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Are Seed Oils Actually Bad for Your Health?

7 minutes read

In recent years, seed oils have been called “evil,” “toxic,” and “poisonous” — and accused of causing inflammation, gut issues, and dementia. Influencers and media outlets have even dubbed a group of seed oils “the hateful eight,” and advised people to steer clear of them. But do any of these claims hold up?

Rather than villainize or promote seed oils, the purpose of this article is to review the science related to seed oil claims. The decision of whether to consume seed oils (or any food item) is a personal choice and should be an informed one based on the available research.

So let’s see what science says.

What are seed oils?

Seed oils are edible oils from plant seeds, such as sunflower, cottonseed, safflower, canola, soy, rice bran, corn, and grapeseed [1].

While seed oils vary in nutritional makeup, they each contain saturated fats, monounsaturated fats, and polyunsaturated fats (which include omega-6 and omega-3 fatty acids). The fatty acid profile of seed oils, from highest to lowest percentage, is omega-6 fatty acids (typically in the form of linoleic acid), monounsaturated fats, saturated fats, and a small amount of omega-3 fatty acids [2].

From a basic nutritional profile, seed oils are rich in fatty acids that are a critical micronutrient that your body needs to function [3]. However, some health influencers have blamed the way seed oils are made for their so-called inflammatory and toxic properties.

But what exactly are the claims that people are making about seed oils? Let’s walk through the five biggest arguments against seed oils, and set the record straight.

Argument #1: The refining of seed oils makes them rancid and toxic.

A common anti-seed oil argument is that the refining of seed oils makes them toxic because it creates polar compounds and rancidity.

But the research says otherwise.

First, let’s talk about refining...

Most seed oils on the market are refined, meaning solvents (i.e., the liquid that substances dissolve in) and heat are used to extract the oil from the seeds. Refining gives oils a light color and flavor, and makes them more stable, meaning they have a longer shelf life and are less likely to go bad.

In other words, refining seed oils makes seed oils last longer in a stable state, reducing the likelihood of rancidity.

But what about polar compounds and inflammation?

The anti-seed oil crowd will often claim that when heated, seed oils produce polar compounds that can be bad for our health because they cause oxidation, inflammation, and are toxic.

While certain seed oils, like canola oil, grapeseed oil, and sunflower oils do produce the most polar compounds when heated, they also have the highest smoke point, or point at which they will start to burn — which is the point when these polar compounds begin to form [4].

This means it takes much higher heat (over 500 degrees Fahrenheit) for these oils to become threatening to your health — a much higher temperature than you’d use to cook anything.

Not to mention the amount of time it takes to produce polar compounds. Studies show that these oils have to be heated for multiple hours at their smoke points to produce high quantities of polar compounds — again, longer than you’d ever allow an oil to burn while cooking in your own home.

Meanwhile, unrefined, highly flavorful oils, like flaxseed, walnut, and pumpkin seed oil, actually are less stable, and have lower smoke points, meaning they are easier to burn and produce harmful compounds. This includes extra-virgin olive oil.

The takeaway? For the best taste, health, and cooking results, choose your oils based on what your goal is. If you’re frying food, use oils like canola oil, sunflower oil, or oils with high smoke points for higher-heat cooking. For things like making salad dressings, use your flavorful, less stable oils. Above all — don’t burn your oils, and you have nothing to worry about!

Ultimately, this incorrect portrayal of seed oils has more to do with the abundance of fried foods in today’s diet and, more specifically, the techniques of deep frying at restaurants. If restaurants are not filtering and changing their frying oil frequently, there may be more of a buildup of leftover food in the oil, which can reduce the smoke point, making it more likely that the oils will produce polar compounds that are harmful to the body [5].

So if you’re concerned about seed oils, just skip fried restaurant foods. Though it is worth noting that overused and old oil can change the taste and smell of the food for the worse — which restaurants don’t want. Chefs recommend frying oil be changed every two days in restaurants [6].

All of this to say: fried food in general is best to be avoided, but having it once in a while won’t kill you.

Argument #2: The use of hexane for refining is unsafe for health.

Hexane is a chemical compound made from crude oil that is sometimes used in the extraction process of oils, which is why some argue that seed oils are toxic — they have hexane in them, and exposure to hexane can have alarming negative health effects in humans.

But this is another claim that lacks scientific support. Hexane has been used as a chemical solvent to extract oils from plants for almost a century now in food manufacturing [7]. During extraction, hexane is added to the crushed plants and a chemical reaction allows from the oil from the plant to separate out — and the hexane is then removed.

It’s important to remember that only oils that are refined use hexane, so if it’s hexane you’re worried about, skip refined oils.

But hexane use in refined oils shouldn’t be that concerning either. Studies show that leftover hexane (if there’s any at all — and there often isn’t) in refined oils is less than one millionth of a milligram per each kilogram of oil — essentially, a neglible amount. To put that into perspective, we’re exposed to far more hexane by inhaling gasoline fumes from cars in a day than we are from consuming seed oils [7].

This isn’t to make you more concerned about hexane exposure, but to say that residual hexane is not a health concern, and that research tells us there is not currently any evidence of negative health effects from consuming trace amounts of hexane [8].

Argument #3: Hydrogenation makes seed oils unhealthy.

Another word thrown around in the seed oil discourse is hydrogenation, and the idea that hydrogenation makes seed oils unhealthy.

There’s truth to this claim: research indicates that hydrogenation can have negative consequences on your cardiovascular health [9].

Let’s take a look at the science.

Partial hydrogenation vs. full hydrogenation

Hydrogenation is the process of adding hydrogen molecules to oils to take them from a liquid to a solid. There are two types: partial and full. Partial hydrogenation was popular for decades in products like Crisco and margarine because it added to the taste of the fat, prolonged shelf life, and could be used in certain baked goods and cooking for flavor and texture.

However, partial hydrogenation creates trans fats, a type of fatty acid. While trans fats naturally occur in small amounts in animal products like meat and dairy, the trans fats formed during partial hydrogenation (artificial trans fats) can raise your bad cholesterol levels and increase your risk of heart attack and stroke, which is why the FDA banned them (along with all partially hydrogenated oils) in 2018 [10, 11].

Fully hydrogenated oils, on the other hand, don’t contain trans fats and are still legal to include in foods. That said, fully hydrogenated oils are high in stearic acid, a type of saturated fatty acid that has been linked to the developement of cardiovascular disease [12].

The takeaway: not all seed oils are hydrogenated. If you’re buying a seed oil in its liquid form, you don’t have to worry about trans fats or high levels of stearic acid. However, you might want to steer clear of fully hydrogenated (or solid) seed oils like margarine to protect your heart health.

Argument #4: The high amount of omega-6 fatty acids (linoleic acid) in seeds oils causes inflammation.

There’s a common argument that circulates frequently that says that seed oils cause inflammation. More specifically, people claim that omega-6 fatty acids, which can be found in sunflower, safflower, soy, sesame, and corn oils, are said to be linked to inflammation.

This claim is based on animal studies, which have shown that linoleic acid (a common omega-6 fatty acid) increases pro-inflammatory cytokines (i.e., proteins that help control inflammation in the body) [13].

However, what the influencers and wellness gurus leave out when talking about seed oils being “inflammatory” is that omega-6 fatty acids, like linoleic acid, are involved in both inflammatory and anti-inflammatory pathways in healthy humans [14]. That is to say, omega-6s can stimulate inflammation, or prevent inflammation depending on the conditions.

It’s also important to recall that inflammatory processes play an important role in the immune system’s response to stimuli such as harmful pathogens or cell damage. Anti-inflammatory pathways help us fight against things that cause inflammation, such as bacteria trying to enter our body through a cut. Hence, context is key and many of these claims are grounded in animal studies, not humans.

So, do seed oils cause inflammation in humans?

Beyond the concerns about inflammation in animal studies, a close look at rigorous human studies debunks the claim that seed oils increase inflammation [15].

A 2021 meta-analysis of 83 controlled human trials, showed that increasing consumption of omega-3 fatty acids, omega-6 fatty acids, polyunsaturated fatty acids had little to no effect on inflammatory markers or disease risk [16]. Another meta-analysis of 15 randomized controlled trials (the gold standard of establishing a causal relationship) found that there is no evidence that adding omega-6 fatty acids to the diet increases inflammation [17].

Some studies even showed that omega-6 intake can be anti-inflammatory and support metabolic health. One study in obese people found that omega-6 fatty acid intake reduced liver fat, reduced weight, and did not cause any inflammation [18].

Another randomized control trial in obese women with type 2 diabetes found that 8 grams of safflower oil daily (a seed oil high in omega-6) improved levels of blood sugar, blood fats, and inflammation. This result shows that higher omega-6 levels may result in reduced inflammatory status [19].

In other words, there is no evidence of seed oils causing inflammation in humans. More interestingly, there has been evidence to show the opposite effect: seed oils decrease inflammation.

Argument #5: The omega-6 content of seed oils causes heart disease.

What about heart disease? You may have heard that the high omega-6 polyunsaturated fat content of seed oils can increase the risk of heart disease and cardiometabolic problems.

Again, this is because of the argument that increased omega-6 fatty acids leads to increased inflammation, which is linked to heart disease.

However, several studies and well-conducted meta-analyses (here, here, and here) have found that increasing intake of omega-6’s by consuming more linoleic acid from vegetable oil actually reduced negative cardiometabolic outcomes, and reduced the risk of death by cardiovascular disease [20-22]. Another review of human trials found that omega-6 linoleic acid decreased instances of CVD outcomes in healthy people [23].

There’s currently no evidence from human trials to support that omega-6 content of seed oils is a major driver of inflammation and heart disease.

Seed oils in fast foods and packaged foods

Ultimately, seed oils aren’t the root of the problem — rather, it’s the packaged foods and fast foods that contain them.

While fast foods (i.e., fried foods) and ultra-processed foods (UPFs) do contain high amounts of seed oils, it’s often the hydrogenated (i.e., solid and spreadable) type like margarine and shortening, both of which can lead to cardiovascular disease and other health problems.

Corporations like to use hydrogenated fats for a few key reasons:

they’re cheap
they enhance the texture and taste of foods — especially desserts and fried foods

But while the use of seed oils has grown alongside the increase in popularity of processed foods, it’s not accurate to say that seed oils are responsible for the global obesity epidemic and rising health consequences of UPFs.

Put another way, there’s a correlation between the increased prevalance of seed oils and UPFs in the American diet, but this doesn’t mean there’s a causal link.

In fact, it’s not really about seed oils at all...

Fast foods and UPFs are typically high in sugar, salt, and fat — a combination engineered to be maximally palatable and keep you hooked, while at the same time setting you up for weight gain, disrupted satiety, cardiovascular disease, and even cognitive decline. UPFs also tend to be low in fiber and micronutrients, making them nutritionally lacking and common triggers for blood glucose spikes.

This is particularly alarming as processed food consumption has increased to 57% among U.S. adults [24]. Put another way, more than half of the U.S. diet contains UPFs.

This isn’t by accident: ultra-processed foods make up 73% of the U.S. supply chain and are, on average, 52% cheaper than minimally processed alternatives, making them more convenient and easily accessible to the majority of Americans, especially those living in food deserts where fresh produce is scarce [25].

For most people, scaling back on processed foods and fast foods, which are high in sugar, salt, and fat that can lead to metabolic health problems will provide the greatest benefit.

Key takeaways

From a health perspective, there seems to be no evidence to back up the claims that seed oils are toxic, cause inflammation, or are rancid.

Ultimately, the decision of whether to consume seed oils is a personal choice and should be made on the best available evidence and not what a social media influencer says.

Does that mean seed oils are explicitly healthy? It's hard to say, but what's clear from the evidence is that it is primarily highly processed foods which are contributing to the metabolic health crisis, rather than seed oils in isolation.

Here’s a recap:

Refining seed oils does not contribute to rancidity. In fact, it makes them last longer in a stable state — reducing the likelihood of rancidity.
Seed oils only release polar compounds at 500+ degrees Fahrenheit — a much higher temperature than you’d typically reach when cooking at home. Plus, they need to be heated for multiple hours for polar compounds to even form. You may want to skip fried foods at restaurants (where frying oil may not be changed as often) if you’re concerned, but once in a while is completely fine.
The residual hexane (if any) leftover from refining seed oils is so negligible that there is no impact on health whatsoever. If you’re concerned, just skip refined oils and opt for unrefined instead.
Seed oils in liquid form are not hydrogenated. But fully hydrogenated fats like margarine or spreadable shortenings (commonly used to improve taste and texture) are high in stearic acid and can increase your risk of heart disease, so avoid those.
The omega-6s found in some seed oils affect both inflammatory and anti-inflammatory pathways in the body. Emerging research even suggests that eating omega-6s from some seed oils can help reduce inflammation and the risk of cardiovascular disease in humans, rather than increase it, as wellness gurus tend to claim.
Processed foods, which are high in sugar, salt, fat, and other additives, are much more likely to lead to inflammation, microbiome disruptions, cardiovascular health problems, obesity, and more.

If you desire to eat a diet that contains minimally processed ingredients, you can still enjoy seed oils in moderation. Not all are refined (i.e., processed using hexane and other solvents) — just look out for cold-pressed options at the store like grapeseed, flaxseed, and hemp seed. You can also use olive oil, avocado oil, and coconut oil at home.

If you’re still concerned about omega-6s, canola oil, flaxseed oil, and soybean oil tend to have higher amounts of omega-3s — another essential fatty acid [26].

The point is: moderation is always key. If the majority of seed oils in your diet comes from deep-fried meals at restaurants, seed oils may pose a problem — but for most people, the issue has less to do with seed oils and more to do with the consumption of sugar, salt, and fat from processed foods.

References:

Written by: Peyton Lessard, MS

Reviewed by: Emily Johnson, MSc RD

Table of Contents

What are seed oils?
Argument #1: The refining of seed oils makes them rancid and toxic.
Argument #2: The use of hexane for refining is unsafe for health.
Argument #3: Hydrogenation makes seed oils unhealthy.
Argument #4: The high amount of omega-6 fatty acids (linoleic acid) in seeds oils causes inflammation.
Argument #5: The omega-6 content of seed oils causes heart disease.
Seed oils in fast foods and packaged foods
Key takeaways

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