The Truth About Seed Oil Toxicity

This will no doubt be one of my most hated articles, but I'm here to show you the evidence, and let the evidence speak for itself.

The internet is awash with videos and articles warning about the dangers of seed oils — that they're toxic, inflammatory, and terrible for our health.

In this article, I'll explain where these concerns came from, including worries about heating seed oils and the omega-3 to omega-6 ratio, why the evidence contradicts the popular opinion, and how to actually reduce our heart disease risks.

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, we need omega-6 fats because our body can't make them on its own, so we have to get them from food — nuts, seeds, meat, and eggs.

But over the years, concerns have grown about whether the massive increase in seed oil consumption is actually harming our 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.

Sounds bizarre, doesn't it? Why would these oils be added to the food supply in the first place?

In the 1960s and 1970s, the conversation around fat and heart health really started to heat up. Many health experts began telling people to cut back on saturated fats — butter, lard, and other animal fats — because studies suggested that these fats raised cholesterol levels and therefore increased the risk of heart disease.

So, 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 our diet meant we were consuming a lot 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 harming us? Especially if those oils aren't manufactured properly and contain nasty impurities like trans-fats or oxidized fats?

The omega-6 inflammation debate

First, let's address the concerns about the balance between omega-6 and omega-3 fatty acids in our 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 crucial for heart health, brain function, and overall well-being. That's why you often hear about the benefits of eating fatty fish like salmon or taking omega-3 supplements.

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 we consume omega-6 fats, they can be broken down into molecules that might cause inflammation in the body. This process is part of our normal immune response, but if we consume too much omega-6 and not enough omega-3, the concern is that the body could shift toward a more inflammatory state.

And chronic inflammation has been linked to a whole host of health problems, including heart disease, diabetes, arthritis, and even cancer.

The idea is that our modern diet, full of seed oils and processed foods, has way more omega-6 than omega-3, which throws off the balance between these fats and leads to inflammation.

But is it really true that eating more omega-6 leads to more inflammation, or is this concern overblown? The answer is nuanced.

What the evidence actually shows

The theory that omega-6 fatty acids cause inflammation sounds logical at first if we extrapolate what happens in a lab when testing single cells in a petri dish. But we want to know what happens in the real world, and a fantastic source of evidence comes from human randomized controlled trials.

For example, this randomized controlled trial of obese individuals fed one group a diet high in omega-6 seed oils, and the other group was fed a diet high in saturated fat mainly from butter. Both groups ate the same amount of calories, and the macros — 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 very interesting as it measured the amount of linoleic acid (the omega-6 fatty acid) in the blood, so the scientists could see if the seed oil group were actually following the protocol and eating more seed oils.

But that's just one study. And if we cherry-pick data we can easily be led astray. Instead, it's far better to look at a meta-analysis — where the randomized controlled trials are pooled together, combined, then analysed. It means we look at the entirety of the evidence, rather than cherry-picking.

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 pointing in the same direction, and no publication bias was detected. Nor did the authors of this analysis have any conflicts of interest to declare.

Randomized controlled trials are expensive to run and usually don't last a long time. So if we observe the population instead, we see that high dietary intakes of linoleic acid are associated with reduced risks of diabetes [3].

And reduced risks of heart disease, which is what we would expect from the randomized controlled trial results [4].

Again, the authors of these analyses had no conflicts of interest to declare.

Now, some studies that compare the ratio of omega-6 to omega-3 fats show that having more omega-6 compared to omega-3 can be linked to worse health. 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 our bodies doesn't hold up when we look at the scientific evidence.

Instead, when we replace saturated fats — like the ones in butter — with polyunsaturated fats from seed oils, we see real benefits for our hearts, as evidenced by a massive Cochrane review which pooled together 15 studies with over 56,000 participants. It found that when people cut down on saturated fats and used polyunsaturated fats instead, there was a significant drop in heart disease [6].

But there's an important piece of history we need to address. In the past, many people switched from butter to margarine, believing it was a healthier choice. The problem — and this is tragic — 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 big concerns people have about seed oils is whether cooking with them — especially at high temperatures — can make them more harmful. You might have heard claims that heating seed oils creates dangerous compounds like trans fats or oxidized oils, which could then lead to inflammation or other health problems.

Let me explain. 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.

But 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 we call double bonds. These double bonds make the fats weaker, so they can break down more easily. When the oil breaks down, it creates harmful stuff like free radicals and oxidized fats, which can damage the cells in your body.

So the concern is that when you heat seed oils — for example, for frying — these polyunsaturated fats with all their double bonds can break down and form these harmful compounds. What does the evidence show?

In 2022, a meta-analysis that combined 33 studies together found that heating seed oils to temperatures less than 200°C did not create trans-fatty acids. Above that temperature, though, and during prolonged heating, yes, trans-fats were created [7].

The analysis finishes with the conclusion: "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."

And to explore the concerns about oxidation, 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 make an increase in the 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

Should we be worried about consuming seed oils that have been heated? Well, this brings me onto the real problem with seed oils.

If someone is consuming seed oils that have been exposed to very high temperatures over a prolonged period of time, they are probably consuming takeaways, fries, and other processed foods.

So when people say that they've given up seed oils and they are feeling amazing, what they've actually done is stopped eating junk food. Which is wonderful — I hope everyone does that.

But that's entirely different to someone drizzling fresh seed oils over a home-made salad, or cooking at home using seed oils, or replacing butter with modern margarines.

As explained earlier, we have robust evidence from human randomized controlled trials that when people switch from butter to unsaturated fats, we see reduced inflammation levels [2], improved blood cholesterol, and reduced heart disease [6].

So, here's 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 oils. If we focus on eating a diet full of whole foods — like fruits, vegetables, lean meats, and whole grains — then seed oils on salads, or using seed oils in home cooking, is great for us.

That's what the evidence shows, and even though this article will be hated, it's the evidence that matters. Not opinions.

A closer look at cholesterol and CRP

If we have a brief look 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].

And a separate Cochrane review concluded that increasing polyunsaturated fatty acid intake probably slightly reduces our 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 profound increase of dietary linoleic acid intake, CRP tended to rise, while in those with low linoleic acid supplementation, 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. So the message stands: moderate intake from whole foods and home cooking is fine. The problems start 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/

3. https://diabetesjournals.org/care/article/44/9/2173/138865/Dietary-Intake-of-Linoleic-Acid-Its-Concentrations

4. https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.118.038908

5. https://pubmed.ncbi.nlm.nih.gov/29599053/

6. https://www.cochrane.org/CD011737/VASC_effect-cutting-down-saturated-fat-we-eat-our-risk-heart-disease

7. https://www.mdpi.com/2072-6643/14/7/1489

8. https://www.sciencedirect.com/science/article/abs/pii/S0271531707000899

9. https://onlinelibrary.wiley.com/doi/abs/10.1002/mnfr.201100533

10. https://www.mdpi.com/2304-8158/12/11/2129

11. https://www.cochrane.org/CD012345/VASC_polyunsaturated-fatty-acids-prevention-and-treatment-diseases-heart-and-circulation