Rates of chronic diseases have spiked in recent decades. Over the same time period, the food supply has shifted toward more use of oils made from seeds, such as canola and soybean.
Some people—U.S. Health Secretary Robert F. Kennedy Jr., most prominently—have connected these developments. “Seeds oils are one of the most unhealthy ingredients we have in foods,” he said in a Fox News interview late last year. Kennedy thinks federal regulators and companies should move swiftly to address this problem. As a result of this negative attention on seed oils, Sweetgreen and Steak n’ Shake have vowed to remove seed oils from menu items.
However, seed-oil consumption is not the only thing that’s changed since the 1980s. Other notable trends during this period include binge-watching tv shows, online dating, and pop-country music. Correlation isn’t causation, and most nutrition researchers and dietitians say that seed oils aren’t related to upticks in chronic illness.
Here’s what to know about arguments for and against seed oils.
What are seed oils?
Seeds naturally contain edible oils. Most seed oils found on grocery shelves have gone through industrial processes, such as mechanical crushing and chemical extraction, to recover as much oil from the seeds as possible.
Oils from seeds contain a type of fatty acid, polyunsaturated, that we obtain only from food. Most seed oils are especially rich in one type of polyunsaturated fat called omega-6.
Non-seed oils, like olive and avocado, are high in another type of fat, monounsaturated, and lower in omega-6. Monounsaturated fats and antioxidants help to protect these oils from going rancid. Unlike seed oils, olive oil is often sold in its natural, unrefined forms—cold pressed and extra virgin. Extra virgin olive oil is one of the few oils that are commonly consumed without refining.
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Several physicians, mostly unaffiliated with mainstream research institutions, argue that seed oils’ high omega-6 content, combined with its instability and chemicals formed during oil processing, are primary culprits for increased chronic disease. Such critics lament the so-called “Hateful 8”—canola, corn, cottonseed, soybean, sunflower, safflower, grapeseed, and rice bran oil.
They point to a handful of studies, including a 2013 analysis by researchers at the National Institutes of Health (NIH) finding that replacing saturated fats with omega-6 fat increased the risk of dying from chronic diseases. The concerns have fomented a crusade against seed oils on social media.
But mainstream researchers view these oils differently. “Seeds oils are really healthy foods,” says Dr. Dariush Mozaffarian, director of the Food is Medicine Institute at Tufts University. “There is overwhelming evidence for benefit and very little evidence for harm.”
The omega-6 fat debate
Some seed-oil critics are alarmed that American diets are loaded with omega-6 but much less of another fat type, omega-3, found in healthy foods like salmon and nuts.
Studies in mice show that excess omega-6 fat causes inflammation. However, “these effects just haven’t been shown in humans,” says Eric Decker, a food scientist at the University of Massachusetts. A large study in 2017 found that eating more omega-6 didn’t change people’s inflammatory markers.
Research pointing to the harms of omega-6, including concerns about the ratio of omega-6 to omega-3, is flawed, Mozaffarian says. More important is getting plenty of omega-3s, regardless of your omega-6 intake. “It’s all driven by the omega-3s,” he says. “Omega 6 isn’t necessarily bad,” says Jason Ewoldt, a Mayo Clinic dietitian, “but omega-3s seem to be better.”
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Contradicting the 2013 analysis by NIH, other research demonstrates that modest intake of omega-6 fats isn’t linked to heart disease, diabetes, or obesity. The NIH paper’s conclusions may have been skewed, Decker says, because it lumped together people who ate seed oils together with those who ate margarine with trans fatty acids, which are now banned for health reasons.
Many studies find benefits of omega-6 and polyunsaturated fats, according to Mozaffarian. For example, omega-6 reduces heart disease risk. “Omega-6 will lower your bad cholesterol,” Decker says. “Human trials have proved this biological effect.”
Industrial processes remove some beneficial compounds in these oils. Unprocessed, extra-virgin seed oils have more antioxidants—but these versions are expensive. Regardless, seed oils may be healthier than other alternatives such as butter, according to a new study.
What about the chemicals in seed oils?
Another argument against seed oils is that they harbor toxic chemicals, partly because they’re often heavily processed. Decker says we need more research on hexane, a liquid chemical that pulls out the oil from seeds.
After hexane does its job, companies try to remove it, but trace amounts may remain. Hexane has been linked to neurological damage in factory workers inhaling the chemical.
The FDA doesn’t monitor hexane levels in seed oils. “We don’t actually know how much is in there or how much is harmful to human health,” says Alison Kane, a dietitian at Massachusetts General Hospital. Decker adds that more studies are needed, though there’s most likely very little hexane in the actual products. “It’s probably not a big risk,” he says. Hexane processing isn’t allowed for bottles certified organic.
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Another potential red flag: compared to olive oil, seed oils may be more prone to oxidation and going rancid. This generates harmful compounds that could, in theory, drive chronic disease. “Oxidation of lipids certainly produces compounds that could harm health,” Decker says, but without more research, “it’s hard to make conclusions.”
Seed oils are also diverse. Compared to other seed oils, canola has more heart-healthy monounsaturated fats—abundant in olive and avocado oil—and omega-3s. The monounsaturated fat makes canola less susceptible to oxidation, Kane adds. Soybean oil, the most purchased seed oil in the U.S., is another option that’s higher in healthy omega-3s than some of other frequently used seed oils.
The risks may rise with frying
Nina Teicholz, founder of the Nutrition Coalition, is concerned about oxidation in seed oils in general and especially when cooking with them. “Heat speeds up the chemical reactions and oxidation,” Teicholz says. In her book, she describes a study showing higher markers of oxidative stress after eating food cooked with safflower oil, compared to olive oil. Due to its higher content of monounsaturated fat, canola oil is “a better option” for cooking than other seed oils, Teicholz says.
Another potential issue: deep-frying vats at restaurants. They reuse the same seed oil, which may eventually produce cancer-causing chemicals. This is less of a problem at large fast-food chains with safety checks and cooking technologies that help minimize these chemicals, but smaller restaurants may not have these precautions, Decker explains.
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For restaurant preparation of non-fried food, seed oils typically “go out with the product” on the plate, instead of being reused for the next meal, Decker says. In addition, restaurants sometimes drizzle oil over the food, giving it a shiny appearance.
Finishing with these oils may ratchet up calories and fats to unacceptably high levels, even without frying. “These oils are primarily fat,” Kane says. “There is such a thing as too much.”
The American Heart Association suggests capping one’s omega 6 fats, including seed oils, at 5-10% of total calories. This equals about 11-22 grams of omega-6 fats per day. Seed oil critics recommend much lower consumption.
A solution
Americans are frustrated by how the food supply is impacting health. But seed oils are “a culprit by association,” Ewoldt says, because they’re often found in unhealthy ultra-processed foods. “It’s not necessarily the seed oils driving obesity, heart disease, and cancer. It’s the processed foods with high calories, salt, fat, and sugar.”
Prioritize a broader diet of mostly whole foods. “It’s a disservice to blame one single thing as the root cause for diseases,” Kane says. “The real problem is an overall unhealthy dietary pattern.”
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