Kendra Pierre-Louis: For Scientific American’s Science Quickly, I’m Kendra Pierre-Louis, in for Rachel Feltman.
What do ready-to-bake cinnamon rolls, most french fries, and many premade salad dressings have in common? They’re all made with seed oils.
Until fairly recently many of us might have been concerned about how much oil those foods contained rather than what kind. But these days a lot of people seem to have a problem with seed oils.
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Everyone from so-called wellness influencers on social media to Robert F. Kennedy Jr., the secretary of health and human services, wants us to believe that seed oils are uniquely bad for us. The health secretary has gone so far as to say, without evidence, that seed oils are toxic. And some restaurants, including the salad chain Sweetgreen, are increasingly boasting of seed-oil-free options.
So today we dig into seed oils: What are they, and are they secretly killing us? To answer these questions we reached out to Eric Decker. He’s a professor at the Center for Agriculture, Food and the Environment at the University of Massachusetts Amherst.
Hi, thanks for taking the time to speak with us today.
Eric Decker: No problem. It’s my pleasure.
Pierre-Louis: One of the reasons we wanted to speak with you is you’re something of an expert on seed oils, which have in recent years become the villain of, you know, quote, unquote, “wellness” social media. For me that raises a pretty natural question, which is: What is a seed oil?
Decker: Basically, the fats we eat could come from animals, so that would be things like lard and tallow and butter; or they can come from fruit, and that would be things like olive and avocado; or they can come from seeds, and those seeds could be soybean, corn, canola, sunflower.
Pierre-Louis: So something like corn or soybean oil would be a seed oil …
Decker: Correct.
Pierre-Louis: But something like olive oil or—my personal nemesis because I’m allergic to it—avocado oil, that is not a seed oil.
Decker: Correct.
Pierre-Louis: But when I’m looking in my kitchen cabinet I’m seeing, for the most part, liquid oils that all look exactly the same. Are seed oils different, like, on a chemical level from non-seed oils?
Decker: Yeah, they’re gonna have a couple differences. So what makes olive oil unique is it’s not a refined oil—extra-virgin olive oil …
Pierre-Louis: Mm-hmm.
Decker: Avocado oil, sometimes it’s refined; sometimes it’s not refined. Most of the seed oils that we would buy at the supermarket would be refined. There are some cold-pressed seed oils, but they’re pretty uncommon.
So because they’re refined or not refined could be one of the reasons that their composition will change. But the other thing is, is that the genetics of the plant dictates the type of fatty acids that are in the oil.
Pierre-Louis: Mm-hmm.
Decker: And so olive and avocado oil, very high in oleic acid …
Pierre-Louis: Mm-hmm.
Decker: Which is a monounsaturated fatty acid.
Pierre-Louis: Mm-hmm.
Decker: The seed oils, you can find a very broad range of fatty acid composition. So canola oil would be very high in monounsaturated …
Pierre-Louis: Mm-hmm.
Decker: Like olive oil and avocado oil. But then the others, like corn and soybean and the safflower or sunflower, these are high in polyunsaturated fatty acids, and the most common one of those is linoleic acid …
Pierre-Louis: Mm-hmm.
Decker: Which has two double bonds, versus the monounsaturates with one double bond.
Pierre-Louis: Okay, so for a regular person who’s just trying to figure out what they should eat in the grocery store aisle, these fatty acids, do they matter?
Decker: Yeah, so there’s really good human clinical data that show that the polyunsaturated fatty acids will decrease the LDL cholesterol in your blood. And this is important because LDL cholesterol’s been associated with cardiovascular disease and heart attacks …
Pierre-Louis: Mm-hmm.
Decker: So people that—especially people that have high LDL …
Pierre-Louis: Mm-hmm.
Decker: Would benefit greatly by using a seed oil that has a high linoleic-acid concentration.
Pierre-Louis: Wait, what? Because I have it on good authority from our human health secretary, Robert F. Kennedy Jr., that Americans are being “unknowingly poisoned” by seed oils. Here’s what he said in an interview on Fox & Friends in 2024.
[CLIP: RFK Jr. speaks in an August 24, 2024, interview on Fox & Friends Weekend: “Seed oils are one of the, the most unhealthy ingredients that we have in foods…. Seed oils, they are associated with all kinds of very, very serious illnesses, including body-wide inflammation.”]
Decker: Yeah, there’s a lot of theories about this. There’s one theory called the omega-3/omega-6 ratio …
Pierre-Louis: Mm-hmm.
Decker: And this theory—the omega-6, which is the polyunsaturated linoleic acid, is thought to increase inflammation …
Pierre-Louis: Mm-hmm.
Decker: Whereas the omega-3s, most of which we would get from fish oil …
Pierre-Louis: Mm-hmm.
Decker: Would decrease inflammation. So even though this, this is a theory and it’s been shown in animals, when you actually look at human clinical trials …
Pierre-Louis: Mm-hmm.
Decker: The linoleic acid, the omega-6, has not been shown to increase inflammation. So right now the research shows that the benefit of the linoleic acid in decreasing your LDL cholesterol is much greater than the potential risk of that fatty acid increasing inflammation.
I mean the other controversy on the seed oil is they’re extracted with hexane …
Pierre-Louis: Mm-hmm.
Decker: Which is a chemical solvent. But there’s pretty good data that shows that for the consumer that’s not really a high risk. Almost all the hexane’s removed from the oil when they process it. And really, the only time you ever see hexane being a, a problem is with workers in factories that are exposed …
Pierre-Louis: Yeah.
Decker: To very high levels of hexane.
Pierre-Louis: From the outside it often seems, when I go to the grocery store and I look at, you know, these seed oils, they’re often lower-cost than some of the other oils that we’re being pushed to consume instead. And I’m wondering—it feels like there’s a little bit of classism at play here and this idea that just because it’s expensive, it must be better for us.
Decker: So the best example is extra-virgin olive oil. They just press out the fat, and when they press out the fat they don’t get all the fat out of the olive …
Pierre-Louis: Mm-hmm.
Decker: So the yield is somewhat low. Plus, olives can only be grown in warm climates, so there’s …
Pierre-Louis: Mm-hmm.
Decker: Limitation of how much that’s out—there’s actually some shortages that are occurring right now because of climate change. So they typically will be more expensive. And when you think about olive oil it’s really—has a lot of flavor to it, and that flavor is very desirable in the olive oil. And so that’s why you don’t wanna refine it, because it takes all the flavor out of it.
The seed oils, on the other hand, are extracted with hexane …
Pierre-Louis: Mm-hmm.
Decker: And when they’re extracted with hexane this gets all the oil out, so it’s very, very efficient. But it takes out some other fats—it takes out things like phospholipids and free fatty acids and fats that aren’t desirable in the oil. So the refining process removes these other lipid components from the oil, and this makes the oil very bland in flavor; it gives it really good cooking properties, like very high smoke point; and so it makes these oils very, very versatile. But because they are so efficient at extracting it, it also makes the costs much lower.
Pierre-Louis: And isn’t that also why people love deep-frying in peanut oil, because it has a high smoke point?
Decker: Yeah, they’re all pretty similar; all the refined oils are pretty similar. When you get into high saturated fats they would be a little bit higher …
Pierre-Louis: Right.
Decker: But, you know, peanut oil is a good one—’cause in a lot of those applications when you’re cooking, you want bland, right? You don’t wanna make your sugar cookies with olive oil.
Pierre-Louis: [Laughs.]
Decker: [Laughs.] So, so you want, you know, you want a bland—or maybe in the case of a cookie you want butter, right? But in some applications …
Pierre-Louis: Mm-hmm.
Decker: You don’t want the oil to have flavor.
One thing about olive oil, it’s actually not good to high-temperature-cook olive oil ’cause it’s got a low smoke point and you’ll lose a lot of the flavor that you’re paying for. So it’s, like, you should use it in cold applications or in—you know, you can use it in a frying pan, which doesn’t get too hot.
I think a, a risk in all oils …
Pierre-Louis: Mm-hmm.
Decker: Is if you heat ’em too high and they start to smoke, you’re now decomposing the oil into other compounds, which have health risks. So really managing your oil when you’re cooking with it and not letting it get too high of temperature and start to smoke is also very important.
Pierre-Louis: Right, and that’s where it comes into handy to know how to cook a little and to look for the smoke point. But I guess it feels like there’s a health halo around kind of the more expensive oils and we’re vilifying sort of the cheaper, more accessible oils.
Decker: Yeah, I mean, extra-virgin olive oil does have this halo, and there’s a lot of good research that shows that it has health benefits beyond just normal oils. A really good contrast is your favorite, avocado oil.
Pierre-Louis: [Laughs.] Which, to be clear, I am only vilifying because it wants me dead, so. [Laughs.]
Decker: [Laughs.] So avocado oil doesn’t really have any data that shows that it has major health benefits, and actually, its fatty acid composition, in terms of the amount of monounsaturated fatty acids, is very similar to canola oil. So—and when you go to the store and you look at olive oil—or avocado oil, you find that a lot of times it’s refined; it’s not cold-pressed. And so if it’s refined, then you’re taking out other potential beneficial compounds in the oil, and it becomes even more like canola oil, except it’s probably four times more expensive.
So it’s more that people are making these cold-pressed oils into being these superfoods when there’s really no evidence to show, especially with avocado oil, that that’s true.
Pierre-Louis: That’s, A, awesome to know, and, B, I hope the word gets out because people are putting avocado oil in everything for no reason [Laughs] as someone who’s allergic.
But something I’ve noticed is that the people who really hate seed oils aren’t just pushing other plant-based oils, like olive or palm oils. They’re really pushing for us to use animal fats, like butter or rendered beef, a.k.a. tallow.
You’ve probably heard that the fast-food chain Steak ’n Shake, for example, has said it’s gonna eliminate seed oils in favor of beef tallow. And actually, if you go on the website, they’re selling jars of beef tallow. And this kind of makes me think—Steak ’n Shake is just fundamentally processed fast foods. And I can’t imagine that even if you were to switch whatever oil they were using before for a, quote, unquote, “healthier” oil, it’s gonna make that much big of a difference in the health of ordinary people.
Decker: Yeah, there’s two parts of this controversy. One is that some people don’t feel that saturated fats are that bad for you, but again, there’s pretty good clinical evidence to show that saturated fats increase your LDL cholesterol. They do the opposite of the polyunsaturated fats. So especially if you were to replace polyunsaturated fats with saturated fats, you would decrease the benefit of the healthy polyunsaturated fats in your diet.
However, on the other side, there’s an oxidation that occurs in oils, and when those oils oxidize they start to decompose into a whole variety of compounds, some of which, in animal studies, have shown to have some toxicity. So if you do something, like, in a food-service operations and you deep-fry foods with these highly polyunsaturated fatty acids, they can oxidize and produce these products, where if you use something like tallow, which is saturated, it’s much more stable against the oxidation and doesn’t present, again, that risk.
Now, as long as you take care of the oil that you’re using in the fryer, there’s no real risk there. The risk becomes—is when the oil is not used properly.
Pierre-Louis: And what does “used properly” mean?
Decker: It just means you have to make sure that you maintain the oil in the freshest state possible. Good food-service operations will actually filter and clean their oil on a regular basis. And then you have to monitor the oil, and when it gets to be older you need to discard and start with fresh oil.
Pierre-Louis: In a way you’re saying that using beef tallow gives fast-food restaurants an easier margin, so they’re able to, essentially, use older oil than they would if they were using a seed oil.
Decker: Yeah, the, the tallow would last a lot longer. And it costs a lot of money to fill up that fryer with oil, as you can imagine, so the longer you can make your oil last, the more money you’ll save.
Now, another part of beef tallow is that beef tallow can have very positive flavors to it.
Pierre-Louis: Mm-hmm.
Decker: So for many years McDonald’s used beef tallow. And then what happened is people were concerned about the cholesterol in the beef tallow, so McDonald’s stopped using the beef tallow. But many times if you go to fancy restaurants, you can get fries that are cooked in beef tallow—or even duck fat …
Pierre-Louis: Yeah.
Decker: Sometimes you see duck-fat fries. And that’s part of the advantage of that, is you’re getting a flavor in addition to the stability of the fat.
Pierre-Louis: And it also just sort of seems like for your everyday, you’re not looking for a flavor, maybe you’re making a stir-fry, a seed oil is fine. If you really want, like, a strong—like, maybe you’re making a pasta dish or a salad, that’s a really good place to use your extra-virgin olive oil because it adds a really good flavor profile.
Decker: Exactly.
Pierre-Louis: And to be clear, I love a good duck-fat french fry; they are very tasty. But I’m also not under the belief system that a potato deep-fried in duck fat is heart-healthy. [Laughs.] I’m not making a health claim. And I think the thing that, with seed oils in particular, is many of the people who are vilifying them is they’re making an explicit health claim and telling people to eat diets that are really heavy in these saturated fats and to really pivot away from eating, you know, canola oil or oils that are, for many people, lower-cost, more accessible and—it seems to be, based on the available evidence—better for our health.
Decker: Yeah, I mean, I would say that the influencers who have had so many negative comments about seed oil, those aren’t really based on strong scientific evidence.
And you know, like you said, the recommendation should really be, “Eat less fried food,” right, because fried food’s gonna be, generally, very high in calories. If it’s potatoes or chicken nuggets, it’s not gonna be very high in positive nutrients.
So the messaging is—seems to be all messed up. We shouldn’t worry about, “Is seed oil better than tallow?” We should be worried about people that eat so much fried food in their diet.
Pierre-Louis: That’s our show. Join us on Friday, when we’ll dig into Venezuela’s oil reserves and why the U.S. seems to want it.
Science Quickly is produced by me, Kendra Pierre-Louis, along with Fonda Mwangi, Sushmita Pathak and Jeff DelViscio. This episode was edited by Alex Sugiura. Shayna Posses and Aaron Shattuck fact-check our show. Our theme music was composed by Dominic Smith. Subscribe to Scientific American for more up-to-date and in-depth science news.
For Scientific American, this is Kendra Pierre-Louis. See you next time!
