Updated 1st November 2022

Are seed oils bad for you?

Seed oils include canola (or rapeseed in the United Kingdom), soybean, sunflower, cottonseed, corn, grapeseed, rice bran, and safflower. In fact, most vegetable oils are seed oils.

You may have read that these products are big trouble and that they’re linked to an increased risk of heart disease, weight gain, or other diseases.

Some even refer to these oils as the “hateful eight.”

Let’s find out where this anger comes from and check what the science has to say.

What are the seed oil haters saying?

According to a lot of folk on the internet, the issue with seed oils is that they contain high levels of omega-6 fatty acids.

The argument goes that one of the most common omega-6s — linoleic acid — is converted into arachidonic acid in your body. This, they suggest, leads to the formation of other compounds that increase levels of inflammation throughout the body.

And that’s where the health warnings come in. A wide variety of conditions — from diabetes to depression and from cardiovascular disease to cancer — involve inflammation. So, if seed oils really do boost inflammation, that would be a worry.

These concerns appeared decades ago, and in the last few years, their volume has intensified.

However, you’ll be glad to hear that the latest research doesn’t back up this claim. Before we get cracking, let’s outline what linoleic acid is.

What is linoleic acid?

Linoleic acid is an essential fatty acid. This means that your body needs it but can’t produce it. So, you need to get it through food.

It's in seed oils, of course, but also in nuts, meats, and eggs.

Linoleic acid plays a wide variety of roles in the body, including maintaining healthy skin and forming a part of your cell membranes.

As we’ve learned, linoleic acid is converted into arachidonic acid, which is then converted into a wide range of other important compounds.

These compounds are involved in lots of processes, such as wound healing, blood clotting, and regulating inflammation.  

Although inflammation is a dirty word — it’s associated with chronic diseases, after all — we should also remember that it’s an essential response to injury and infection.

Our health would struggle without it. It's only when it's in the wrong place at the wrong time and in excess that it’s a problem.

Recent omega-6 evidence

Over the last few years, a number of reviews of human studies have failed to identify any health concerns linked to consuming linoleic acid in realistic doses. We’ll take a quick look at a few of these here.

Firstly, a 2020 review concluded that higher levels of omega-6s, particularly linoleic acid, are linked to a reduced risk of cardiovascular disease. 

Similarly, they found that these fatty acids are associated with a reduced risk of developing metabolic diseases and type 2 diabetes. And they found that higher levels of linoleic acid are linked with better glucose control.

Another review found that people who consume more linoleic acid or have more linoleic acid in their blood don’t have higher levels of inflammation.

In fact, the authors explain that in many studies, participants consuming the most linoleic acid had the lowest levels of inflammation.

They write that linoleic acid and arachidonic acid are involved in both pro- and anti-inflammatory pathways. And they don’t recommend for people to reduce their intake.

One review looked at 15 studies on linoleic acid consumption and a range of markers for inflammation. Again, they found “virtually no evidence” that this compound increased inflammation. 

What’s this about ratios?

Scientists have shown that humans evolved consuming a diet where omega-3 and omega-6 were closer to a 1-to-1 ratio. Currently, though, most people consume much more omega-6 than omega-3 — around 16:1.

This, some believe, increases inflammation because omega-3s are anti-inflammatory and omega-6s are pro-inflammatory. 

Below, we’ll give you a deep dive into the reasoning behind the ratio concerns. It’s a bit complicated, but it’s good to lay it all out. This way, when you see it explained elsewhere by seed-oil detractors, you’ll know what they’re going on about. 

Buckle up, there will be many acronyms.

The devilish details

OK, let’s go. The most common omega-3 in plant-based foods is alpha-linolenic acid (ALA). The most common omega-6 is linoleic acid, as we’ve seen.

In your body, you have enzymes that break ALA into eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). 

EPA and DHA are long-chain fatty acids, and they’re really important for health. You need those. 

The same enzymes that convert ALA into EPA and DHA also convert linoleic acid (the omega-6) into arachidonic acid, which we’ve already met.

Arachidonic acid, EPA, and DHA all go on to form compounds called eicosanoids.

The eicosanoids produced by EPA and DHA are anti-inflammatory, and the eicosanoids produced by arachidonic acid are slightly pro-inflammatory.

The argument goes that ALA and linoleic acid “compete” for the enzymes.

So, if you consume fewer omega-3s than omega-6s, the omega-6s hog the enzymes and, therefore, your body produces more of the slightly pro-inflammatory eicosanoids. 

In principle, that sounds reasonable. But does it stand up to real-world scrutiny?

Testing the ratio hypothesis

Some studies have looked at links between the ratio of omega-3 and -6 and inflammation. But, as far as inflammation is concerned, the ratio doesn’t seem to be too important.

As long as you're consuming a reasonable amount of omega-3, the ratio doesn’t matter.

A 2008 report on fats and fatty acids from the Food and Agriculture Organization of the United Nations concluded: 

“Based on the evidence and conceptual limitation, there is no rational[e] for a specific recommendation for [omega-6 to omega-3] ratio.”

For instance, one study involving more than 800 people found that omega-6 didn’t block omega-3’s anti-inflammatory action, but that people who had high levels of both had the lowest levels of inflammation.

Another study with more than 1,100 participants found that higher levels of either omega-3s or -6s had lower levels of inflammatory compounds.

Omega-6s and health

Over the years, a number of large, well-conducted studies have found links between consuming omega-6s and positive health outcomes.

For instance, a study in Finland recruited almost 2,500 men aged 42–60. They followed them for an average of 22.4 years.

The scientists found that higher levels of linoleic acid in the blood were associated with a lower risk of death from cardiovascular disease and death from any cause.

Levels of arachidonic acid had similar positive links, but they were less strong.

Another study pooled data from 30 studies that came from 13 countries and almost 70,000 participants. 

The authors found that higher levels of linoleic acid were associated with a lower risk of cardiovascular disease, death from cardiovascular disease, and one type of stroke.

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Also, arachidonic acid levels weren’t associated with a higher risk of cardiovascular disease. In fact, individuals with the highest levels had a reduced risk.

Another large study used data from more than 120,000 people. This time, the researchers focused on type 2 diabetes. 

They concluded that linoleic acid intake is associated with a lower risk of type 2 diabetes.

We spoke with ZOE’s chief scientist, Dr. Sarah Berry, an associate professor in the Department of Nutritional Sciences at King's College London in the U.K.

She explained that “as long as you’re consuming adequate omega-3, it’s perfectly fine to be consuming omega-6."

"Based on current evidence," Sarah continued, "it will not cause an unfavorable inflammatory response. Omega-6 has a protective effect against many chronic diseases.”

Additives, trans fats, and other nasties

Some of you might have seen claims that seed oils contain a lot of nasties, including hexane, synthetic antioxidants, trans fats, and peroxides. 

The reality is that the oil produced in the U.K. and United States goes through rigorous quality checks.

This weeds out any chemicals that might cause health issues. For instance, hexane — a solvent — is used to process oil, but it’s all removed during processing.

As for trans fats, Sarah told us:

“Industrially produced fats in the U.S. and U.K. no longer contain these, or only contain trace amounts. Most trans fats in our diet come from dairy, and some evidence suggests that these particular fats might even be healthy."

"This myth that seed oils have trans fats is totally outdated.”

It’s true that if you heat seed oils to a high temperature and reuse them repeatedly, toxic compounds can build up. 

However, this is only really relevant for restaurants and industrial deep fryers. And there are laws in place to protect the consumers in these cases. 

Importantly, when you’re frying an egg at home, this isn’t an issue. You won’t generally reach these high temperatures, and you won’t be reusing the oil.

A true link beyond the graph

Some of you might have seen social media posts where a graph plots seed oil use against the prevalence of obesity or type 2 diabetes. 

Lo and behold, as seed oil usage rises over the years, so does chronic disease. This, they claim, proves that seed oils cause these chronic conditions.

But is this a fair use of the data? As Sarah carefully explained to us:

“It’s a load of nutribabble.”

Everyone loves a graph. They tell you a story at a glance. But the story they tell is about correlation, not causation. 

As an example, on that same graph, you could add a line charting the steady rise in mobile phones or the number of horror movies released each year. These lines might also correlate with increased disease, but no one is saying that the Scream franchise is linked to type 2 diabetes.

Another line you could add to that graph would be the amount of ultra-processed foods the average person eats. That, too, might rise in step with chronic disease.

And that might be a much more telling correlation.

And here’s where some confusion can enter: A lot of ultra-processed foods contain seed oils. But they also contain high levels of sugar, salt, and other additives, while lacking fiber and other nutrients.

So, eating a lot of ultra-processed foods that contain seed oils might well be linked to an increased risk of obesity and other conditions, but it’s not the seed oils that are to blame.

What to do

At the end of the day, if you want to avoid seed oils just in case, that’s fine. The evidence so far suggests that there are no issues with them, but everyone gets to choose.

Many seed oil detractors suggest replacing them with animal-based fats, like tallow or butter. However, these types of fats are linked to poorer health outcomes. 

So, if you still want to switch away from seed oils, go for olive oil. For more information about oils, listen to a recent episode of the ZOE Science & Nutrition podcast about cooking oils.

While researching this article, we came across a range of conspiracy theories. Some suggested that “Big Farmer” is hiding the “truth” about seed oils from the general public so they can make more money. 

In reality, it’s more likely that people who are selling seed oil-free products are hiding the truth to ensure they continue to sell their products.

Sources

Associations Between Linoleic Acid Intake and Incident Type 2 Diabetes Among U.S. Men and Women. Diabetes Care. (2019). https://diabetesjournals.org/care/article/42/8/1406/36195/Associations-Between-Linoleic-Acid-Intake-and 

Biomarkers of dietary omega-6 fatty acids and incident cardiovascular disease and mortality. Circulation. (2019). https://pubmed.ncbi.nlm.nih.gov/30971107/ 

Dietary linoleic acid and human health: Focus on cardiovascular and cardiometabolic effects. Atherosclerosis. (2020). https://www.sciencedirect.com/science/article/abs/pii/S0021915019315758 

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. (2012). https://www.sciencedirect.com/science/article/abs/pii/S2212267212004649 

Evolutionary aspects of diet, the omega-6/omega-3 ratio and genetic variation: Nutritional implications for chronic diseases. Biomedicine & Pharmacotherapy. (2006). https://www.sciencedirect.com/science/article/abs/pii/S0753332206002435 

Fats and fatty acids in human nutrition. Report of an expert consultation. (2008). https://pubmed.ncbi.nlm.nih.gov/21812367/ 

Habitual dietary intake of n-3 and n-6 fatty acids in relation to inflammatory markers among US men and women. Circulation. (2003). https://pubmed.ncbi.nlm.nih.gov/12821543/ 

Linoleic acid. (n.d.). https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/linoleic-acid 

Linoleic acid. Advances in Nutrition. (2013). https://academic.oup.com/advances/article/4/3/311/4644566 

Relationship of plasma polyunsaturated fatty acids to circulating inflammatory markers. The Journal of Clinical Endocrinology & Metabolism. (2006). https://pubmed.ncbi.nlm.nih.gov/16234304/ 

Serum n-6 polyunsaturated fatty acids and risk of death: the Kuopio Ischaemic Heart Disease Risk Factor Study. The American Journal of Clinical Nutrition. (2018). https://pubmed.ncbi.nlm.nih.gov/29566193/ 

The omega-6/omega-3 ratio and cardiovascular disease risk: uses and abuses. Current Atherosclerosis Reports. (2006). https://pubmed.ncbi.nlm.nih.gov/17045070/ 

Too much linoleic acid promotes inflammation—doesn’t it? Prostaglandins, Leukotrienes, and Essential Fatty Acids. (2008). https://www.sciencedirect.com/science/article/abs/pii/S0952327808001324

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