Seed oils probably aren’t something you’ve given much thought to, but they make up a large part of our daily diets and can be found in everything from frozen meals and bread to baby food and canned foods.
According to 2023 research in the journal Nutrients, more than 25 percent of the calories in our diets come from eight seed oils: canola, sunflower, flaxseed, corn, grapeseed, soybean, rice bran and safflower oil.
But there is growing concern that they could be bad for our health. They are thought to increase inflammation, which in turn is linked to conditions such as type 2 diabetes, heart disease, depression and Alzheimer’s disease. And two new studies have suggested that seed oils may increase our cancer rates.
So how bad are seed oils? We spoke to the experts to find out.
What are seed oils?
Comes from the seeds of plants, which have been dried, steamed and then processed to release the oil. They contain a high level of omega-6 fatty acids.
The problem is that while mechanically pressing seed oils – where the oil is literally pressed from the seed – tends to retain the nutrients and flavors, more modern methods of producing the oils on a large scale and with a longer shelf life can mean that complicated steps and use chemicals such as peroxides, which create a bland-tasting oil and toxic byproducts.
Seed oils provide an inexpensive source of fat, which is why they are so widely used, especially in ultra-processed foods (or UPFs), including convenience foods.
Seed oils make up a large part of our daily diets and can be found in everything from frozen meals and bread to baby food and canned foods.
Are they ‘bad’ for you?
The two new studies certainly make for alarming reading.
When researchers at the University of South Florida analyzed colon cancer tissue from 81 people, they found it contained high amounts of inflammatory omega 6, microscopic fatty compounds produced when the body breaks down seed oils.
They are thought to promote inflammation, which causes cancers to grow, and prevent the body from fighting the tumors.
Tim Yeatman, professor of surgery and lead author of the study, told Good Health: ‘Eating excess omega 6 fatty acids results in an excess of pro-inflammatory lipid (i.e. fat) mediators, resulting in an immunosuppressed environment of colon cancer. And I think it allows the cancer to grow and thrive and reproduce.”
Another study, published in Clinical Oncology, found that in men with early prostate cancer, those who eliminated seed oils from their diet (including processed foods) and consumed more foods rich in omega-3 fatty acids, such as salmon, had a slower progression of the disease. .
“Our findings suggest that something as simple as changing your diet can slow cancer growth and extend the time before more aggressive interventions are needed,” said William Aronson, professor of urology at the University of California, Los Angeles, and lead author of the study. study.
Is it linked to diabetes/dementia type 2?
The rise in obesity and type 2 diabetes has been linked to our increased intake of seed oils, but Sarah Berry, professor at the Department of Nutritional Sciences at King’s College London, suggests that the growing amount of processed food in our diets may be the real cause. cause.
“Many ultra-processed foods contain seed oils, but they are also high in sugar, salt and other additives, while lacking nutrients,” she explains.
“So eating a lot of ultra-processed foods that contain seed oils may 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 laboratory research shows that vegetable oil is more likely than olive oil to cause plaque buildup in the brain.
Dr. Catherine Shanahan, an American doctor and author of Dark Calories: How Vegetable Oils Destroy Our Health And How We Can Get It Back, previously wrote in the Mail: ‘Plant oils can make you feel foggy and tired, trigger migraines and even eventually cause diseases such as dementia.
‘Vegetable oil causes oxidative stress, which damages membranes and results in the formation of plaque in the brain.’
But isn’t eating seeds good for you?
In the right amounts, omega-6 fatty acids are beneficial, even essential, to our health. For example, linoleic acid – the most common form of omega 6 found in seed oils – helps maintain healthy skin and is 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 is also thought to help lower levels of ‘bad’ cholesterol and improve blood sugar control – ‘omega 6 fatty acids are associated with a reduced risk of developing diseases such as metabolic syndrome and type 2 diabetes’ , she says, adding, “It’s only to the extent that it’s a problem.”
This is partly because linoleic acid is converted in the body into arachidonic acid, which has a pro-inflammatory effect. ‘Many experiments have shown that arachidonic acid promotes obesity, inflammation and tumor growth,’ adds Professor Yeatman.
Processing seed oils can also lead to more toxins, he says.
More than 25 percent of the calories in our diet come from eight seed oils: rapeseed, sunflower, linseed, corn, grapeseed, soy, rice bran and safflower oil.
“Commercial processing may include bleaching and deodorization steps that break down the seed oils and produce hazardous byproducts.”
The problem is that most people consume much more omega-6 fatty acids than omega-3 fatty acids – 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 will cause inflammation in the skin. body. However, this is not clear.
Professor Berry says: ‘As long as you get enough omega 3, it’s fine to consume omega 6. And it has a protective effect against many chronic diseases.’
A minimum of 250 mg and a maximum of 4,000 mg omega 3 is usually mentioned as beneficial.
Professor Yeatman adds that eating seed oils in moderation is healthy. “I’m not demonizing seed oil,” he says. “But if you eat it at breakfast, at lunch and in your salad dressing at dinner, it adds up and that’s the problem.”
He recommends avoiding UPFs and reading food labels.
Is it okay to cook with it at home?
Heating seed oils to high temperatures, such as frying, can increase the levels of toxins, according to Professor Yeatman.
A particular problem is if you heat seed oils to a high temperature and then reuse them repeatedly, as levels of toxins can build up, Dr. Shanahan said.
‘A fundamental problem with these oils is that they contain very high levels of polyunsaturated fatty acids (PUFAs). These compounds are sensitive to reactions with oxygen, a process called oxidation.’ This oxidation creates new compounds called lipid oxidation products (LOPs), which are toxic.
‘Toxicologists who conduct field tests on vegetable oils in people’s homes and restaurants are discovering that even before cooking, the oils contain higher concentrations of toxins than when they were first bottled. Even more toxins are created when the oil is heated to make food,” says Dr. Shanahan.
More toxins will form as the leftovers are “reheated,” she adds.
Another problem is aldehydes; these form when seed oils are heated to a high temperature.
Martin Grootveld, professor of bioanalytical chemistry and chemical pathology at 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 health and nutrition stakeholders took acrylamides and their toxicological properties very seriously – but heated PUFA-derived acrolein, which inflames the lungs when inhaled, is almost never mentioned.’
In 2019, he published a study in Nature showing that a 150ml portion of fries cooked in vegetable oil contained 25 times more carcinogenic aldehydes than the World Health Organization’s tolerable upper exposure limit.
Dr. Shanahan’s advice is to avoid fried foods when eating out.
Should I switch to olive oil?
Professor Yeatman suggests using other oils with more omega-3 fatty acids, such as olive oil and avocado oil.
‘Olive oil has many proven health benefits and is a central part of the Mediterranean diet, which has consistently been shown to be the healthiest eating pattern. Avocado oil also has benefits for heart and eye health, and much more.”
You may have heard people talk about an oil’s “smoke point”: when it starts to smoke during cooking and the fatty acids in it begin to break down, creating harmful compounds.
Professor Berry says there is a lot of misinformation about it: ‘It is true that heating certain cooking oils at high temperatures for long periods of time can produce compounds that are unhealthy,’ she says.
“But in a home kitchen, this isn’t something to worry about.” Reusing oil can have the same effect, but that’s something people never do at home, and there is legislation to ensure restaurants don’t do it either.”
Dr. Shanahan argues that the food oil industry has “inculcated the false idea that in order for an oil to be suitable for cooking, it must have a high smoke point” – “unfortunately, even if an oil has a high smoke point, the PUFAs remain in the ( ‘natural’ vegetable) oil…breaks down invisibly in a harmful way, long before they start smoking’.
What about butter?
The current advice from the British Heart Foundation is that it is better for our hearts to replace saturated fats with unsaturated fats: ‘Simply switching from butter to margarine spread is one way to do this.’
Other experts strongly disagree and argue against processed oils.
Dr. Shanahan advocates 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 oil (almond, hazelnut, pecan oil, etc.).