If you’re a regular on social media, you may have heard that seed oils are unhealthy—or even toxic! According to a certain subset of internet influencers, seed-derived cooking oils can cause everything from heart disease to inflammation to fatigue to skin problems. Yet, contrary to posts demonizing common ingredients, plenty of scientific research disagrees. Here’s how to make sense of this health “scare.”
What are seed oils?
There are many types of vegetable cooking oils, but when people talk about seed oils, they’re often referring to a list of eight oils: canola, corn, cottonseed, soybean, sunflower, safflower, grapeseed, and rice bran. (Note that oils like olive, avocado, and coconut are missing from this list.) All eight of these oils contain fat and therefore fatty acids (a group of essential major nutrients). And many (but not all) of these seed oils contain a relatively high proportion of omega-6 fatty acids.
A quick aside on chemistry: Fatty acids are the building blocks of triglycerides, or complete fat molecules. They are organic compounds consisting primarily of carbon and hydrogen chains with an acidic group at the end. In saturated fats, each carbon, except the terminal carbons, is bonded to two hydrogens. In unsaturated fats, some of these hydrogens are replaced by double bonds between adjacent carbons. Omega-6 fatty acids are unsaturated, and the first of these double bonds occurs at the 6th carbon from the end, hence their name.
There are several types of omega-6 compounds, but one particular type, called linoleic acid, is the source of most of the criticism against seed soils. Linoleic acid is, again, an essential nutrient that our bodies need. We can’t synthesize it, and we need it to support healthy cell signaling, function, and immune systems.
But seed oil critics say we’re ingesting way too much linoleic acid, leading to the buildup of byproducts like arachidonic acid, which they say causes inflammation and also counteracts the benefits of consuming omega-3 fatty acids. The domino effect of all this, seed oil advocates say, is an increased risk of diseases like type 2 diabetes, heart disease, and cancer.
A kernel of truth
There are some elements of truth in the criticism of seed oils. Eating fried and processed foods in excess is generally bad for your health. So if avoiding seed oils means eating fewer chips and cakes, you may feel better.
Additionally, if you eat a typical Western diet, you’re probably not at risk for linoleic acid deficiency, and you’re probably getting 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 soybean, sunflower, or safflower oils, and pet foods now contain a lot of soy, which translates into higher linoleic acid levels in meat and dairy products, says Philip Calder, a nutritional scientist and professor at the University of Southampton in England. “Linoleic acid has permeated the food chain over the last 50 to 60 years,” he says. Popular science.
Additionally, Calder explains that there is “theoretical evidence” that linoleic acid may be partially converted to arachidonic acid, which is then partially converted to compounds associated with inflammation. Additionally, omega-6s and omega-3s may compete for the same metabolic pathways. All of these biological mechanisms exist in the human body.
But here’s where things get complicated: This theoretical argument doesn’t hold up to scientific observation. “It just doesn’t happen in real life,” says Guy H. Johnson, a nutritional scientist and assistant professor of food science and human nutrition at the University of Illinois at Urbana-Champaign. “If you have enough omega-3s, the inflammatory environment is not enhanced by omega-6s.”
What the research says
Calder agrees. “Most human studies show either no association between linoleic acid intake and inflammatory biomarkers or the relationship is the opposite of what you might think. We see that higher levels of linoleic acid and arachidonic acid are associated with lower levels of inflammatory biomarkers,” he says. He co-authored a 2018 review study evaluating the published literature on inflammation and omega-6s and came to the same conclusion.
“We didn’t find anything that demonstrated a detrimental association between omega-6s and inflammatory markers in humans,” he adds. A 2012 study co-authored by Johnson concluded the same thing.
Many other studies and meta-analyses have come to similar conclusions, and have found benefits where one might expect harms. “Every time anyone looks at blood omega-6 levels and health outcomes—and we’ve done this many times with thousands of people…they find that the people with the highest levels of omega-6 have the best outcomes,” says William S. Harris, a professor at the University of South Dakota Sanford School of Medicine and president of the Fatty Acid Research Institute.
Harris has co-authored several human cohort studies as well as large review articles evaluating the health impacts of omega-6 fatty acids. In a 2017 meta-analysis, he and his co-authors found that omega-6 consumption actually reduces the risk of type 2 diabetes. In a 2020 review of 30 observational studies, Harris and colleagues concluded that higher levels of linoleic acid are associated with a reduced risk of cardiovascular disease.
In fact, higher linoleic acid intake is associated with a lower risk of death from all causes, including heart disease and cancer, according to another 2020 meta-analysis that evaluated 38 different studies. I could go on and on about studies—there are dozens, but you probably get the idea.
The way fatty acids and metabolic processes work in the body is complex. “There’s a nuanced interplay between omega-6s, omega-3s, and a variety of other metabolites,” Harris says. The idea that omega-3s are good and omega-6s are bad “is not true and is way too simplistic,” he adds.
There are a few legitimate conflicting studies, Harris and Calder say. Among them are two oft-cited papers published by lead author Christopher Ramsden, chief of the Lipid Peroxidation Unit at the National Institute on Aging. In those studies, Ramsden found unpublished research from the late 1960s and early 1970s in which two groups of people fed a diet high in seed oils and margarine had worse health outcomes.
There are several caveats to these findings, however. For one thing, the study participants were fed much higher amounts of omega-6 oils than are common in today’s diets, Harris notes. Additionally, many of the solid margarines used in the study likely contained high amounts of trans fats, all of which are considered harmful to human health, Calder adds.
Seed oil skeptics also worry about the use of hexane in production. “It’s true that hexane is used to extract vegetable oils, regardless of their source,” notes Johnson, who has written numerous petitions for health claims on various oils. “But the product that consumers buy at the grocery store doesn’t contain any hexane at all. It’s gone,” he adds — removed during processing.
Overall, the vast majority of scientific evidence indicates that cooking with oils containing omega-6 is safe and probably good for you.
So what should you eat?
Given the above, it might seem like you’re starting to consume safflower oil, but that’s not exactly the case. Since the Western diet already includes plenty of omega-6s, you’re probably covered. “We consume a lot of omega-6s. I don’t really recommend that people start supplementing their diet with omega-6s,” Harris says. “But I would say that efforts to reduce omega-6 intake are going to have a negative impact on health,” he adds. That’s because a decrease in omega-6 intake means a decrease in the observed protective benefits of linoleic acid, Harris explains.
It’s also possible that those looking to replace seed oils may end up inadvertently opting for less healthy alternatives. Often, influencers combine their disdain for seed oils with other health fads, such as promoting the “carnivore diet,” anti-sunscreen sentiment, or sometimes all three at once. This pile of misinformation would encourage viewers to avoid sunscreen and vegetables, while subsisting on whole steaks and sticks of butter every day. There’s nothing in the vast body of scientific research on human health and nutrition that suggests that any of the above ideas are a good idea.
Saturated fat may not be as bad for heart health as previously thought, but a diet high in saturated fat and animal products can still increase your risk of high cholesterol and cardiovascular disease. And again, we already eat a lot of saturated fat. Calder says omega-3s are the best bet, omega-6s are a close second, and saturated fats are at the bottom of the pyramid of foods you need to eat more of.
Harris also recommends that people try to eat more omega-3s, especially those found in seafood (seaweed and algae can be a plant-based source for vegans and vegetarians).
Basically, the best way to follow a healthy diet is probably the one you’d expect. Calder and Johnson say a diet rich in fruits and vegetables, with whole grains and plenty of fiber is the best choice. “That’s what your mother told you,” Johnson adds. Moving more and eating a little less in general are probably also good ideas for most Americans, Harris notes. “It’s not sexy, but that’s just the way it is.”
Finally, to stay vigilant, be mindful of health claims you see online. Always remember that correlation does not equal causation, one person’s experience does not equal a solid scientific study, and there is no magic bullet for every health problem. “If something seems too good or too simple to be true, it probably is,” Johnson says.
