Seed oils are not ‘toxic’: separating fact from fiction

If you’re active on social media in the health and nutrition space, it’s very likely you’ve seen health advice over the last few years warning you about the health harms of seed oils. These seed oils are toxic don’t you know? And they’re blamed for a whole host of health ills such as inflammation, lowered immunity, heart disease, type 2 diabetes, cancer and more. Oh my. But the reality about seed oils is far less scary and there are few health concerns you need to worry about. And in fact, quite a few health benefits to be gained by having them in your diet. In this blog post, I’ll look at what seed oils are, explain where all those health warnings have come from and put this into context for what it means for your health and the foods you choose to eat.

Seed oils. They’re the new gluten. Or should that be carbohydrates? Or maybe GMOs? I don’t know, so many nutrition scare campaigns, so little time to keep up with them.

Seed oils are the topic for today’s post so let’s give a bit of background on what they are. Seed oils are oils extracted from the seeds of plants. The common ones include:

• Canola oil
• Sunflower oil
• Soybean oil
• Safflower oil
• Sesame oil
• Corn oil
• Flaxseed oil
• Grapeseed oil

These seed oils are often used in cooking and food manufacturing due to their high levels of unsaturated fatty acids and low levels of saturated fatty acids. As well as their use in cooking and baking, you’ll find them in a range of more highly processed foods like deep-fried foods, baked goods, salad dressings, margarines and baked biscuits. And hold that information on the many foods that can contain seed oils in your mind as I will call it back later in the post in putting the scary health alarms about them into context.

Many of these seed oils are fairly recent additions to our modern diet thanks to solvent technology to extract them from plants became available. This contrasts with the traditional way of extracting oil from plants such as crushing or cold pressing. The process of making extra virgin olive oil is a classic example of this process that has been used for thousands of years.

But solvents used in the extraction of seed oils such as hexane only remain in trace amounts in the oil after processing. And hexane has been used for nearly a century to extract oil, and there is no evidence that it’s harmful in the trace amounts found in processed seed oils. In fact, you probably inhale more hexane when you’re filling your car with petrol than you’ll ever get from seed oils.

What are omega-3s and omega-6s?

So on to the whole ‘seed oils are toxic’ argument. What drives this is the commentary you may read from those advising you to cut them out of your diet. It is all about the types of fatty acids found in seed oils – in particular the omega-6s. More about what omega-6s are shortly.

All oils contain a combination of polyunsaturated, monounsaturated, and saturated fats to varying degrees. In most seed oils, it is the omega-6 polyunsaturated fatty acids that dominate. So, first, let’s give a bit of background into this whole omega fatty acid thing.

Omega fatty acids are a group of fatty acids found in food and in our bodies with unique chemical and health properties. It is the omega-3 and omega-6 fatty acids that are of the most interest for their health benefits. Both fatty acids are ‘essential’ because the body cannot make them, so they must come from our diet.

Omega-3s explained

The two most important omega-3s are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). You’ll find them mostly in oily fish such as salmon and herring. Another type of omega-3 fatty acid is called alpha-linolenic acid (ALA), which is found in plant foods such as walnuts, soybeans and flaxseeds. The body can convert a small amount of ALA into EPA into DHA.

Omega-3 fatty acids are important parts of the structure of cell walls. They are abundant in the eyes and brain, and this links to their key role in development and vision, especially in the early stages of life. Low levels of omega-3s during pregnancy are associated with preterm birth and potentially delayed cognitive development. In older adults, people who have higher amounts of omega-3s in their diet have a lower risk of developing age-related macular degeneration.

Omega-3s also have a crucial role in helping to keep the heart and blood vessels healthy and the immune system working normally. A diet rich in foods high in omega-3 fatty acids is linked with a decreased risk of heart disease. Regular consumption of foods high in omega-3 fatty acids may help to lower the risk of blood clots and irregular heartbeats, improve blood lipids and lower blood pressure.

Omega-6s and their role

So, that’s the omega-3s. What about omega-6s? Just like omega-3s, omega-6 fatty acids are also essential fatty acids with the main one of interest called arachidonic acid. Animal foods such as meat and eggs are the main dietary sources of arachidonic acid. The body can also make arachidonic acid from another omega-6 fatty acid called linoleic acid which is found in many plant oils – and here, seed oils are a major source.

Omega-6 fatty acids have more of a counterbalancing action to omega-3s. They can promote inflammation, blood clotting, and the constriction of blood vessels. But it is too simplistic to say that omega-3s are ‘good’ and omega-6s are ‘bad’. You want a balance between the two for good health and where omega-6s are just as important for healthy brain development, heart health and a robust immune system as the omega-3s.

Too many omega-6s?

So we need both omega-3s and omega-6s, but here the argument that brings in seeds oil goes that we’re getting too many omega-6s and this sets the stage for chronic inflammation, which is at the root of many diseases. And as a population, the ratio of omega-6s to omega-3s in our diet has been increasing over time (which is true) and this has led to all manner of chronic diseases caused by inflammation – or so the story goes. So cut out seed oils and get back to good health. Sounds simple. But it’s also wrong. As the scientific evidence says a very different thing.

To start with, of all those omega-6s from seed oils, only a tiny amount is converted to the omega-6 essential fatty acid of arachidonic acid. And not all of the metabolic by-products of arachidonic acid can easily be labelled as harmful – it can have anti-inflammatory effects too.

And when you go looking for human evidence that having a high amount of omega-6s in your diet will cause inflammation and heart disease, you struggle to find it. Sure, you can cherrypick the odd animal or in vitro study to support your case like those pushing the ‘seed oils are toxic’ narrative do, but that isn’t science – that is severely biased misinformation.

And in fact, when you look at all the human evidence taken together, the story about seed oils and their omega-6s changes. For instance, in an analysis of 30 studies published in 2019, researchers found no association between levels of linoleic acid or arachidonic acid in the blood and the risk for heart disease. In fact, people with higher amounts of linoleic acid in their bloodstream were 7 percent less likely to develop heart disease. The complete opposite of what would be the case if these seed oils were indeed harmful. And this wasn’t one cherry-picked study, but a meta-analysis of 30 individual studies.

But in fairness, these were observational studies that can have issues of bias even though the conclusion was pretty clear. So what about the gold-standard randomised controlled trials?

Well, here we have a Cochrane review of 19 randomised-controlled trials involving over 6,000 people and which ran from 1 to 8 years where more omega-6s were added to the diet. And there clearly was no evidence that omega-6s increased the risk of cardiovascular disease. In fact, even some evidence that it lowered the risk of heart attacks.

And then we have a 2021 review looking at 83 studies with over 41,000 people who had inflammatory bowel disease and which found no link between supplementing omega-6 fatty acids and markers of inflammation.

So it is a clear fail for the thesis that these omega-6s in your diet from seed oils are going to give you the inflammations and make you drop dead from a heart attack. If anything, you probably could do with having some more of them in your diet to keep your heart healthy.

As a rationale for why you need to have fewer omega-6s, you may read that it is because the ratio of omega-6s to omega-3s in our diet is too high – likely at around 10:1 when some advocate it should be lower at 4:1. But it isn’t the omega-6 content of our diet that is the issue, it more that we have an absolute low level of consumption of omega-3s so eating more of these will have additional health benefits on top of the benefits that omega-6s already give us.

Food sources of omega-6s

But now for the nuance. Remember that point I made earlier about some of the major food sources of omega-6 seed oils in our diet? Well, there is some truth to the statement that seed oils aren’t good for us. But that’s because of the foods they can be found in – not the omega-6s themselves.

And here we’re talking about a variety of packaged products—chips, biscuits, baked goods, salad dressings, and deep-fried food to name a few. Pretty clear isn’t it? Many of the foods that contain seed oils are also high in refined carbohydrates, salt and sugar. We already know about the link between having a diet high in these sorts of foods and ill health. But it is these other components, not the seed oils themselves, that are the culprit behind weight gain and poor health.

It’s not too much of a stretch to say that people who eat a lot of refined seed oils are also eating a lot of highly processed food. So their diet quality won’t be the best. Everything behind the ‘cut out seed oils’ message is just another way of saying: eat less highly processed food. But with the price paid for it of making people needlessly fearful when they happen to use some canola or sunflower oil in their cooking heaven forbid.

The right oil for the right job: smoke point explained

But as always, there can be a small kernel of truth in any health scare campaign as there can be an issue with seed oils themselves, though it does apply equally to all cooking oils. And that’s what happens when you heat and reheat oils time and again as happens in the commercial cooking world.

Here, it is important to bring up the topic of smoke point. The smoke point of an oil is the temperature at which a cooking fat or oil begins to break down and give off smoke. You may have seen it yourself in your own kitchen when you leave the oil in a frying pan unattended for a while with no food in it – the smoke turns blue and black and it smells. That’s the smoke point and every cooking oil has one. At the smoke point, there is a deterioration of flavour in the oil and the nutritional quality begins to decline.

The smoke point determines what an oil can be used for, dictating the maximum useable and safe temperature of the oil. A high smoke point is ideal for deep frying. But when you use an oil repeatedly, it will also make it smoke sooner because it lowers its smoke point. And this is the issue with commercial foods where you aren’t going to know the quality of the oil they use and if it has undergone some level of degradation from reuse.

To give you a bit of a guide to the smoke points of oils, extra virgin olive oil has a smoke point of around 190^oC (374^oF) but it can be higher thanks to the presence of many different types of antioxidants. Canola oil is 204^oC (399^oF), grapeseed oil 216^oC (421^oF), sunflower oil 232^oC (450^oF) and safflower oil 266^oC (511^oF).

So you can see why seed oils are popular choices for deep frying foods because of their high smoke point. But when you’re deep frying, it is typically foods that aren’t always going to be bursting with healthy nutrition. And if those oils are used in a commercial outlet who are not strict on its oil reuse policy, then it can affect the quality of the oil that goes into your food.

There is merit though in using a variety of oils and fats in your cooking rather than relying on one ‘superfood’ to cure all your health problems be that olive oil or coconut oil. Okay, that was a joke about coconut oil being a superfood. But each of these oils has its place in the kitchen: coconut oil would suck as a salad dressing but works great in protein balls. And the opposite is true for olive oil.

For seed oils, their shelf stability, fairly neutral flavour and cheaper price make them a good choice for some types of food preparation. And their high smoke point also makes them a good choice for high-temperature cooking – just don’t go reusing the oil that’s all.

What it all means

The whole ‘seed oils are toxic’ thing is just another in a long line of scary messages trying to simplify nutrition to just one factor, that if you only cut from your diet, will give you the best of health. But no one eats seed oils in isolation; they eat a whole range of foods. And that’s what is important: the overall quality of your diet.

If you’re eating lots of highly processed foods and take away deep-fried foods, you’ll also be having quite a lot of omega-6s from seed oil. And then if you make positive changes to eating more foods close to their natural state and doing most of the food preparation yourself, you likely won’t be having as many seed oils in your diet. But the health benefits you get here are because of your whole diet changes and have very little to do with having fewer seed oils. And if you do use them in your cooking, then do so in the confidence that you’re likely doing even more to benefit your health.