Seed oils: Are they healthy or harmful?

A couple of years ago, most people had never heard the term “seed oils.” But you can’t escape it now. If you believe the rumors, you might assume that seed oils are pro-inflammatory wrecking balls, devoid of nutrients, and toxic for your health. Disdain for seed oils runs the gamut—from online influencers to leaders of the Make America Healthy Again movement. For example, Health Secretary Robert F. Kennedy, Jr. has claimed that seed oils are poisoning Americans. Even restaurant chains like Sweetgreen, Steak ’n Shake, and True Food Kitchen have nixed seed oils from some of their menu items or made commitments to go seed-oil-free. But those companies may just be capitalizing on a trend rather than improving your health. In fact, they may be making it worse.  

What does the evidence say? Do seed oils cause inflammation or harm you in some other way? How are they processed? What are the best cooking oils? We’ve got answers to your most pressing seed oil questions. 

What are seed oils? 

Seed oils come from the seeds of plants. Think soy, canola, corn, sunflower, and safflower. Other oils, like olive, coconut, palm, and avocado come from fruits.  

What kinds of fats are in oils? 

Oils contain a mix of saturated, monounsaturated, and polyunsaturated fatty acids. (Polys are further broken down into omega-6 and omega-3 fatty acids.) Most oils are described by their predominant fatty acid. 

For example, coconut oil, which clocks in with roughly 90 percent saturated fatty acids, is considered a saturated fat. Olive, avocado, and canola oils are mostly monounsaturated fats, while soybean, grapeseed, and corn oils are particularly high in polyunsaturated omega-6 fats. 

What about those bottles of “vegetable oil” on supermarket shelves? That’s a generic term for any oil made from plants (as opposed to, say, butter, lard, or other animal fats). But the term typically refers to refined seed oils that have a pale color and neutral flavor that serve as a blank canvas for cooking. It’s typically soybean oil (or a blend of soybean and other oils). 

Seed oils and health 

Are seed oils bad for you?  

In a word, no. In studies that ask thousands of people what they eat, then follow them for decades, those who report eating more unsaturated, plant-based fats have better health outcomes than those who say they eat more saturated, animal-based fats (like butter). 

For example, in one recent study that followed more than 220,000 people for up to 33 years, participants with the highest intake of plant-based oils (which included olive and seed oils) had a lower risk of dying during the study. Replacing about two teaspoons of butter with plant-based oils each day was linked to a 17 percent lower risk of early death. And the benefits held true when the researchers looked at not just all plant-based oils, but at canola, soybean, and olive oils separately. 

Those results jibe with the American Heart Association’s Presidential Advisory on dietary fats and cardiovascular disease. “The best intervention studies show that rates of heart disease are nearly 30 percent lower when you replace animal fats with plant oils, primarily soybean oil,” says Alice H. Lichtenstein, a member of the Presidential Advisory writing group and leader of the Diet & Chronic Disease Prevention Directive at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. 

The advisory also noted that, in studies that follow thousands of people for decades, those who consumed the most polyunsaturated or monounsaturated fats had a lower risk of heart disease than those who consumed the most saturated fat. 

Those results were backed up by studies reporting that the arteries of nonhuman primates (like rhesus monkeys) were more clogged when fed saturated fats than when fed polyunsaturated fats. What’s more, it’s clear from clinical trials in people that replacing saturated fats with unsaturated fats lowers LDL—the “bad” cholesterol that promotes artery clogging. 

Taken together, the research makes it crystal clear that seed oils are healthy, not harmful. 

Is it better to replace saturated fats with polyunsaturated fats or with monounsaturated fats? 

Are you better off replacing saturated fats (like butter, palm, or coconut oil) with polyunsaturated fats (like soybean or grapeseed oil) or with monounsaturated fats (like olive, canola, or avocado oil)? 

While both are good, Lichtenstein points out, “there seems to be more of a benefit of polys than monos for lowering the risk of heart disease. But we also have a lot more data from clinical trials and animal studies on polys than monos.”  

Her bottom line: “I think we’re splitting hairs. It’s like arguing whether blueberries are better than blackberries, when you just want people to eat more fruit.”  

Do I need to improve my ratio of omega-6 to omega-3 fats to reduce inflammation? 

For decades, some people have argued that your health depends on how much omega-6 fats you eat compared to how much omega-3 fats you eat. 

“Historically, human diets contained roughly equal amounts of omega-6 and omega-3 fats, or at most, about four times as much omega-6,” writes functional medicine doctor Mark Hyman. “But with the rise of seed oils and processed foods, that ratio has skyrocketed—often reaching 10 times, 20 times, or even higher.” 

Why might that matter? 

“Omega-6 and omega-3 fatty acids compete for certain enzymes,” explains Lichtenstein. Those enzymes can convert the omega-6 fat linoleic acid into another omega-6 fat called arachidonic acid, which can get converted into other compounds that promote inflammation. Those same enzymes also convert the most abundant omega-3 fat in our diet, alpha-linolenic acid, into other omega-3 fats that have anti-inflammatory effects.  

“So if you accept that omega-6 fatty acids are pro-inflammatory and omega-3 fatty acids are anti-inflammatory, then the ratio seems important to focus on,” says Lichtenstein. “It sounds credible. But it’s not evidence-based.” 

In reality, inflammatory processes are far more complex. 

For starters, only 0.2 percent of the linoleic acid we eat is actually converted to arachidonic acid. And in trials that slashed linoleic acid intake by up to 90 percent or boosted it nearly six-fold, levels of arachidonic acid in the blood didn’t budge. 

So the focus on linoleic acid’s effect on arachidonic acid is misplaced. 

What’s more, an analysis of 30 randomized controlled trials found that eating more linoleic acid was not linked to higher blood levels of inflammatory markers. In another analysis on data from nearly 70,000 people, higher blood levels of both linoleic acid and arachidonic acid were linked to a lower risk of cardiovascular disease. (That study was partially funded by Unilever, which makes mayonnaise and previously made margarine. Both are high in omega-6 fats.) 

Bottom line: Omega-6 fatty acids from your diet aren’t pro-inflammatory, so you can ignore claims about the ratio of omega-6 to omega-3 fats.

Seed oil processing 

One of the charges against seed oils is that the refining processes used to make them introduce toxic chemicals and strip the oils of their nutrients. That’s an exaggeration. Here’s what to know about how oils are made. 

Why do oils need to be refined? 

“A crude vegetable oil is pretty inedible,” says Eric Decker, professor of food science at the University of Massachusetts Amherst. “It’s not something you’d want to consume.”  

The refining process removes components of crude vegetable oils that can lead to rancidity, splattering, unpleasant flavors, or a dark color. The result: a neutral-tasting oil with a long shelf life that can be used at high heat without smoking. But the refining process also removes some important nutrients like antioxidants (more on that below). 

What does it mean for an oil to be extra-virgin, expeller-pressed, or cold-pressed? 

Many cooking oils are produced by using a press to force the oil out of the seed or fruit.  “There are a bunch of different presses manufacturers can use,” says Decker. “An expeller press is one of them.” And “cold-pressed” simply means that the fruit or seed has been pressed at a lower temperature, typically below 120º F. 

Mechanical extraction without additional refining steps is what’s used to make extra-virgin olive oil as well as “finishing oils” like toasted sesame or walnut, which you add after you’ve finished cooking to add flavor or can use to make salad dressings. The result is a flavorful oil that tends to have a lower smoke point, a shorter shelf life, and a higher price than refined oils. 

Why is hexane used to make refined oils? 

While extra-virgin and finishing oils are extracted mechanically, most other oils are extracted using chemicals like hexane or a combination of mechanical and chemical methods.  

In the case of chemical extraction, “hexane is used because the manufacturer is trying to recover 100 percent of the oil from the fruit or seed,” Decker explains. “When cold pressing, you leave a lot of the oil in the pomace, which is the solid material that’s left behind. Hexane is just the most efficient way to get everything out.” 

After the oil is extracted, the hexane is evaporated off. Very little, if any, remains. Also, “if you cook with the oil, you’re probably going to evaporate off any remaining hexane,” says Decker. That said, the Food and Drug Administration hasn’t set a hexane limit  for cooking oils. “It would be great if there was more transparency,” says Decker. “I'm pretty confident that the industry knows how much residual hexane is in the oil, but they’ve chosen not to make that data public.” 

Is hexane harmful? It can cause neurological problems—like muscle weakness, numbness, and decreased sensation—in people who work with hexane and inhale it regularly, like in shoe factories where hexane is used as a component of glue. “But there’s no evidence that the amount that you would find in your cooking oil would be a concern,” says Decker. 

Even so, the European Food Safety Authority is re-evaluating the use of hexane in food production. So stay tuned. In the meantime, if you don’t want to run any possible risk of being exposed to hexane in refined oils, you can buy extra-virgin olive oil or any organic oil. (Organic oils can’t be made using hexane.) 

What other techniques are used to make a refined oil? 

There are four major steps, explains Decker.  

1. Degumming. That takes out compounds called phospholipids, which can cause cloudiness and off-flavors and make the oil less stable. “Oil processors use citric acid, which is what you find in citrus fruit, to help the phospholipids separate from the oil,” Decker explains. “They sell those phospholipids as lecithin, which is an emulsifier that you’ll see on a lot of food labels.” For example, lecithin inhibits crystallization in chocolate.
2. Neutralization. That removes free fatty acids, which can produce off-flavors and reduce an oil’s shelf life. To do that, manufacturers add sodium hydroxide to the oil, which separates the fatty acids from the rest of the oil. “They recover those fatty acids and sell them to the soap industry,” says Decker.
3. Bleaching. “The purpose is to get a consistent color, as well as remove chlorophyll, a green pigment found in plants that will accelerate the oil going rancid,” says Decker. A filter aid like bentonite clay is added to the oil to bind to the pigments, then the clay and the pigments are filtered out.
4. Deodorization. “Refiners inject steam into the oil, hold it under a vacuum, then pull out the steam,” Decker explains. “That removes any off-aromas that would be in the oil.” 

Does refining strip oils of all their nutrients? 

The downside of the refining process is that it’s hard to remove the stuff you don’t want—like gums, off-flavors, and compounds that can make the oil go rancid—and keep the stuff you do want, like antioxidants. But you don’t lose everything in a refined oil. 

“People say refining strips oils of all their antioxidants,” says Decker. “That’s wrong.” The refining process does remove some tocopherols, which are forms of vitamin E that have antioxidant properties. “But manufacturers recover those tocopherols and can add them back to get to the optimum level.” 

That’s not necessary with soybean oil. “Soybean oil has more tocopherols than the optimum level,” says Decker. “So they take that extra tocopherol that’s removed during refining and sell it as a vitamin E supplement or as an antioxidant to be used in foods.” 

And despite what you may have heard, all oils (even refined ones) contain tocopherols. “They provide resistance to oxidation,” Decker explains. “If they weren’t there, the oil would oxidize in the bottle in just a couple days.” 

What is “light” olive oil? 

Terms like “light tasting,” “light in flavor,” “pure,” or simply “olive oil” (as opposed to “extra virgin olive oil”) refer to olive oil that has been refined. (In this case, “light” means that the oil has a milder taste, not fewer calories.) Like any other refined oil (i.e., any seed oil), “light” olive oil has a neutral flavor, high smoke point, and long shelf life, and it lacks the flavor, aroma, and additional antioxidants you’d find in extra-virgin olive oil. You can use it interchangeably with any other vegetable oil. 

Why do seed oils get such a bad rap?  

It’s not just claims about inflammation, toxic chemicals, and lost nutrients. Seed oils may also be guilty by association, because they’re often used in ultra-processed foods. (Vegetable oils themselves are “processed culinary ingredients,” not ultra-processed foods, according to the NOVA system, which is the source of the generally recognized definition of “ultra-processed.”) But there’s no reason to assume that seed oils explain why people who eat more ultra-processed foods have a higher risk of some illnesses. Something else about those foods—which are often high in calories, white flour, added sugars, and salt—or something else about the people who eat them could explain those higher risks. 

“A lot of the criticism of seed oils is because of their use in frying oil,” says Decker. “Yet nobody seems to be saying, ‘Eat less fried food.’” Some companies are making things worse by switching from seed oils to fats that are truly unhealthy. (The Steak ’n Shake chain, for example, recently replaced seed oils with cholesterol-raising beef tallow as the frying fat for its fries, onion rings, and chicken tenders.)  

Bottom line: Don’t worry about seed oils in your mayonnaise, salad dressing, buttery spreads, or cooking oil. Replacing ultra-processed foods with fruits, vegetables, beans, whole grains, and other unprocessed foods is a good idea. But fries, onion rings, and chicken that are fried in beef tallow rather than seed oils are no gift to your arteries.

Cooking oils

What oils should I use for cooking?

A refined vegetable oil or extra-virgin olive oil are all healthy options that you can use for nearly all home cooking. 

Some other points to consider when choosing an oil:  

• Extra-virgin olive oil. It starts to smoke at a lower temperature—about 350º to 415º F—than most refined oils, but almost all home cooking takes place below those temperatures. Note: Just because your oven temperature is higher than 350º doesn’t mean that the oil or food is getting that hot. That said, your oil may get hotter than that when stir-frying in a wok or when deep-fat frying, so a refined oil is better for those purposes.
• Refined seed oils (like canola, soybean, sunflower, safflower, etc.). They’re good anytime, but are especially good for stir-frying, baking, or any dish where you don’t want the oil to impart a noticeable flavor or color.
• Avocado oil. Like olive and canola oils, it’s high in monounsaturated fatty acids, “which are very slow to oxidize,” says Decker. That’s good because it means that the oil will last longer. But avocado oil lacks the unique blend of antioxidants that you’ll find in extra-virgin olive oil and is far more expensive than canola oil, he points out. “There’s very little research to suggest that it’s healthier than other oils.”
• Cold- or expeller-pressed oils. They haven’t been refined, so they start smoking at a lower temperature and their flavor can degrade when they’re heated. So oils like toasted sesame or pumpkin seed or walnut or flaxseed are best used in a salad dressing or as a finishing oil (to add flavor after you’re done cooking).
• Price. “These oils that are being promoted, like extra-virgin olive and avocado, are really expensive,” Decker points out. “They cost four to five times more than many seed oils, so they’re going to be out of reach for a lot of consumers. It’s kind of an elitist recommendation in my mind.” So buy extra-virgin olive and avocado oil if you like—and can afford—them. (Or use extra-virgin olive oil only when you want its flavor.) But if they’re out of your price range, don’t worry that you’re sacrificing your health by choosing a far less expensive seed oil. 

How can I tell if my oil has gone bad? 

“If it’s a refined oil, it should have no smell,” says Decker. “If it has any odor, it’s typically going to smell like paint. That means it’s gone rancid.” Toss it. It’s harder to pick up on that smell in an unrefined oil like extra-virgin olive, says Decker, because the compounds that give the oil its aroma can mask the paint smell. 

How should I store my oils? 

At a minimum, you should store your cooking oils in a dark place, like a cupboard or pantry, not on your countertop. Even better: “Keep most of your oils in the refrigerator,” says Decker. “They’ll last a lot longer.” That’s especially true for finishing oils—like toasted sesame or walnut—which aren’t refined and can go rancid faster. 

One caveat: Olive oil will solidify if you keep it in the refrigerator. “Nothing bad will happen to it, though,” says Decker. (You can thaw your solidified oil on the countertop, though it may take hours to re-liquefy.)

Decker’s approach: “Olive oil prices have been going up and they’re probably going to get worse with tariffs, so I try to stock up on it and store the extra in the freezer.”