Toxic Seed Oils Turn French Fries Toxic, Sadly
Table of Contents
French fries are a beloved indulgence, but the toxic seed oils used to fry them can transform this treat into a health hazard.
You may have seen this photo of cigarettes in a French fry cup before. It’s an AI-generated image. But it’s not just a made-up meme. It started circulating shortly after I was interviewed for this movie. During the interview, I made the rather shocking comparison between cigarettes and French fries.
That comes from a study called: Toxic Lipid Oxidation Product (LOP) Generation in Culinary Frying Oils (CFOs) During High-Temperature Frying Practices: Passage into Fried Foods. In it, the author warns that using seed oils instead of tallow in deep fryers poses “severe health hazards.”
Dietitians often describe me as an “influencer” seeking attention rather than a physician with decades of clinical experience helping people change their lives for the better. These people don’t seem to know much about toxicology. Nor do they seem to have any interest. I do.
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To understand any food’s impact on our health, we have to know what happens between the field and our plate.
Edible oil industry scientists warn that see oils, like soybean, corn, and canola, easily deteriorate into toxins with time. Their fragile fatty acids (the polyunsaturated kind) react with oxygen in the air, generating toxic compounds called aldehydes—chemicals also found in cigarette smoke—that make fried foods like French fries harmful. Oxidation reactions mutate the fatty acids, transforming them from natural and potentially nutritious into toxic aldehydes such as crotonaldehyde and n-hexanal. The result? Toxic oil.
The high temperatures required for frying accelerate reactions, so toxins form much faster.
Because restaurants reuse the oils in their deep fryers many times, oil toxicity becomes alarmingly potent. The study estimates that a 154-gram serving of potato chips (about a large serving) fried in seed oils used for an estimated 3-4 days contains aldehyde levels comparable to smoking 25 cigarettes a day. Yes, the same toxic chemicals in cigarette smoke—crotonaldehyde (1.8–5.7 mg) and n-hexanal (2.5–9.5 mg)—are absorbed into your fries and chips, transferring directly into your body when you eat them.
These aldehydes wreak havoc by causing oxidative stress, a process where free radicals damage cells, proteins, and DNA. Oxidative stress is a root cause of chronic diseases, including heart disease, cancer, and Alzheimer’s, which I discuss in depth in my book, Dark Calories. Unlike stable fats like beef tallow or butter, which resist oxidation, seed oils dominate commercial frying because they’re cheap and widely available. This makes them a public health hazard, as fried foods are a dietary staple for many.
Influential dietitians want you to believe these toxic oils have no meaningful health effects.
On a recent popular podcast, a dietitian named Sarah Berry dismisses the potential for foods deep-fried in these toxic oils to harm our health. She has a PhD in Nutritional Sciences and works for a processed food company, called Zoe. Zoe sells foods made with seed oils. When the host pressed her to address the concern that seed oils oxidize easily, she says:
“The worst thing that’s going to happen is it will taste pretty rubbish. The second worst thing that’s going to happen is you will have deactivated some of the beneficial compounds that are found in seed oils.”
It’s frankly irresponsible for any health expert to dismiss foodborne toxins out of hand. People who say things like this are usually financially conflicted in one way or another. They’re often also in the business of writing nutrition articles that don’t pass the serious-science test. Indeed, that is the case here.
Dr. Berry’s Flawed Muffin Study Convinced Her Seed Oils Are Not Toxic
Dr. Berry was involved in a randomized human clinical trial that supposedly shows deep-fried seed oils do not hurt our health. But the study was deeply flawed.
In this study, 19 healthy males aged 18-40 years old were given 50g of oil (mostly sunflower oil) in a muffin. The muffins were either made with “fresh” oil or “nasty” oil, used to fry potato chips 5 times a day for 10 days. First, the men consumed muffins with fresh oil and underwent testing. Then, on another day, they consumed muffins with the nasty oil and underwent the same testing.
The study tested something called the flow-mediated dilation, as a measure of how blood vessels functioned three hours after consuming the oil. On the podcast, Berry says, “There was no difference between the fresh oil and this rather nasty-sounding oil that we’d repeatedly fried.” That prompted the host to ask, “So it doesn’t really matter in real life, is what you’re saying?” And Berry nodded, saying, “It doesn’t matter in real life.”
But Both Muffins Were Toxic
Dr. Berry drew the conclusion that seed-oil toxins don’t harm us because both muffins caused similar effects. But her study was fatally flawed. The study compared two toxic oil-containing muffins to each other. They should have compared them to a third, one containing healthy fats.
You see, neither muffin contained fresh oil. All the hateful eight seed oils are heated and stripped of their protective antioxidants in the factory. And let’s not forget, making the muffins heats them again. As a result, both muffins tested positive for a kind of toxin called polar compounds. The less heated muffins contained less, 7 percent. The more heated muffins contained more, 28 percent.
And here’s what you need to know:
Polar compounds are just one class of toxins commonly found in seed oils.
There are many sorts of toxins in heated seed oils. But the authors only measured this one category. And it happens that this one category develops AFTER the most harmful toxins—the kind that do the most damage to our body—have appeared and disappeared. Most of those disappearing toxins are similar to toxins in cigarette smoke, those “alpha beta unsaturated aldehydes,” mentioned above. In other words, they didn’t measure the worst toxins!
(This is not to say the older, most reheated oils are less toxic. They’re also bad for us. But food changes over time, so the toxins in seed oils heated for 10 days are different from the toxins in oils heated for 3 days, which are different from the toxins in oils only heated once after bottling. Different toxins will have different health effects. And it may just be that this particular arterial function test does not detect the effects of the toxins in 10-day-heated oils.)
Protect Yourself From the Whole Collection of Toxins!
You can protect yourself by avoiding the Hateful Eight seed oils. At home, fry with stable fats like coconut oil or lard, which resist oxidation. When eating out, avoid deep-fried foods and ask for butter or olive oil. Better yet, support restaurants that avoid seed oils entirely. Some will proudly say so on their menus, thanks to the growing no-seed-oil movement. By understanding the toxic potential of seed oils, you can make informed choices to safeguard your health.
French fries don’t have to be toxic, but as long as seed oils are the default, they’re a risky indulgence.
Learn more about seed oils and oxidative stress in my book, Dark Calories, at DrCate.co
