Do Seed Oils Make You Sick?

Critics say they raise your risk for weight gain, heart issues, and more. But the science doesn't support those claims.

By Sharon Liao

May 31, 2022

detail of person holding bottle of oil in grocery aisle with shelves of different oils behind them

If you’re on social media, chances are you’ve heard about the evils of seed oils. Twitter, Instagram, and Tik Tok are full of posts and memes claiming that canola (rapeseed), corn, cottonseed, grapeseed, rice bran, safflower, soy, and sunflower oils are toxic. Critics have dubbed them “the hateful eight” and they’re blamed for a host of ills, such as headaches, foggy thinking, lowered immunity, heart disease, type 2 diabetes, and more. And on the flip side, #seedoilfree proponents say that steering clear of these vegetable oils can fight disease, help you lose weight, and ramp up energy.

But these oils are rich in unsaturated fats, which experts have told us for years are better choices because they don’t clog arteries the way saturated fats do.

“There’s a lot of confusion about seed oils,” says Dariush Mozaffarian, MD, DrPH, a cardiologist and the Jean Mayer professor of nutrition at Tufts University Friedman School of Nutrition Science & Policy. The internet is full of so-called “experts” citing convincing arguments, he says. “But if you actually read the research, you’ll find that the evidence [against seed oils] is incorrect or incomplete.” It can be tricky filtering out the truth. We looked into the claims to see if you really need an oil change.

The Claim: Fats in Seed Oils Are Bad Fats

All oils contain a combination of polyunsaturated, monounsaturated, and saturated fats to varying degrees. With most seed oils, a type of polyunsaturated fats called omega-6s dominate. The high proportion is one of the main reasons some people suggest you skip seed oils.

The argument is that getting too much of a type of omega-6 in these oils, linoleic acid, sets the stage for chronic inflammation, which is thought to be at the root of many diseases. While linoleic acid isn’t inflammatory itself, the body converts it to arachidonic acid, a building block for inflammatory compounds.

“But it’s a flawed argument,” says Mozaffarian. Only a small amount of the linoleic acid we eat—about 0.2 percent—turns into arachidonic acid, and not all of the compounds it produces cause inflammation. “Arachidonic acid is a complicated molecule,” says Mozaffarian. “It also has anti-inflammatory effects.”

Seed oil critics point to laboratory research done on animals as proof but there is little evidence from studies in people that linoleic acid is inflammatory. “We aren’t built like mice and rats,” says Mozaffarian.

In fact, current evidence indicates that linoleic acid may lower inflammation in humans, which can protect against inflammatory diseases like heart disease. For example, in an analysis of 30 studies published in 2019 in the journal Circulation, scientists found that linoleic and arachidonic acids levels didn’t raise the risk for heart disease. In fact, those with higher amounts of linoleic acid in their bloodstream were 7 percent less likely to develop it.

Another study published in 2018 in the American Journal of Clinical Nutrition that followed nearly 2,500 men for an average of 22 years found that those with the highest blood levels of linoleic acid had a 43 percent lower risk of dying from any disease during the study period than those with the lowest. Higher blood levels of arachidonic acid reduced the risk of dying by 20 percent.

In addition, linoleic acid is an essential fatty acid, meaning we need to consume it because the body can’t make it itself. Linoleic acid is important for the production of cell membranes and skin health. Cooking oils are a major source, but you can also get it from nuts, seeds, meat, and eggs.

Those who suggest you avoid seed oils advise using avocado, coconut, or olive oils instead. Avocado and olive oils are mostly monounsaturated fat, which is heart healthy. But 82 percent of the fat in coconut oil is saturated, so it should be kept to a minimum in your diet.

Claim: Our Omega-6 to Omega-3 Ratio Is Out of Whack

You’ve probably heard about the heart-healthy benefits of omega-3s—the polyunsaturated fats found in fish, walnuts, and flaxseed. As with linoleic acid, we can’t make omega-3s and need to get them from food.

The line of reasoning goes that human ancestors ate a diet in which omega-6s and omega-3s were more in balance than they are today, so we should aim for the same. It’s true that we eat far more omega-6 fats than omega-3s, but not as much omega-6s as some claim. You often hear that omega-6 intake is 20 to 50 times higher than omega-3 intake, but research suggests it’s closer to a 10:1 ratio.

There’s no question that omega-3s are beneficial and we should get more of them in our diets, but cutting back on omega-6s isn’t a smart move, says Mozaffarian. “The science is very clear: Omega-6s are good for health.” Study after study shows that a diet high in these fats can help lower cholesterol and blood sugar, says Mozaffarian. According to an American Heart Association analysis, getting at least 5 to 10 percent of your daily calories from omega-6s reduces your risk of heart disease.

The Claim: Seed Oils Are Toxic

Another accusation against seed oils is that they’re full of toxic byproducts. Manufacturers use heat and solvents, such as hexane, to extract the oil from seeds. This process supposedly introduces chemical additives and unstable molescules, which can turn the polyunsaturated fats into harmful trans fats.

Hexane may pose a risk to the environment and to workers if they inhale it, but it is less clear whether the trace amounts in oils can cause health problems. And they’re only heated for short periods, so they have fewer trans fats than milk or butter, says Guy Crosby, PhD, an adjunct associate professor of nutrition at the Harvard T.H. Chan School of Public Health.

Cold-pressed oils (made without heat or chemicals) and expeller-pressed oils (made without chemicals) are an option, but they come with a heftier price tag. Because heat can deplete some nutrients, cold-pressed oils may have a slight advantage. But the bottom line is that the regular oils are healthy, too, says Crosby.

The bigger concern is when you heat up seed oils time and again. When you bring unsaturated fats repeatedly to high temperatures, you’ll get a buildup of damaging chemicals, says Crosby. The main problem is restaurant and factory deep-fryers that only get changed once or twice a week. “Cooking with seed oils at home isn’t an issue,” he says.

The Claim: The Foods Seed Oils Are in Aren't Good for You

This is partly true. Because these oils are relatively inexpensive and neutral in flavor, they’re used in a variety of packaged products—chips, crackers, baked goods, salad dressings, mayonnaise, and margarine to name a few—and they’re used to fry foods in restaurants.

Some people who stop eating foods like these say they have more energy and that they lost weight, attributing those benefits to their lower intake of seed oils. But many of these foods contribute plenty of calories, refined carbohydrates, sodium, and sugars. “Sure, if you cut back on these foods, chances are you’re going to feel better,” says Crosby, and even lose weight. But it’s likely because you’re eating fewer processed foods in general; the seed oils themselves aren’t the culprit. There’s no reason to avoid cooking with them or to stop eating whole foods that contain omega-6 fatty acids, like nuts or seeds.

Sharon Liao

Sharon Liao is a writer and editor specializing in health, nutrition, and fitness. She lives in Redondo Beach, Calif.