Truth about cooking oils and cancer: They’ve also been linked to dementia and diabetes. Now doctors reveal exactly what temperature fills them with cancerous toxins, which are the worst… and how to protect yourself

Seed oils are probably not something you’ve given much thought to – but they make up a huge part of our everyday diets, found in everything from frozen meals and bread to baby formula and tinned food.

In fact, more than 25 per cent of the calories in our diets come from eight seed oils: rapeseed, sunflower, flaxseed, corn, grapeseed, soybean, rice bran and safflower, according to 2023 research in the journal Nutrients.

But there’s growing concern they may be bad for our health – they’re thought to increase inflammation, which in turn is linked to conditions such as type 2 diabetes, heart disease, depression and Alzheimer’s. And two new studies have suggested seed oils may be driving up our cancer rates.

So how bad are seed oils? We spoke to the experts to find out.

What are seed oils?

Derived from the seeds of plants, which have been dried, steamed and then processed to release the oil. They contain high levels of omega-6 fatty acids.

The problem is that while mechanical pressing of seed oils – where the oil is literally squeezed out of the seed – tends to retain the nutrients and flavours, more modern methods to produce the oils on a large scale and with a longer shelf-life can involve complicated steps and use chemicals such as peroxides, which create a bland-tasting oil and toxic by-products.

Seed oils provide a cheap source of fat, which is why they’re used so widely, especially in ultra-processed foods (or UPFs), including ready meals.

Are they ‘bad’ for you?

The two new studies certainly make for alarming reading.

When researchers at the University of South Florida analysed colon cancer tissue from 81 people, they found it had large amounts of inflammatory omega 6, microscopic fatty compounds, which are produced when the body breaks down seed oils. 

They’re thought to promote inflammation, which helps cancers grow, and prevent the body from fighting the tumours.

Tim Yeatman, a professor of surgery and lead author of the study, told Good Health: ‘Eating excess omega 6s results in excess pro-inflammatory lipid [i.e. fats] mediators, which results in an immuno-suppressed environment of the colon cancer. And I think it allows [the cancer] to grow and prosper and propagate.’

Another study, published in Clinical Oncology, found that in men with early prostate cancer, those who removed seed oils from their diet (including processed foods) and consumed more foods rich in omega 3s, such as salmon, the disease grew more slowly.

‘Our findings suggest something as simple as adjusting your diet could slow cancer growth and extend the time before more aggressive interventions are needed,’ said William Aronson, a professor of Urology at University of California, Los Angeles, and the study’s lead author.

Is it linked to type 2 diabetes/dementia?

The rise in obesity and type 2 diabetes has been linked to our increased intake of seed oils, but Sarah Berry, a professor in the Department of Nutritional Sciences at King’s College London, suggests the growing amount of processed food in our diets may be the actual cause.

‘A lot of ultra-processed foods contain seed oils – but they also contain high levels of sugar, salt and other additives, while lacking nutrients,’ she explains. 

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

When it comes to dementia, emerging lab-based research shows vegetable oil is more likely than olive oil to cause plaque build-up in the brain.

Writing previously in the Mail, Dr Catherine Shanahan, a US physician and author of Dark Calories: How Vegetable Oils Destroy Our Health And How We Can Get It Back, said: ‘Vegetable oils can make you feel foggy and fatigued, cause migraines and even trigger diseases such as dementia down the line.

‘Vegetable oil causes oxidative stress, which damages membranes and results in plaque building up on the brain’.

But isn’t eating seeds good for you?

In the right amounts, omega 6s are beneficial, even essential, for our health. For example, linoleic acid – the most common form of omega 6 in seed oils – helps maintain healthy skin and forms a part of cell membranes.

‘And evidence shows that people who have a higher intake of linoleic acid have a lower risk of dying from heart disease,’ Professor Berry told Good Health.

It’s also thought to help lower levels of ‘bad’ cholesterol, and improve control of blood sugar levels – ‘omega 6s are associated with a reduced risk of developing diseases such as metabolic syndrome and type 2 diabetes’, she says, adding: ‘It is only in excess that it’s a problem.’

That’s partly because linoleic acid is converted into arachidonic acid in the body, which is pro-inflammatory. ‘Many experiments have shown that arachidonic acid promotes obesity, inflammation and tumour growth,’ adds Professor Yeatman.

The processing of seed oils can also lead to more toxins, he says.

‘Commercial processing can include bleaching and deodorising steps which degrade the seed oils and produce dangerous by-products.’

The issue is most people consume a lot more omega 6s than omega 3s – the estimated ratio in the modern western diet is around 16 to 1: it should be closer to 1 to 1. If not, the argument goes, this causes inflammation in the body. However, this is not clear- cut. 

Professor Berry says: ‘As long as you are consuming adequate omega 3, it’s perfectly fine to be consuming omega 6. And it has a protective effect against many chronic diseases.’

A minimum of 250mg and a maximum of 4,000mg of omega 3 is typically cited as beneficial.

Professor Yeatman adds that eating seed oils in moderation is healthy. ‘I’m not demonising seed oil,’ he says. ‘But when you eat it at breakfast, at lunch, and in your salad dressing at dinner, it adds up and that’s the problem.’

He advises avoiding UPFs and reading food labels.

Is it ok to cook with it at home?

Heating seed oils to high temperatures, as in deep frying, can increase levels of toxins, according to Professor Yeatman.

A particular issue is if you heat seed oils to a high temperature then reuse them repeatedly, as levels of toxic compounds can build up, said Dr Shanahan.

‘A basic problem with these oils is that they are very high in polyunsaturated fatty acids (PUFAs). These compounds are prone to reacting with oxygen — a process called oxidation.’ This oxidation creates new compounds called lipid oxidation products (LOPs), which are toxic.

‘Toxicologists who perform real-world tests on vegetable oils in people’s homes and restaurants find that even before cooking, the oils contain higher concentrations of toxins than when they were first bottled. Even more toxins form when the oil is heated to make food,’ says Dr Shanahan.

More toxins will form if the leftovers are ‘reheated’, she adds.

Another concern is aldehydes – these form when seed oils are heated to a high temperature.

Martin Grootveld, a professor of bio analytical chemistry and chemical pathology at De Montfort University, has been studying the formation of aldehydes in heated seed oils for decades.

He told Good Health: ‘The most toxic aldehyde, acrolein, is more toxic than acrylamide.

‘All those involved in the health and nutrition fields took acrylamides and their toxicological properties very seriously – but heated PUFA-derived acrolein, which inflames lungs when inhaled, is hardly ever mentioned.’

In 2019, he published a study in Nature showing a 5oz serving of French fries cooked in vegetable oil contained 25 times more carcinogenic aldehydes than the World Health Organisation’s tolerable upper limit for exposure.

Dr Shanahan’s advice is to avoid deep-fried food if eating out.

Should I swap to olive oil?

Professor Yeatman suggests using other oils with more omega 3s such as olive and avocado oil.

‘Olive oil has many proven health benefits and is a central component in the Mediterranean diet consistently shown to be the healthiest eating pattern. Avocado oil, too, has benefits for heart and eye health and more.’

You might have heard people talk about the ‘smoke point’ of an oil – when it starts to smoke on cooking and the fatty acids within it start to break down, forming harmful compounds.

Professor Berry says there’s a lot of misinformation about it: ‘It’s true that heating some cooking oils for long periods at high heat can produce compounds that are unhealthy,’ she says.

‘But in a home kitchen, this isn’t something to worry about. Re-using oil can have the same effect, but that’s not something people ever do at home, and there is legislation in place to make sure restaurants don’t do it either.’

Dr Shanahan argues the food oil industry ‘has drilled home this false idea that for an oil to be suitable for cooking it needs to have a high smoke point’ – ‘unfortunately, even if an oil has a high smoke point, the PUFAs in the [‘natural’ vegetable] oil…are invisibly breaking down in a harmful way long before they start smoking’.

What about butter?

Current advice from the British Heart Foundation is that it’s better for our hearts to replace saturated fats with unsaturated: ‘Making the simple swap from butter to margarine spreads is one way to do this.’

Other experts strongly disagree, arguing against processed oils.

Dr Shanahan argues for using butter, ghee, extra-virgin olive oil or unfiltered refined olive oil, unrefined peanut oil, unrefined coconut oil, unrefined avocado oil, sesame oil, unrefined palm oil, bacon fat, tallow, lard, chicken fat or unrefined tree nut oils (almond, hazelnut, pecan, etc).