Seed oils: why they're good for your heart

What are seed oils?

Seed oils are oils made from the seeds of plants, including rapeseed, sunflower, soyabeans and corn.

Seed oils are a type of vegetable oil, but not all vegetable oils come from seeds. Other vegetable oils like olive oil and avocado oil are made from the fruit’s flesh.

In the UK, the most common seed oils are rapeseed oil, often sold as just ‘vegetable oil’, and sunflower oil.

Are seed oils healthy?

Seed oils are mainly made of fat, which means they are high in calories, so consuming too much of them could lead to weight gain.

However, a small amount of fat is essential as part of a healthy diet. And seed oils may be a healthier choice than other cooking fats.

The UK government’s Eatwell Guide recommends using vegetable and seed oils, which contain mostly unsaturated fats, instead of cooking fats high in saturated fat, such as butter, ghee and coconut oil.

This is based on a wide body of research that has found eating too much saturated fat can lead to high cholesterol levels, which can raise the risk of having a heart attack or stroke.

• Find out more about the different types of fats.

Why are seed oils controversial?

Since seed oils were first developed in the early 20th century, they have become a large part of many people’s diets.

In recent years, there have been claims on social media that seed oils can increase the risk of cardiovascular disease, despite their low levels of saturated fat.

Some critics say that seed oils promote inflammation, which is your body’s response to illness or injury, because they contain omega-6 fatty acids – a type of polyunsaturated fat.

Research has shown that chronic inflammation has been linked to an increased risk of cardiovascular disease. However, there is not much reliable evidence that seed oils cause this type of inflammation in the first place.

Others claim that; the wide use of seed oils in foods is linked to increased rates of chronic diseases, such as cardiovascular disease, they contain harmful toxins from industrial refining, and can become toxic through heating during cooking.

But even though these ideas have become popular online, there’s no evidence that seed oils are harmful when eaten as part of a balanced diet.

Below we take a look at 4 incorrect claims that seed oils are bad for you in more detail.

1. Do seed oils and linoleic acid cause inflammation? 

As mentioned previously, seed oils are high in a type of unsaturated fat called polyunsaturated fat. There are 2 main types of polyunsaturated fats: omega-6 and omega-3 fatty acids. 

These are both essential fatty acids. This means that your body cannot make them on its own, so you need to get them through your diet. 

Omega-3s have been found to lower inflammation. They’re found in oily fish such as salmon or mackerel, walnuts, and some seeds and their oils, including rapeseed, flaxseed and soyabean.

However, seed oils often contain a lot of omega-6 fatty acids, which unlike omega-3s most people already have enough of in their diet. They often include one particular omega-6 called linoleic acid.

Previously, it was thought that linoleic acid and other omega-6s increased inflammation in the body. 

This was based on the idea that linoleic acid can become arachidonic acid in the body. This can then go on to form compounds that can contribute to inflammation.

This is why some scientists suggest that a diet with too much omega-6, and not enough omega-3s, could lead to inflammation.

However, there is a lack of conclusive evidence that omega-6 fats directly cause inflammation in humans. Some research suggests that they may even be slightly anti-inflammatory.

For example, a 2023 study in the journal Nutrients of nearly 2,800 people in the United States aged 66 years old on average looked at the effect of omega-6 fatty acids on the level of 10 different inflammatory biomarkers (substances in the body that are a sign of inflammation).

It found that higher levels of linoleic acid and arachidonic acid in the blood were not associated with a higher level of any of these biomarkers and were even linked to a lower level for 5 of them.

2. Can seed oils raise the risk of chronic disease?

Some critics of seed oils have pointed out that the rates of obesity and chronic conditions like coronary heart disease and type 2 diabetes in the Western world have grown as intake of seed oils and in turn omega-6s and linoleic acid has increased.

But this is an example of what scientists call correlation, not causation. This means that we do not know if seed oils have caused an increase in these diseases, or if it’s something else.

In an attempt to clarify the question, researchers at Harvard University in the United States examined 38 studies into the topic that included more than 810,000 people.

They found those with the highest intake of linoleic acid were 13 per cent less likely to die from any cause, and 13 per cent less likely to die from cardiovascular diseases, like heart attacks and strokes, during the study than those with the lowest intake.

The findings were reported in The American Journal of Clinical Nutrition in 2020.

They suggest that, while omega-3s have a stronger anti-inflammatory effect than omega-6s they may still have some benefits to health.

3. Do seed oils contain toxins like hexane?

To obtain large quantities of oil from seeds, food manufacturers usually use a chemical process that involves mixing the seeds with a chemical called hexane to extract the liquid oil.

Hexane is a compound found in petroleum that can be toxic in humans, so it may seem concerning that it’s used to make seed oils.

However, the oils go through a process to remove almost all the hexane before they’re sold to the public.

This means that only trace amounts of hexane are left in the oil, which is tightly regulated and considered safe by UK food safety authorities.

If you are concerned about hexane, cold-pressed oils are a good alternative. These oils are extracted mechanically, without the use of chemical solvents like hexane.

4. Do seed oils become toxic at high temperatures?

Some people claim that seed oils can become toxic when they are used during cooking.

This is because linoleic acids can break down into harmful compounds called aldehydes when they are heated to high temperatures.

However, this is more of a concern when seed oils are repeatedly heated to a very high temperature. It should not be an issue when cooking with seed oils at home, but make sure not to re-use oils after heating them.

There have also been concerns that heating seed oils produces another type of fat called trans fats. Like saturated fats, eating too many trans fats can raise your blood cholesterol levels, which can increase the risk of heart attack and stroke.

However, a meta-analysis of 33 studies published in 2022 in Nutrients found that heating oils up to 200 degrees Celsius did not increase the level of trans fats.

Are seed oils used in ultra-processed foods?

Seed oils are sometimes seen as unhealthy because they’re often found in ultra-processed foods.

These foods have had a high level of processing and contain ingredients you would not find in the average kitchen.

Recent research has found that people who eat more ultra-processed foods may have a higher risk of developing cardiovascular disease.

But this is thought to be caused by the high levels of sugar, salt and saturated fat often found in ultra-processed foods, not because they contain seed oils.

The BHF verdict

There is not enough evidence behind claims that seed oils contribute to inflammation and increase the risk of cardiovascular disease.

“There’s no need to completely avoid seed oils. In fact, many are rich in vitamin E and unsaturated fats,” says BHF Senior Dietitian Tracy Parker.

“While seed oils are high in calories, they’re fine in moderation. And they’re a healthier choice than fats high in saturated fat like butter, palm oil and coconut oil, which are known to increase cholesterol levels.

“Consuming a variety of fats, including olive oil, nuts, seeds and oily fish that contain healthy omega-3 fatty acids, as well as seed oils, is best for your overall health.”

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