Are seed oils unhealthy? Why are so many influencers speaking out against them?

Randomised trials of omega-6s do not show an increase in inflammation caused by the fatty acid

If you’ve been online lately, you’ve probably been bombarded with the discourse about seed oils. Once a completely innocuous food that no one really talked about, they have become the enemy of the moment. Robert F Kennedy Jr, the secretary of health and human services in the US, has called them “toxic” and vowed to act against them. There are countless activewear-clad influencers online saying that nothing harms your health more than these products.

Which is odd, because the Australian dietary guidelines say seed oils are basically fine. You should limit them because they’re high in calories but otherwise there’s no major reason – according to the guidelines and most experts – why you should avoid them. As Prof Lauren Ball, director of the University of Queensland’s Centre for Community Health and Wellbeing, says: “There’s no convincing evidence that eating seed oils like canola or sunflower oil in normal dietary amounts harms our health. In fact, these oils contain unsaturated fats, which are generally beneficial for cardiovascular health.” Dr Jason Wu, a professor at the University of New South Wales, agrees: “The evidence base consistently points to health benefits of eating seed oils.”

Seed oils are a group of cooking oils that are, unsurprisingly, extracted from seeds. They’re generally quite highly refined – although you can get less refined versions of many of them – and include canola oil, sunflower seed oil, rice bran oil, soya bean oil and corn oil. Most of the time vegetable oil is composed of seed oils – it’s often a mixture of sunflower seed and canola oils.

The concerns about seed oils boil down to one thing: inflammation. It is asserted that the omega-6 fatty acids contained in most seed oils such as linoleic acid can be harmful in large doses and may cause the body to become more inflamed over time. This isn’t entirely crazy – there is some evidence from rodent studies that omega-6s cause inflammation, and that having too many of them can have long-term negative outcomes.

The problem, of course, is that lab rats and mice aren’t people. Very high levels of omega-6s can cause liver dysfunction and gut issues in rats but that doesn’t mean that a bit of sunflower seed oil is going to destroy your intestines.

Data from humans doesn’t really show this connection. People who eat more linoleic acid have substantially lower risks of cardiovascular disease than those who eat more. Studies looking at supplementing seed oils in people’s diets have shown no major impacts – and sometimes even modest benefits – compared with placebos.

Human studies also don’t support the inflammation link. Randomised trials of omega-6s do not show an increase in inflammation caused by the fatty acids. A randomised crossover study in which the same people were randomly given either omega-3 or omega-6 fatty acids found that there was no difference in how they affected inflammatory markers. Evidence from careful analyses of genetics and causality in humans show little indication that omega-6 fatty acids are inflammatory. As Wu puts it, the pro-inflammatory effects of omega-6 fatty acids are “not supported by evidence”.

On the other hand, we know that animal fats are probably bad for you. They contain high levels of low-density lipoprotein – bad cholesterol – which goes on to clog arteries and cause heart attacks. Ball notes: “Replacing saturated fats, such as those in butter, with unsaturated fats in oils like canola and sunflower can lower LDL cholesterol, an important risk factor for cardiovascular disease.”

In other words, the scientific perspective is simple: seed oils are fine. They’re high in calories but otherwise perfectly healthy, and if you’re eating them to replace saturated fat then they’re probably going to improve your health. So why are there crazy-eyed men yelling at canola oil in the supermarket aisle on Instagram?

The story here is mostly about dietary fads. Favourite diets online now are quite high in animal fats. If you’re promoting your new book about keto diets, it’s not great for sales if everyone thinks that eating too many steaks is going to give them heart disease. Yes, there were some scientific concerns but those have been largely disproven in the literature already.

There are a lot of people scaremongering about seed oils online, which makes sense. You get a lot more attention for yelling that we’re all being poisoned than by calmly explaining the science. I doubt the furore about seed oils is going away any time soon.

But from a health perspective you’re probably better off eating seed oils than any other fats. You should limit consumption, because too many calories is not great, but whether you’re frying, making mayonnaise or whatever, it’s likely that the healthiest choice is a seed oil of some kind.

Dr Gideon Meyerowitz-Katz is an epidemiologist and science communicator and a senior research fellow at the University of Wollongong

Antiviral is a fortnightly column that interrogates the evidence behind the health headlines and factchecks popular wellness claims