Seed oils have suddenly become a topic of hot debate in health circles. First came unsubstantiated claims by the incoming head of the US Department of Health, Robert F. Kennedy Jr, that Americans are being “poisoned” and should switch to cooking with animal fat, then, earlier this month, two studies published in respected scientific journals appeared to suggest seed oils might be implicated in cancer growth.
‘Seed oils’ are refined oils used in cooking, including rapeseed oil – better known as canola oil in the US – as well as sunflower, safflower, grapeseed, sesame and peanut. Modern diets include more of them than ever before, as they are widely used in commercial kitchens (think restaurant and takeaway food) and in many of the processed and ultra-processed foods we buy in the supermarket.
So what’s the truth about them?
What the latest studies really tell us
Colorectal or bowel cancer is the third most common cause of cancer-related death in the UK, and among young people, rates are rising. From the early 1990s to 2018, the number of adults aged 25 to 49 diagnosed with bowel cancer in the UK increased by 22 per cent, leading a global trend. So it’s understandable that searching for a ‘cause’ is at the forefront of many minds.
As part of this search, scientists at the University of Florida inspected 162 samples from human colorectal tumours and looked at the ‘lipid mediators’ within them. Lipid mediators are cell-signalling molecules in the body largely derived from the foods we consume: the researchers found within the tumours an excess number of inflammation-promoting lipid mediators, paired with a shortage of the type that promote healing. These inflammatory molecules include omega-6 fatty acids, while the healing molecules, omega-3s, were practically absent.
Omega-6 fatty acids are also present in large amounts in, you guessed it, seed oils. The results of the study led its lead author, Professor Timothy Yeatman, to state that “a human’s immune system can be extremely powerful and drastically impact the tumour microenvironment, which is great if harnessed correctly for health and wellness, but not if it’s suppressed by inflammatory lipids from processed foods.
“We now see this inflammation in the colon tumours themselves, and cancer is like a chronic wound that won’t heal – if your body is living off of daily ultra-processed foods, its ability to heal that wound decreases due to the inflammation and suppression of the immune system that ultimately allows the cancer to grow.”
However, while what we eat does have a role in the production and growth of every cell in the body, the link established in this particular study is far from a direct one, says Dr Megan Rossi, a dietitian and postdoctoral research fellow at King’s College London.
“This study has not directly connected the fats found in the tumours to actual foods or to these seed oils. It may be a stepping stone to such a discovery, but looking just at tumour biopsies, there are too many inferences that need to be made in order to suggest that this is all down to having some sunflower oil in your food.”
It’s also worth noting that Professor Yeatman pointed to ultra-processed foods (UPFs) – and the seed oils within UPFs specifically – as being a concern, rather than to seed oils alone.
Another study, which hit the headlines around the same time last week, provided additional context to the University of Florida findings.
Published in the Journal of Clinical Oncology, research by medics at the University of California Los Angeles showed that a diet low in omega-6 and high in omega-3 fatty acids, combined with fish oil supplements, significantly reduced the growth rate of prostate cancer cells in men with early stages of the disease. After a year, the group of patients on the high omega-3 diet had a 15 per cent decrease in a marker of cancer progression, while the control group – who had no changes to their diet – saw a 24 per cent increase.
“In many ways this study is more about the benefits of omega-3s, which we absolutely do know are anti-inflammatory – these are the kinds of fatty acids you see plenty of in olive oil and in the Mediterranean diet we always recommend,” says Dr Rossi.
“But it’s important to note that this study saw people take out a load of ultra-processed foods from their diets and increase the amount of healthful foods, including omega-3 supplementation. So more has changed than just the omega-6 to omega-3 ratio: the amount of fibre would have changed, the polyphenol [plant chemical] content would have changed, all the things that we know feed the microbiome, which then has a role in reducing inflammation, too.”
Is it seed oils or UPFs we should worry about?
Scientists – including the authors of this latest research – are open about the fact that there may be greater complexities at play behind the results of these studies.
The fact is, we actually need omega-6 fatty acids in our diets – they are not something optional that we can completely erase. “The main omega-6 fatty acid is linoleic acid, and that’s an essential fatty acid – our bodies need it for certain metabolic functions,” explains Dr Rossi. “And when we’ve looked at studies supplementing with isolated linoleic acid, it doesn’t seem to have this inflammatory cascade at all.”
Many clinical trials have shown that replacing saturated fat with linoleic acid is actually good for us, decreasing levels of bad cholesterol – hence advice from heart health experts to switch animal fats for polyunsaturated fats like seed oils and olive oil.
But there is an ideal ratio of omega-6 to omega-3, thought to be between 2:1 and 4:1, say the experts. The problem may be that we are now consuming omega-6s far in excess of this, with a typical Western diet seeing intakes of between 15:1 and 17:1.
Why are our diets so heavily in favour of omega-6? It’s not because we’re cooking with a little bit of rapeseed or sunflower oil at home now and then: it’s because of the way our entire culture of eating has changed in the last half a century. Many experts agree that ultra-processed food – combined with the heavy use of seed oils and their like as part of their production – are the reason. “In the ultra-processed food world, ratios of omega-6 to omega-3 are more like 20:1,” adds Dr Rossi.
Processed oat milks, ready meals, biscuits and even crackers and breads may all contain high levels of omega-6 fatty acids that if made from scratch, would contain none at all.
The lead author of the gut study recognised this when he said: “I’m not demonising seed oil. But when you eat it at breakfast, at lunch, and in your salad dressing at dinner, it adds up – and that’s the problem.”
The thing is, unless we scrutinise the label of every processed food we eat, we’d never know.
And the broader scientific community still has lots of questions they want answered before they state anything as definitive as that seed oils, on their own, cause cancer.
“Is it just the omega-six to omega-3 ratio? Or is it the way those oils get processed and change during processing? Is it the fact that ultra-processed foods contain so many emulsifiers? Is it that they contain hardly any fibre, when fibre intake has been shown to greatly reduce the risk of bowel cancer? The take home for me is: remove or switch out these ultra-processed foods, which happen to have the seed oils in them, and eat more wholesome foods,” says Dr Rossi.
“The same goes with cooking oil. We’ve always known that extra virgin olive oil is the ultimate oil, for its omega-3 and polyphenol content – choosing it over seed oils where possible can only be a good choice.”
So what oils should we use for cooking our foods?
Different oils have different qualities, and choosing the right one depends on the intended purpose, says dietitian Nichola Ludlam-Raine, author of How Not To Eat Ultra-Processed. Alongside Dr Rossi, she says extra virgin olive oil – sometimes referred to as EVOO – is the number one oil most of us should choose for cooking and eating. “It’s great for dressings, drizzling, and low to medium-heat cooking, as it’s rich in monounsaturated fats and antioxidants. It’s a versatile oil that’s been well-studied for heart health benefits,” she says.
Following that, Ludlam-Raine recommends avocado oil – “good for high-heat cooking like frying due to its high smoke point, and it’s rich in monounsaturated fats” – and in fact, next on her list come three seed oils: rapeseed oil for high heat cooking, sesame and flaxseed for dressings. She recommends cold pressed or extra virgin versions of these oils, which will ensure they are the most nutritious version of the product available.
What you shouldn’t do, urges Dr Rossi, is what many seed oil detractors suggest – swap cooking with seed oils for cooking with animal fats such as butter.
“If you’re going to ditch seed oils for olive oil then I say great, go for it – it can only be beneficial for your health, whereas currently I would consider cooking with seed oils more of a ‘neutral’ health choice. But if you’re going to switch seed oils for drenching everything in butter, I’d be concerned. Consistently the evidence still shows that replacing butter or other saturated fats with seed oils – unsaturated and polyunsaturated fats – is better for reducing inflammation in your body, among other health benefits.”
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