Are Seed Oils Toxic and Inflammatory?

This no doubt will be my most hated blog post, 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. People claim they're toxic, inflammatory, and terrible for our health. But is that really true?

In this post, we'll explore where these concerns about seed oils come from, including worries about heating seed oils and the omega-3 to omega-6 ratio. I'll show you why the evidence contradicts the popular opinion and then chart a path forward that can help us reduce heart disease risks.

1. What Are Seed Oils and Where Have the Concerns Come From?

Seed oils are extracted from seeds like soybeans, corn, and sunflowers, and they've become a staple in modern cooking and processed foods. It's important to note that olive oil isn't a seed oil. Olive oil comes from the fruit of the olive tree, not the seeds, and is mostly made up of monounsaturated fats.

Some seed oils, like canola oil, contain a lot of monounsaturated fats too. But generally, most seed oils are higher in polyunsaturated fats, especially omega-6 fatty acids like linoleic acid.

Just like omega-3 fats, we need omega-6 fats because our bodies can't make them on their own. We have to get them from the food we eat—sources like nuts, seeds, meat, and eggs.

But over the years, concerns have grown about whether the massive increase in seed oil consumption is harming our health. Some people claim that seed oils are causing widespread inflammation, leading to weight gain and heart disease. But are these fears justified, or is the truth being twisted?

2. The Rise of Seed Oils in Our Diet

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—yes, you read that right. These oils were first made for things like machine lubrication, making candles, and 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. That's when they started appearing in our food supply.

This might sound bizarre. Why would these oils be added to the food supply in the first place?

Well, in the 1960s and 1970s, the conversation around fat and heart health really started heating up. Many health experts at the time began advising people to cut back on saturated fats—things like butter, lard, and other animal fats—because studies suggested that these fats raised cholesterol levels, thereby increasing the risk of heart disease.

So, people began looking for healthier 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.

Suddenly, seed oils became the go-to replacement for animal fats in many kitchens. People swapped butter for margarine, which was made from seed oils. Seed oils were increasingly used in processed foods, from salad dressings to snacks like chips and cookies.

This massive shift in our diet meant that 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?

3. The Omega-3 to Omega-6 Ratio: What's the Big Deal?

Omega-3 fatty acids, found in foods like fish, flaxseeds, and walnuts, are well-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.

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.

And don't worry—we'll address heating seed oils shortly.

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

4. Inflammation: Lab Findings vs. Real-World Evidence

The theory that omega-6 fatty acids cause inflammation sounds logical at first, especially 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, in a randomized controlled trial involving obese individuals, one group was fed 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 macronutrients—protein, carbs, and fat—were the same.

After the 10-week study period, the seed oil group had less liver fat, reduced inflammation, and lower insulin levels compared to the saturated fat group ( 1).

This study also measured the amount of linoleic acid (remember, that's an omega-6 fatty acid) in the participants' blood. This allowed the scientists to verify whether the seed oil group followed the protocol and were indeed eating more seed oils ( 1).

But that's just one study, and we can easily be led astray by cherry-picking data. It's far better to look at meta-analyses, where randomized controlled trials are pooled together, analyzed, and examined for overall trends. This provides a more complete picture.

In a 2017 meta-analysis combining 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 meta-analysis examined inflammation markers such as C-reactive protein (CRP), interleukin-6, and tumor necrosis factor. No significant effect was observed. What's more, no publication bias was detected, and the authors of the analysis had no conflicts of interest to declare ( 2).

But here's something to keep in mind: randomized controlled trials are expensive to run, and they typically don't last a long time.

So, if we observe populations over time, we find that high dietary intakes of linoleic acid are associated with reduced risks of diabetes ( 3) and heart disease ( 4).

Again, the authors of these analyses have no conflicts of interest to declare ( 3, 4).

Now, some studies that compare the ratio of omega-6 to omega-3 fats suggest that having more omega-6 compared to omega-3 can be linked to worse health outcomes. But—and this is key—this is often due to a lack of omega-3 intake, not because people are eating too much omega-6.

The real issue is that people aren't consuming enough omega-3s. A critical appraisal of the omega-6 ratio recommended focusing on increasing eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) intake—key omega-3 fatty acids found in fatty fish—rather than worrying about the omega-6 to omega-3 ratio. The authors argued that focusing on the ratio alone is misleading, and that it's the lack of omega-3s that's the real problem in most Western diets ( 5).

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.

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 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.

5. What About Heating Seed Oils?

One of the major concerns people have about seed oils is whether cooking with them—especially at high temperatures—makes them more harmful. You've probably heard claims that heating seed oils creates dangerous compounds like trans fats or oxidized oils, which could then lead to inflammation and other health problems.

Here's why this happens: when seed oils are exposed to heat, light, or air, they can start to break down. This process, called oxidation, is accelerated during cooking. It's particularly problematic for seed oils because they contain a lot of polyunsaturated fats, which have double bonds in their chemical structure that are more prone to breaking down under heat. When these fats break down, they can form harmful compounds like free radicals and oxidized fats, which can damage cells in the body.

So, should we be concerned about cooking with seed oils? The evidence shows that it depends on how high the temperature is and how long the oils are heated.

A 2022 meta-analysis reviewed 33 studies to examine the effects of heating oils on trans fat formation. The analysis found that heating seed oils to temperatures below 200°C did not create significant levels of trans fats. However, when heated above 200°C, especially for prolonged periods, trans fats did form. The researchers concluded that heating oils to very high temperatures or for extended periods should be avoided ( 6).

What about oxidation? One study heated sunflower oil for 1 hour at 180°C, comparing it to unheated oil. The study found a small increase in oxidation but not enough to raise oxidized LDL levels or affect the inner lining of the blood vessels ( 7). However, when the same seed oil was heated 20 times (to simulate conditions in fast food fryers), inflammatory markers in the participants increased significantly ( 8).

6. Conclusion: Seed Oils Aren't the Villain

So, should we be worried about consuming seed oils, especially when they're heated?

The real issue with seed oils isn't the oils themselves but how they're used in our diets. If someone gives up seed oils and starts feeling better, it's usually because they've stopped eating junk food, fast food, and other processed foods that are typically made with highly heated and reused seed oils. Cutting out those types of foods is undoubtedly beneficial for health, but it's not the seed oils alone that are causing harm.

If you're using fresh seed oils at home in salad dressings or for light cooking, there's no evidence that these oils will negatively impact your health. In fact, we have robust evidence from human randomized controlled trials showing that when people replace saturated fats—like butter—with polyunsaturated fats from seed oils, inflammation is reduced, cholesterol levels improve, and the risk of heart disease decreases ( 2, 8).

In summary, seed oils by themselves aren't the problem. The real issue is the overconsumption of ultra-processed foods and takeaways that contain highly processed and reheated oils. If we focus on eating a diet full of whole foods—like fruits, vegetables, lean meats, and whole grains—then seed oils can be a part of a healthy diet.

That's what the evidence shows, and even though this post might be hated, it's the evidence that matters, not opinions.

Make sure to check out the next post on the 5 blood tests everyone should get to help protect and optimize their health.

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.mdpi.com/2072-6643/14/7/1489
7. https://www.sciencedirect.com/science/article/abs/pii/S0271531707000899
8. https://onlinelibrary.wiley.com/doi/abs/10.1002/mnfr.201100533
9. https://www.cochrane.org/CD011737/VASC_effect-cutting-down-saturated-fat-we-eat-our-risk-heart-disease