rates of heart disease [6].
The takeaway: seed oils by themselves aren't the problem. The real issue is eating too many ultra-processed foods and takeaways that contain these repeatedly heated industrial oils. A diet built around whole foods — like fruits, vegetables, lean meats, and whole grains — can include seed oils on salads or in home cooking without harm. That's what the evidence shows.
A closer look at cholesterol and CRP
Looking specifically at the effect of linoleic acid intake on cholesterol: a 2023 analysis combined 40 randomized controlled trials and found that linoleic acid decreased LDL cholesterol by about 3.26 mg/dL [10].
A separate Cochrane review concluded that increasing polyunsaturated fatty acid intake probably slightly reduces the risk of heart disease events and of combined heart and stroke events [11].
Interestingly, when a subgroup analysis was done in the 2017 inflammation meta-analysis, there did appear to be a link between very large intakes of linoleic acid and an inflammatory marker called CRP. In subjects with a more pronounced increase of dietary linoleic acid intake, CRP tended to rise, while in those with a modest linoleic acid increase, CRP was unaffected [2].
The authors explain that a more profound increase of dietary linoleic acid may be more effective and drastic in changing the tissue composition of both n-6 and n-3 PUFAs and also their inflammation-related derivatives. The message is consistent: moderate intake from whole foods and home cooking is supported by the evidence. The problems begin with extreme intakes and repeatedly heated industrial oils.
References
1. https://pubmed.ncbi.nlm.nih.gov/22492369/
2. https://pubmed.ncbi.nlm.nih.gov/28752873/
4. https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.118.038908
5. https://pubmed.ncbi.nlm.nih.gov/29599053/
7. https://www.mdpi.com/2072-6643/14/7/1489
8. https://www.sciencedirect.com/article/abs/pii/S0271531707000899
9. https://onlinelibrary.wiley.com/doi/abs/10.1002/mnfr.201100533
