What science actually says about seed oils

If you consume social media, you may have heard: Seed oils are terrible for your health–even toxic! Cooking oils derived from seeds cause everything from heart disease to inflammation to fatigue to bad skin–according to a certain subset of Internet influencers. Yet contrary to the posts demonizing the common ingredients, a bevy of scientific research disagrees. Here’s how to understand the health “scare.”

What are seed oils?

There’re many different types of plant-based cooking oils, but when people talk about seed oils, they’re often referencing a list of eight: Canola, corn, cottonseed, soybean, sunflower, safflower, grapeseed, and rice bran oils. (Note that things like olive, avocado, and coconut oil are absent from this list.) All of these eight oils contain fat and therefore fatty acids (an essential major nutrient group). And many (though not all) of these seed oils contain a relatively high proportion of omega-6 fatty acids.

A quick chemistry aside: fatty acids are the building blocks of triglycerides, or complete fat molecules. They are organic compounds made up of predominantly carbon and hydrogen chains with an acid group on the end. In saturated fats, every carbon except for the terminal ones have two hydrogens bonded to it. In unsaturated fats, some of those hydrogens are replaced with double bonds between adjacent carbons instead. Omega-6 fatty acids are unsaturated, and the first of those double bonds occurs at the 6th carbon from the end–hence the name.

There are multiple kinds of omega-6 compounds, but one particular type, called linoleic acid, is at the center of most of the scorn against seed soils. Linoleic acid is, again, an essential nutrient that our bodies need. We cannot synthesize it, and we need it to support healthy cell signaling, function, and immune systems.

But seed oil detractors argue that we are ingesting far too much linoleic acid, leading to the accumulation of byproducts like arachidonic acid, which they claim causes inflammation and also counteracts the benefits of eating omega-3 fatty acids. The domino impact of all of this, anti-seed oil advocates assert, is higher risk of diseases like type 2 diabetes, heart disease, and cancer.

A kernel of truth

Inside the backlash against seed oils are a few kernels of truth. Eating fried and processed foods in excess is generally detrimental to your health. So if avoiding seed oils translates into eating fewer french fries and snack cakes, you might feel better.

Plus, if you eat a typical western diet, you are probably at no risk of linoleic acid deficiency, and you likely ingest more omega-6 fatty acids than omega-3s. In recent decades the amount of linoleic acid in our diets has increased because many processed foods and restaurant meals are made with soy, sunflower, or safflower oils and animal feed now contains a lot of soy, which translates to more linoleic acid inside meat and dairy products, says Philip Calder, a nutrition scientist and professor at the University of Southampton in England. “Linoleic acid has permeated the food chain in the last 50 to 60 years,” he tells Popular Science.

Additionally, Calder explains that there is “theoretical evidence” that linoleic acid can be partially converted into arachidonic acid, which is subsequently partially converted into compounds associated with inflammation. Additionally, omega-6s and omega-3s can compete for the same metabolic pathways. All those biological mechanisms exist in the human body.

Yet here is where things get sticky: that theoretical argument doesn’t stack up to scientific observation. “That just really doesn’t happen in real life,” says Guy H. Johnson, a nutrition scientist and adjunct professor of food science and human nutrition at the University of Illinois Urbana-Champaign. “If you’ve got enough omega-3, the inflammatory environment isn’t increased by omega-6s.”

What the research indicates

Calder agrees. “Most human studies either show there isn’t a relationship between linoleic acid intake and inflammation biomarkers, or that the relationship is the opposite to what you might think would happen. You see higher linoleic acid and higher arachidonic acid are associated with lower levels of inflammation biomarkers,” he says. He co-authored a 2018 review study assessing the published literature on inflammation and omega-6s and concluded as much.

“We didn’t find anything that demonstrates there’s a harmful association between omega-6’s and inflammatory markers in humans,” he adds. A 2012 review co-authored by Johnson found the same thing.

Many other review studies and meta-analyses have come to similar conclusions, and additionally finding pluses where you might expect minuses. “Every time anybody looks at blood levels of omega-6s and health outcomes–and we’ve done this several times with thousands of people… you find that people with the highest levels of omega-6s have the best outcomes,” says William S. Harris, a professor at the University of South Dakota’s Sanford School of Medicine and president of the Fatty Acid Research Institute.

Harris has co-authored multiple human cohort studies as well as large review papers assessing the impacts of omega-6 fatty acids on health. In a 2017 meta-analysis, he and his co-authors found that omega-6 consumption actually lowers risk of type 2 diabetes. In a 2020 review of 30 observational studies, Harris and his colleagues concluded that higher linoleic acid levels are associated with reduced risk of cardiovascular disease.

In fact, higher linoleic acid intake is associated with lower risk of death from all causes including heart disease and cancer, according to another 2020 meta-analysis that assessed 38 different studies. I could keep linking studies–there are dozens of them, but you probably get the point.

The way fatty acids and metabolic processes unfold in the body is complicated. “There’s a nuanced interplay between omega-6s, omega-3s, and a variety of other metabolites,” Harris says. The view that omega-3s are good and omega-6s are bad “is not true and is far too simplistic,” he adds.

There are a couple of legitimate, contrary bits of research out there, say both Harris and Calder. Including two, often-cited papers published by lead author Christopher Ramsden, chief of the Lipid Peroxidation Unit in the National Institute on Aging. In these studies, Ramsden uncovered previously unpublished research from the late 1960’s and early ‘70’s wherein two groups of people fed a diet high in seed oils and margarine showed worse health outcomes.

However, there are big caveats to these findings. For one, the study participants were fed much higher levels of omega-6-containing oils than is common in diets today, notes Harris. Plus many of the solid margarines the study used likely contained high amounts of trans fats, which are uniformly understood to be harmful to human health, says Calder.

Another concern that the seed oil skeptics cite is the use of hexane in production. “It’s true that hexane is used to extract vegetable oils from whatever their source is,” notes Johnson, who has written multiple health claim petitions on various oils. “But the product that consumers buy in the grocery store has no hexane in there at all. It’s gone,” he adds– removed during processing.

All in all, the vast majority of scientific evidence indicates that cooking with omega-6 containing oils is harmless and probably good for you.

So, what should you eat?

Given the above, it might sound like you start chugging safflower oil, but that’s not exactly the case. Since the western diet already includes so much omega-6, you’re probably covered. “We’re getting plenty of omega-6s. I’m not really advocating that people start supplementing their diet with omega-6,” says Harris. “But what I would say is efforts to reduce the intake of omega-6 are going to have an adverse effect on health,” he adds. This because less omega-6 means less of the observed protective benefits of linoleic acid, Harris explains.

And it may also be that those seeking to swap out seed oils inadvertently end up swapping in less healthy alternatives. Often, influencers combine their disdain for seed oils with other health fads, like promoting the “carnivore diet,” anti-sunscreen sentiment, or even sometimes all three in one. This pile of misinformation would have viewers eschewing sun protection and vegetables, while chowing down on whole t-bone steaks and sticks of butter daily. Nothing in the vast amount of scientific research on human health and nutrition indicates any of the above is a good idea.

Saturated fats may not be as harmful to heart health as once thought, but a diet very high in saturated fats and animal products can still raise your risk of high cholesterol and cardiovascular disease. And again, we already eat a lot of saturated fat. In order from most to least healthy in the context of the modern western diet, Calder says that omega-3s are the best option, omega-6s come second, and saturated fats are at the bottom of the pyramid of things you need to eat more of.

Harris, too, recommends people try to eat more omega-3’s, particularly the kind found in seafood (seaweed and algae can provide a plant-based source for vegans and vegetarians).

And broadly, the best path to a healthy diet is probably what you’d expect. A diet rich in fruits and vegetables, with whole grains and lots of fiber, is best, say Calder and Johnson. “It’s what your mother told you,” Johnson adds. Moving more and eating slightly less overall, are probably also good ideas for most Americans, notes Harris. “It’s not sexy, but that’s the way it is.”

Finally, to stay your sharpest, be mindful of the health claims you see online. Always remember correlation doesn’t equal causation, one person’s experience is not equivalent to a robust scientific study, and there is no simple quick-fix for every health problem. “If it sounds too good or simple to be true, then it probably is,” says Johnson.