Are seed oils really bad for you?

Seed oils like canola and sunflower oil have attracted controversial claims about harmful effects in recent times. Is there any truth to them?

You might have a bottle of sunflower oil or canola (rapeseed) oil stashed away in a kitchen cupboard somewhere. Whether you cook with them or drizzle them over salads, seed oils are popular across the world.

But these unassuming seed oils have become the centre of a heated debate online.

In recent years, seed oils have become the target of countless social media posts, with people claiming that they are "toxic", "poisonous" and, ultimately, are damaging our health. Critics have nicknamed some seed oils ‘the hateful eight' – referencing eight popular seed oils, canola, corn, cottonseed, grapeseed, soy, rice bran, sunflower and safflower – and blame them for causing heart disease and type 2 diabetes.

Are seed oils really the enemy, or is the antagonism towards them unjustified?

Much of the recent criticism of seed oils focuses on their high omega-6 fatty acid content.

Omega 6 fatty acids are essential fatty acids, which means we need them, but can't produce them ourselves. In recent years some scientists have argued that omega 6 can cause chronic inflammation (which can increase the risk of developing diseases including heart disease and cancer).

But controlled trials have found that omega-6 fatty acids do not increase inflammation, says Dariush Mozaffarian, professor and director of the Food is Medicine Institute at Tufts University in Massachusetts in the US.

"New research shows that omega-6 fatty acids give rise to unique natural molecules, like lipoxins, that have powerful anti-inflammatory effects in the body," says Mozaffarian.

Recent research studied the diet and health of over 200,000 people in the US for around 30 years. The researchers found that people who consumed more plant oils (including seed oils) were less likely to die from cardiovascular disease or cancer over the course of the study. On the other hand, those with a higher intake of butter were more likely to die during the same period.

There are numerous observational studies looking at how omega 6 effects our heart health – where scientists look at data on diet and health, and find associations between the two.

But some observational studies rely on people's own accounts of what they eat, says Matti Marklund, assistant professor of human nutrition at Johns Hopkins Bloomberg School of Public Health in the US. And this, he adds, can be problematic because people may misremember, or even be dishonest, about their dietary habits.

Another way to measure omega 6 intake is to measure the average amount in the individual components and ingredients in a person's diet. However, Marklund adds, it can be difficult to translate what people say they have eaten into certain quantities.

Numerous studies investigating the effects of omega 6 on our health focus on linoleic acid, an omega 6 fatty acid found in high amounts in seed oil, that has been found to lower the 'bad' LDL cholesterol in our blood.

In a 2019 study, Marklund instead focused on the levels of fatty acids in the blood of participants from around 30 observational studies – some which followed people for up to 30 years – and looked at how many developed cardiovascular disease and died from it. He found that those with the highest levels of linoleic acid in blood had the lowest risk of developing cardiovascular disease.

There is some confusion regarding omega 6 and heart health, says Christopher Gardner, director of nutrition studies at the Stanford Prevention Research Center in the US.

This partly stems from omega 6's role in the process of blood clotting, which Gardner says people mistakenly only associate with strokes and heart attacks. Omega 3, he says, tends to be more blood-thinning. "If you had a wound in your hand, you'd want it to clot," he says. "You need balance."

Meanwhile, scientists concluded in a 2019 analysis of 30 studies that people with higher amounts of linoleic acid in their blood were 7% less likely to develop heart disease.

"Linoleic [acid] might improve cholesterol to reduce the risk of cardiovascular disease, and also improve glucose metabolism, which reduces the risk of type 2 diabetes," Marklund says.

Another common accusation levelled at seed oils is that eating too much omega 6 compared to omega 3 is harmful.  

In the Western world, omega-6 fatty acids account for around 15% of our total energy intake. The average person's ratio of omega 3 to omega 6 can be up to 50:1. However, it should be more like 4:1 to reduce our risk of cardiovascular disease, according to one study.

A 2022 World Health Organization review and meta-analysis reported that a higher omega 6:3 ratio was associated with a greater risk of cognitive decline and ulcerative colitis, a chronic inflammatory bowel disease.

On the other hand, a higher omega 3:6 ratio was also linked to a 26% reduced risk of depression. Overall, the scientists involved in the WHO study concluded that a high intake of omega 6 fatty acids from seed oils is unlikely to increase your risk of death and disease – but say that more high-quality research is needed.

But while some scientists argue that you shouldn't have too much omega 6 compared to omega 3, Marklund says it's better to up your intake of omega 3 rather than consume less omega 6, as both are associated with health benefits. 

Unlike other oils, seed oils are extracted from the seeds of plants. There are some concerns that seed oils are extracted with hexane – a chemical made from crude oil – but there is little evidence so far suggesting that this process can cause issues.

While some evidence suggests hexane could be linked to several health conditions, after being extracted, the oil is then deodorised and bleached to remove and additives. 

"Scientists will say hexane extract is normal in food processing, and the deodorising and bleaching removes potentially harmful compounds," says Gardner.

Cold-pressed seed oils avoid this process entirely, as it involves squeezing the seeds to extract the oil – but this results in a more expensive product.

Despite an abundance of research showing the potential benefits omega 6 might have to our health, researchers have recently found that this fatty acid can fuel the growth of a specific type of breast cancer. The findings may have implications for the impact of omega 6 consumption on other diseases, too.

Cancer cells use nutrients as fuel to grow and multiply, but until now, there was limited research looking at the role omega 6 fatty acids play.

But a study published in March this year found a mechanism by which linoleic acid, an omega 6 fatty acid, helps cancer cells to grow and multiply in patients with triple negative breast cancer (TNBC). This is the most aggressive sub-type of the disease, and one that doesn't respond well to targeted therapies.

Previous studies suggest there is no association with omega 6 fatty acids, or a small increase in risk, says Nikolaos Koundouros, postdoctoral associate at Weill Cornell Medicine research centre in New York. But these studies, he says, haven't taken into account that there are multiple sub-types of breast cancer, and that they all differ in terms of a patient's survival and prognosis, and how they might respond to targeted therapy.

TNBC  seems to respond the most to omega 6 linoleic acid, Koundouros says.

Along with a team of researchers, Koundouros discovered in the lab that, when fed omega 6, TNBC cells activate a protein complex linked to tumour growth and progression. Another protein, which is found in larger amounts in TNBC tumours compared to other breast cancer subtypes, is known to take fatty acids and lipids throughout the body and within cells to exactly where they need to be.

Koundouros explains that these proteins, along with omega 6, may also be relevant in other chronic diseases, such as obesity and type 2 diabetes.

This research could potentially inform treatment options for TNBC patients, but it doesn't necessarily have wider implications for everyone, Koundouros says. "It's important to remember omega 6 fats are essential for a reason; if you completely cut them out, you could have detrimental side effects," he says.

Some seed oils – such as canola oil and soybean oil – have been studied more than others, so have a more rigorous evidence base.

"These each provide a balanced combination of healthy fats, including monounsaturated fats, omega-6 polyunsaturated fats, and omega-3 polyunsaturated fats," Mozaffarian says.

Canola oil, Mozaffarian adds, has similar anti-inflammatory effects and produces better improvements in blood cholesterol levels than olive oil, which has long been hailed the healthiest of all oils.

One meta-analysis of 27 trials found that canola oil was found to significantly reduce LDL cholesterol compared to sunflower oil and saturated fat, while another found that it dramatically reduced body weight, especially in in those with type 2 diabetes.

"Canola oil produces excellent benefits for blood cholesterol levels, and also modestly reduces body weight. The healthy fats in canola oil, especially the omega 6 polyunsaturated fats, also improve blood glucose, insulin resistance, and insulin production," says Mozaffarian.

Soybean oil has also been found to improve cholesterol levels compared to saturated fats. One study found that people who consumed more soybean oil had lower risk of death from all causes; a 6% lower risk for every 5g they consumed daily.

"Seeds are one of nature's most nourishing gifts; a package of beneficial healthy fats," says Mozaffarian.

That something so well studied within nutrition science has had such a backlash has been a source of confusion for some scientists. But this misunderstanding may come from a "misplaced combination of partial truths", Mozaffarian says.

For instance, some people may link seed oils to ultra-processed foods (UPFs), which often contain seed oils, particularly canola, corn, soybean, and sunflower oils. In recent years there has been a huge focus on the health risks of consuming too many UPFs, including increased risk of developing type two diabetes and heart disease.

More like this:

• What is the healthiest cooking oil?

• The truth about saturated fat

• Cod liver oil: The fishy fix that was surprisingly healthy

"But these dangers arise from too much starch, sugar, and salt, loss of natural intact food structure and dozens, or even hundreds, of artificial additives," Mozaffarian says.

Some people have also drawn a correlation with increased consumption of seed oils in recent years and the rise in obesity and diabetes.

"But if you want to draw parallels with people eating more seed oils and unhealthy outcomes, it's because we're eating food that tends to contain a lot of sugar and sodium," Gardner suggests. He says there are many ways to consume seed oils at home that don't involve UPFs, such as with a salad or stir-fry.

"I'd hate to see people tossing out seed oils because of this seed oil war," says Gardner. 

Ultimately, while some scientists call for more rigorous trials looking into the effects of seed oil consumption on our health, others, including Marklund, argue that there is already a wealth of good quality trials showing benefits on blood cholesterol, blood glucose and insulin levels for the general population.

"Omega-6 fatty acids are excellent for health," says Marklund. "They have been linked to lower onset of heart disease, stroke, diabetes, and even death from all causes."

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