It’s curious that something so bland could cause so much controversy. Most of us have a bottle of seed oil, normally called vegetable oil in the UK, in our kitchens – a nearly tasteless but very useful fat that has been a commonplace cooking ingredient for decades.
And yet this previously unremarkable golden liquid has sparked online furore and vicious debate. Nutrition influencers on social media have described it as “toxic”, “inflammatory”, “unnatural” and the root cause of the obesity epidemic.
The US health secretary Robert F Kennedy Jr, who has caused controversy with his views on subjects from vaccines to fluoride in drinking water, has said the population is being “unknowingly poisoned” by seed oils and urged people to revert to “traditional” fats such as butter, lard and beef dripping for better health.
Last month the Wall Street Journal reported that fast food chains were promoting their shift away from seed oils after Kennedy’s criticisms. He even made a televised visit to a branch of Steak ’n Shake to praise its decision to cook fries in beef tallow instead.
So should we really be ditching our bottles of vegetable and sunflower oils and covering everything in lard?
Seed oils have been in widespread use since about the 1950s and, as well as being used for home cooking, are also in many ultra-processed foods. They include rapeseed (known as canola in the US and generally labelled as vegetable oil in the UK), sunflower, soya bean, corn, grapeseed, rice bran, sesame and safflower. While you can buy cold-pressed seed oils, the most common production method involves using a solvent (normally hexane) to extract the oil from the plant. It is correct that hexane is a toxic substance, but it is almost entirely removed from the final product by the refining process – the EU allows a maximum residual limit of 1mg per kilo.
The refining process includes bleaching and deodorising, both of which critics have jumped on to claim that seed oil is “unnatural” and therefore “bad”.
Tom Sanders, emeritus professor of nutrition and dietetics at King’s College London, who has spent his career researching dietary fat and health, explains: “The processing actually takes out potentially toxic material.”
Sarah Berry, professor of nutritional sciences at King’s, agrees: “The end product, in my opinion, is very safe to eat.”
The next allegation against seed oils is that they are “inflammatory”. This assumption is based on a fundamental misunderstanding of the science, says Berry.
Seed oil critics claim that the type of omega-6 fatty acid present in them (called linoleic acid) can be inflammatory, whereas omega-3 – the other essential polyunsaturated fatty acid, found in foodstuffs such as oily fish, flaxseed and chia seeds – can reduce inflammation.
“Because the enzymes used to convert omega-3 into anti-inflammatory chemicals are the same ones used to convert omega- 6, their argument is that having too much seed oil will mean the enzymes are stolen away from the omega-3,” says Berry.
“This isn’t true. It’s true from a theoretical biochemical pathway. It’s true in mice upon unrealistic stimuli. But it is absolutely not true in humans.” In fact, randomised, controlled trials show that linoleic acid has either a neutral or, in most studies, an anti-inflammatory effect in humans.
“The idea that linoleic acid is some sort of toxic thing is absolute nonsense,” says Sanders. “It’s an essential nutrient. Of the essential fatty acids it’s the most important one. If you’re deficient, it impairs immune function and platelet function doesn’t work.”
It also has a potent cholesterol-lowering effect, says Berry, who is chief scientist at nutrition company Zoe. “It has been shown to reduce blood cholesterol significantly. Because of this and based on the current evidence I would say that not only are seed oils not bad for us, they are a healthy part of our diet.”
Sanders attributes much of the decline in cardiovascular disease we’ve seen in the past 50 years to our increased consumption of seed oils. A few weeks ago, a study that followed 200,000 adults over 33 years found that those who replaced a tablespoon of butter a day with the same amount of plant-based oil such as soya bean or rapeseed had a 17% reduction in risk of death from all causes. The study, which was published in JAMA Internal Medicine also found a 17% reduction in risk of death from cancer.
“Our study found that higher butter intake was associated with increased deaths from all causes and cancer, while higher intake of plant-based oils was associated with lower deaths from all causes, cancer and cardiovascular disease,” said lead study author Yu Zhang, a graduate student at Harvard TH Chan School of Public Health.
Priya Tew, from Dietitian UK, says some of the confusion might have come from a 1960s study: “It showed men with heart disease had a higher intake of seed oils. But this was through margarines that also contain trans fats, which we know increase the risk of heart disease.”
A similar logic applies to the argument that, as our intake of seed oils has risen – which it has more than 200-fold over the past 50 to 70 years – so too have our rates of chronic disease.
“Association does not mean causality,” says Berry. “Think what else has changed; our food landscape is almost unrecognisable compared with 70 years ago. It’s estimated 60% of the seed oils we consume come from ultra-processed food which has many other chemicals that are unhealthy for us and processes that affect the healthfulness of the food.”
In other words, it’s not the seed oil that’s the problem.
Berry’s recent statements about seed oils have landed her in hot water. After appearing on a podcast explaining that seed oils are healthy, she received relentless hate mail, including being told she’s “the most hated scientist in America”.
“It nearly got to the point where I was going to stop speaking out on the topic so I didn’t have to be subjected to such horrible comments and meanness. But then I thought, that’s exactly what they want. They want to shut down the real evidence, so it just galvanised me to speak out about it even more.”
As always, with nutrition, it’s better to consider overall diet than to hyper-fixate on one ingredient. But these kind of messages don’t tend to get as much traction. “Human nature is such that we are more susceptible to risk and scare headlines,” Berry says. “They’re going to get more clicks than a balanced, boring nutrition scientist like myself saying seed oils are fine as part of a balanced diet.”
Sanders says you don’t have to ditch your seed oils and you shouldn’t swap them for butter or lard. “The seed oil scare is all just gossip. It’s not based on any good science at all.”
