Are Seed Oils Actually Toxic? The Science Behind Oxidation, LDL, and Chronic Disease
SEED OILS — OXIDATION + LDL + CHRONIC DISEASE
Are Seed Oils Actually Toxic? What the Science Really Shows
Seed oils are often promoted as heart-healthy because they lower LDL cholesterol — but that’s only part of the story. In this long-form breakdown, I walk through the overlooked science on oxidation, linoleic acid, mitochondrial stress, and why focusing on LDL alone misses the real drivers of heart disease, metabolic dysfunction, and long-term chronic illness.
Author
Craig McCloskey
Board-Certified Nutritionist
There’s a war happening in the nutrition world right now — and if you’ve spent any time on social media, you’ve probably seen it.
On one side, we have health professionals and organizations telling people that seed oils like canola, soybean, sunflower, corn, safflower, and cottonseed oil are not only safe, but heart-healthy. On the other side, we have doctors, researchers, and clinicians warning that these same oils may be one of the most damaging ingredients in the modern food supply.
And in the middle? Regular people who just want to know what to eat without wrecking their health.
So what’s the truth? Are seed oils unfairly demonized… or are they genuinely problematic?
Today, I want to slow this conversation down and walk you through the science in a way that actually makes sense — without fear-mongering, without hype, and without reducing everything to a single lab marker like LDL cholesterol.
Because when you zoom out and look at the full picture, seed oils start to look far less like a health food — and far more like a long-term experiment we never consented to.
Watch the Video: Seed Oils Explained
Read the full transcript
What's the deal with seed oils? Are they toxic or are they actually healthy? Because in the nutrition space and on social media today, we have health professionals and influencers just butting heads. And how do you know what to believe? We have health professionals that are shaming and calling other health professionals fear-mongerers because we're trying to highlight the dangers that seed oils come with. But as you're going to learn about today, seed oils are far from a health-promoting food. And they might actually be one of the worst ingredients in our American food system today. But it's not just me that's talking about the dangers of seed oils. There's many decorated health professionals and doctors out there that are sounding the alarm about the dangers of seed oils, including Dr. Kate Shanahan, 13-time New York Times bestselling author Dr. Mark Hyman, Dave Feldman, Stanford graduates Calley Means and Dr. Casey Means, Chris Knobbe, MD — and this is just the very tip of the iceberg. We're all sounding the alarms about the dangers that seed oils have. And today I'm going to be breaking down the science for you so you actually understand why seed oils are a pretty toxic ingredient in our food system. So what does it actually mean for something to be toxic? Well, the definition of toxic is just one word: poisonous. This gives people the impression that for things to be toxic, it needs to be acutely toxic and kill you pretty much instantly. But there's such a thing as long-term toxicity. You’ve probably heard the phrase “the dose makes the poison.” To give you a few examples, take smoking for instance. Smoking cigarettes won’t kill you right away. But if you smoke for years on end, you're more likely to get cancer and even die. Take trans fats for instance. These are considered toxic. These were banned in 2018 by the FDA. But a little bit of trans fats won’t kill you. But if you eat trans fats over a longer period of time — years to decades — these will cause some serious health problems. And this is the same exact problem that we're also having with seed oils. So why do prestigious organizations and health professionals say that things like cottonseed oil, corn oil, canola oil, sunflower oil, soybean oil, and safflower oil are all healthy? Well, I can pretty much sum that up in one sentence. Mainstream health professionals are hyper-fixated on LDL as the culprit and the cause behind cardiovascular disease. If you just have high LDL, then you are at a substantially higher risk of developing plaque inside your arteries and suffering from heart disease. And because seed oils like the ones that I just mentioned lower your LDL — and animal fats like tallow, butter, and ghee raise your LDL — then mainstream professionals say eat seed oils. It’s going to lower your LDL and reduce your risk of cardiovascular disease. But there are major flaws in this line of thinking. As we broke down in our most recent video talking about Whole30 and their inclusion of seed oils in their program, these mainstream organizations and health professionals pretty much only cite a couple different types of studies, forgetting to cite a lot of the research that I'm about to talk about today. They only provide research showing that seed oils lower LDL, or they cite observational studies funded by seed oil companies that show eating more seed oils lowers your risk of cardiovascular disease. But as we've pointed out before, that science just isn’t very good. The problem with this logic is that eating seed oils to lower your LDL does not reduce your risk of heart disease. A study involving almost 137,000 patients hospitalized with a heart attack found that 75% of them had low to normal LDL levels. People are having heart attacks with high LDL and people are having heart attacks with low LDL. Why is that? Because the root cause of cardiovascular disease is not just LDL. It’s when your LDL becomes oxidized. Sure, seed oils might lower your LDL, but they’re going to cause any LDL that you do have to become highly oxidized. Oxidation and oxidative stress are what we are really going to hammer home today. Just by definition, oxidation itself is toxic. And after repeated exposure and enough oxidative stress, many health problems can occur. Much of the outdated view on heart disease focused solely on the amount of LDL. But newer science shows that oxidized LDL is a far better predictor of cardiovascular problems. In fact, evidence suggests atherosclerosis may not even be possible without oxidized LDL. Seed oils become oxidized in several ways: industrial extraction, storage exposure to air and light, breakdown into oxidized linoleic acid metabolites after ingestion, and especially when heated during cooking or deep frying — producing rancid byproducts and even trans fats that the body struggles to eliminate. Seed oils entered the human diet only recently — in the late 1800s and early 1900s — before modern safety testing existed. These oils were grandfathered into GRAS status without long-term testing. One major problem with seed oils is that they deplete antioxidant vitamins like vitamin E and vitamin C. Research from the 1950s and 60s showed that even high doses of vitamin E could not protect against the oxidative damage caused by high-PUFA diets. Over time, fat cells become overloaded with polyunsaturated fats, stop functioning properly, and release inflammatory cytokines — a state known as oxidative stress. Oxidative stress plays a role in heart disease, strokes, Alzheimer’s, Parkinson’s, autoimmune disease, diabetes, cancer, and more. Today, seed oils make up roughly 80% of our fat calories — compared to essentially zero just over a century ago. The core issue is linoleic acid. We need a small amount, but we are consuming massive excess. Linoleic acid has a half-life of about 680 days and accumulates in body fat. Research by Chris Knobbe, MD shows that traditional cultures had body fat with about 2–4% PUFA, while modern Americans are closer to 20–30%. Excess linoleic acid breaks down into oxidized linoleic acid metabolites (OXLAMs), which are highly inflammatory and linked to many chronic diseases. Heating seed oils creates aldehydes like formaldehyde — a known carcinogen. A 2019 study in Nature found that a small serving of fries cooked in seed oil contained aldehyde levels equivalent to smoking 20–25 cigarettes. This is not meant to scare you. It’s meant to inform you. Seed oils damage mitochondria — the energy factories of the body — which helps explain fatigue, brain fog, poor fat burning, insulin resistance, and chronic disease. The real drivers of modern disease are lifestyle-driven: ultra-processed foods, refined sugar, seed oils, sedentary behavior, poor sleep, stress, and environmental toxins. The solution is not fear. The solution is real food, stable fats, movement, sunlight, and lifestyle. Don’t fear animal foods like red meat, butter, tallow, ghee, eggs, and dairy. These foods are not the enemy. Seed oils are one of the most problematic ingredients in the modern food supply — not because they kill you instantly, but because of what they do over time. My goal is not fear. My goal is education and hope — that you can make meaningful changes and reclaim your health. I'm Craig McCloskey. Thanks for being here. If this helped you, leave a like, comment, subscribe, and share this with someone who needs it. I’ll talk to you next time.
What Does “Toxic” Actually Mean?
One of the biggest problems in this debate is how people hear the word toxic.
When most people think of something toxic, they imagine something that kills you immediately — like cyanide or rat poison. But that’s not how toxicity works in real life.
Toxicity exists on a spectrum. There’s acute toxicity, which causes immediate harm, and chronic toxicity, which causes damage slowly, over time, through repeated exposure.
Smoking is a perfect example. One cigarette won’t kill you. But smoking for decades dramatically increases your risk of cancer, heart disease, and early death.
Trans fats were the same way. They didn’t kill people instantly — but over time, they damaged arteries, disrupted metabolism, and contributed to cardiovascular disease. That’s why they were eventually banned.
Seed oils fall into this same category. A little bit here and there probably won’t destroy your health overnight. But years of high exposure? That’s where the problems begin.
As toxicologists like to say:
The dose makes the poison.
Why Mainstream Nutrition Defends Seed Oils
If seed oils are so problematic, why do major health organizations still recommend them?
The answer comes down to one word: LDL.
For decades, conventional cardiology has focused almost exclusively on lowering LDL cholesterol as the primary strategy for preventing heart disease. And here’s the key point:
Seed oils tend to lower LDL
Animal fats tend to raise LDL
So the logic goes like this: if high LDL causes heart disease, and seed oils lower LDL, then seed oils must be heart-healthy.
But there’s a massive flaw in that reasoning.
In other words, people are having heart attacks at every LDL level.
That forces us to ask a better question:
If LDL alone isn’t the problem… what is?
Oxidized LDL: The Missing Piece
The real issue is what happens when LDL becomes oxidized.
This distinction matters — because something can lower LDL while simultaneously making the remaining LDL more prone to oxidation.
And that’s exactly where seed oils come into the picture.
LDL itself is not inherently dangerous — the real risk appears when LDL becomes oxidized. Oxidized LDL is far more predictive of cardiovascular disease than total LDL levels alone.
Why Seed Oils Are So Prone to Oxidation
Seed oils are uniquely vulnerable to oxidative damage for several reasons.
They are extracted using industrial processes that involve high heat, chemical solvents, pressure, and exposure to oxygen. This alone damages their fragile polyunsaturated fatty acids before they ever reach your plate.
After extraction, these oils sit in plastic bottles, under bright lights, on store shelves for months — continuing to oxidize.
When we consume them, their high linoleic acid content breaks down into oxidized linoleic acid metabolites (OXLAMs) inside the body.
And finally — and perhaps worst of all — we heat them.
Cooking with seed oils dramatically increases the formation of toxic aldehydes, trans-fat byproducts, and lipid peroxides — compounds your body struggles to detoxify.
Oxidation isn’t a theory here. By definition, oxidation in this context is chemical damage.
Seed oils are uniquely prone to oxidation due to industrial processing, prolonged storage, and high-heat cooking — creating damaged fats long before they’re even eaten.
A Short History Lesson: Seed Oils Are New
One fact that rarely gets discussed is how new seed oils actually are.
Widespread consumption of industrial seed oils began in the late 1800s to early 1900s — long before modern safety testing standards existed.
These oils were essentially grandfathered into “GRAS” (Generally Recognized As Safe) status without long-term human outcome trials. No generational studies. No chronic toxicity assessments. No mitochondrial research.
Compare that to animal fats like butter, tallow, ghee, and lard — which humans consumed for thousands of years.
Novel foods deserve scrutiny, especially when their consumption explodes alongside chronic disease.
The Antioxidant Depletion Problem
One of the most underappreciated issues with seed oils is how aggressively they deplete antioxidant reserves, particularly vitamin E.
A low-PUFA diet based on lard
A high-PUFA diet based on corn oil
Initially, researchers noticed that the high-PUFA group required three times more vitamin E. But after two years, something alarming happened.
Despite continued vitamin E intake, their blood levels collapsed, and biopsies showed severe oxidative damage to cell membranes.
In simple terms: the antioxidant system couldn’t keep up.
This revealed a critical insight — once PUFA intake crosses a threshold, oxidative stress becomes self-perpetuating.
You can’t “out-supplement” the problem.
Oxidative Stress and Chronic Disease
Oxidative stress isn’t just about heart disease.
It plays a role in nearly every major chronic condition we see today:
Cardiovascular disease
Neurodegenerative diseases like Alzheimer’s and Parkinson’s
Autoimmune conditions
Type 2 diabetes
Cancer
Metabolic dysfunction and obesity
This matters because seed oils now make up 80% of fat calories in the modern diet — compared to virtually zero just over a century ago.
That kind of dietary shift is unprecedented.
Linoleic Acid: The Real Issue
At the center of the seed oil debate is linoleic acid, an omega-6 polyunsaturated fatty acid.
We do need linoleic acid — but only in very small amounts.
The problem is dosage.
Today, linoleic acid makes up 20–30% of total daily calories for many people. That’s far beyond what human physiology evolved to handle.
That difference matters — especially when fat tissue acts as a long-term reservoir.
Linoleic acid accumulates in human fat tissue for years, alters cell membrane structure, and serves as a long-term source of oxidative stress when consumed in excess.
OXLAMs and Aldehydes: The Real Toxins
When linoleic acid oxidizes, it forms oxidized linoleic acid metabolites (OXLAMs) — compounds linked to inflammation, mitochondrial damage, and cellular dysfunction.
Heating seed oils makes things even worse.
That’s not fear-mongering. That’s chemistry.
Mitochondria: Where Everything Breaks Down
Mitochondria are the energy factories of your cells — and they’re especially dense in the heart, brain, eyes, pancreas, and muscles. That’s why we often see many major health issues in these specific parts of the body.
Linoleic acid is structural fat, not fuel. When mitochondria are forced to process excessive linoleic acid alongside refined carbohydrates, oxidative stress skyrockets.
This helps explain:
Low energy
Fatigue
Brain fog
Difficulty losing weight
Metabolic dysfunction
Cataracts
Cardiovascular issues
Certain cancers
It’s not laziness. It’s cellular stress.
Seed oils are not acutely toxic — they are chronically damaging. Over time, repeated exposure drives oxidation, metabolic stress, and cellular dysfunction that compounds across decades.
Why the Medical System Misses This
Most doctors are not malicious or incompetent — they’re operating inside a system that rewards pharmaceutical solutions and oversimplified biomarkers.
LDL must be the villain for statins to make sense.
But chronic disease isn’t caused by a single number. It’s caused by lifestyle mismatches — ultra-processed foods, sedentary living, chronic stress, poor sleep, and environmental toxins.
Seed oils sit right at the center of that storm.
The Takeaway: What Actually Helps
This isn’t about fear.
You don’t need to panic if you’ve eaten seed oils. You don’t need perfection. You don’t need to live in a bubble.
But removing seed oils for 30–60 days is one of the simplest experiments you can run for your health.
Focus on real foods. Traditional fats. Movement. Sunlight. Sleep.
And most importantly — don’t be afraid of the foods humans thrived on long before industrial processing existed.
SHOP RESOURCES
Books to Read About Seed Oils
Tap a book to shop it on Amazon. These are some of the best, research-backed resources on seed oils.
Nutrition
Dark Calories
How Vegetable Oils Destroy Our Health and How We Can Get It Back
Shop on Amazon →
Nutrition
The Ancestral Diet Revolution
How Vegetable Oils and Processed Foods Destroy Our Health - and How to Recover!
Shop on Amazon →
Nutrition
The Big Fat Surprise
Why Butter, Meat and Cheese Belong in a Healthy Diet
Shop on Amazon →
Nutrition
Deep Nutrition
Why Your Genes Need Traditional Food
Shop on Amazon →
Nutrition
Fat For Fuel
A Revolutionary Diet to Combat Cancer, Boost Brain Power, and Increase Your Energy
Shop on Amazon →
Nutrition
Eat Fat, Get Thin
Why the Fat We Eat Is the Key to Sustained Weight Loss and Vibrant Health
Shop on Amazon →
Nutrition
The Fatburn Fix
Boost Energy, End Hunger, and Lose Weight by Using Body Fat for Fuel
Shop on Amazon →
Nutrition
The Great Cholesterol Myth
Why Lowering Your Cholesterol Won't Prevent Heart Disease-and the Statin-Free Plan That Will
Shop on Amazon →
