Are Seed Oils Bad For You?

The boogeyman in your cupboard.

Feb 22, 2025

Nutritional spaces are spending a lot of time warning us of the dangers of seed oils right now. It’s the new problem food on the block, allegedly causing titanic amounts of heart disease, inflammation, and obesity. The new US Secretary of Health, RFK Jr., is particularly critical of seed oils and thinks we should be using beef tallow instead.

Pictured here, next to other respected health experts.

This is an important issue to get right, because seed oils are ubiquitous. Most ingredients lists have one on there somewhere. If they really are as dangerous as people say, replacing them could be an opportunity for enormous health gains. However, if they’re not bad for you, replacing them would be an enormous waste of our time and energy. Time that could be better spent on evidence-backed health interventions, like exercise, reducing red meat intake, or not looking at shirtless pics of Dr Oz (sorry about that).

Inflammation

Probably the most common critique of seed oils is that they cause inflammation. If, like me, you spent years knowing “Inflammation” sounds bad, but weren’t sure what it actually meant, here’s a quick run through.

Inflammation is a response our body makes when something hurts it. When you sprain your ankle and it swells to the size of a watermelon, that’s inflammation. Your body is sending huge amounts of blood to the area, with the goal of removing damaged tissue and starting the repair process. We sometimes think swelling is bad because it coincides with injury, but it’s actually good! If your ankle didn’t swell up like that, it probably wouldn’t be long for this world.

However, we can all have too much of a good thing. We also have low-grade inflammation, which, unlike the acute inflammation in your bum ankle, persists in the body over time. Too much low-grade inflammation is associated with health hazards like blocked arteries and arthritis. So, while inflammation can be handy for injuries, we want to avoid too much of it in the background if we want to make it to 60.

Seed oils contain a high amount of linoleic acid. Some people worry that this is converted to arachidonic acid, which, apart from sounding like a terrifying spider-acid, is pro-inflammatory. As a result, you’ve probably seen people warn online that you need to watch your seed oil intake. The worry is that if you have too much, you’ll convert it all to arachidonic acid, and cause tonnes of inflammation.

The problem, however, is that this worry isn’t supported by evidence. This meta-analysis (a review of many randomised control trials) found that increasing linoleic acid intake did not have a significant effect on the blood concentrations of inflammatory markers.

This is probably because increasing dietary intake of linoleic acid doesn’t result in larger amounts of arachidonic acid. This meta-analysis includes a trial where people’s linoleic intake was increased six-fold, and still they saw no correlation with arachidonic acid. So, the worry people have about this deadly conversion seems unfounded.

This is the problem with relying on mechanistic data instead of outcome data. Many people look at some mechanism, like the conversion of linoleic acid into arachidonic acid, and extrapolate their way to health outcomes. It’s not a bad hypothesis, but we can’t know until we actually check. We should reserve judgement until we see the outcome data because there’ll be many other variables influencing things. Looking at mechanisms alone will never give us the full picture.

Extra Evidence on Inflammation

Effect of dietary linoleic acid on markers of inflammation in healthy persons: a systematic reviews of randomised control trials - “We conclude that virtually no evidence is available from randomized, controlled intervention studies among healthy, noninfant human beings to show that addition of Linoleic Acid to the diet increases the concentration of inflammatory markers.”

The Effect of Plant Derived Bioactive Compounds on Inflammation: A Systematic Review and Meta Analysis - “These findings strongly support the recommendation for diets high in Fruits and Vegetable and plant-oils to help attenuate elevated CRP.” (CRP stands for C-Reactive Protein, which is an inflammatory marker)

Heart Disease

Next up we have Heart disease. Again, a very important thing to get right. It might be a surprise, but heart disease is the leading cause of death globally. I guess that’s good news, because it probably used to be starvation or sabretooth attacks. However, in the immortal words of the girl in the old El Paso advert, “Why don’t we have both?” (Or, I suppose, “neither”, but you get the point.)

Newton. Pasteur. Girl from old El Paso advert.

Now, part of the reason people worry about seed oils causing heart disease is because they worry it causes inflammation, which we’ve already spoken about (remember? from before?). However, there are other concerns, like seed oils oxidizing LDL.

Low-density lipoprotein is also known as “bad cholesterol”. It gets called this because it’s cholesterol, and it’s bad. When LDL oxidizes, it triggers an immune response in your blood. White blood cells move in to engulf it, but they end up getting stuck in the inner lining of your artery. This stiffens them, blocks them, and eventually sends you to the Netherrealm. So, we definitely don’t want to oxidize LDL. If seed oils do that, we’re in trouble.

Good news! They don’t. This might be because seed oils contain antioxidants like vitamin E, which do the exact opposite of what critics of seed oils worry about. Evidence here, here, and here.

This is all a bit of a tangent, because as I said earlier, the best way to know if seed oil consumption increases heart disease risk is to just look at health outcomes. I sound like a broken record, but understanding this will clear out 90% of nutritional misinformation from your life. When looking at outcome data, we can see that not only do seed oils not increase your risk of heart disease, they might protect against it.

This meta-analysis found that replacing saturated fats (*cough* beef tallow *cough*) with polyunsaturated fats (often in the form of vegetable/seed oils) reduced the risk of coronary heart disease events by 19%. This aligns with observational data that people who consume large amounts of linoleic acid have lower rates of coronary heart disease events, and coronary heart disease deaths. Each 5% of energy increment in linoleic acid intake replacing energy from saturated fat intake was associated with 9% lower risk of CHD events.

Extra Evidence on Heart Disease

It’s important to look at totality of evidence when coming to conclusions though. Some research implies that seed oils aren’t beneficial for heart disease, but just have no impact.

The effect of replacing saturated fat with mostly n-6 polyunsaturated fat on coronary heart disease: a meta-analysis of randomised controlled trials - “When pooling results from only the adequately controlled trials there was no effect for major CHD events, total CHD events, CHD mortality and total mortality. ”

Effect of linoleic acid on ischemic heart disease and its risk factors: a Mendelian randomization study - “Our novel study suggests a benefit of linoleic acid for diabetes and lipids but no benefit for IHD, blood pressure, or reticulocyte count”

Obesity

The obesity claims are an odd one, because the physiology of obesity is quite simple. Energy in, energy out. If you consume more calories than you burn, you will gain weight. If you burn more calories than you consume, you will lose weight. So, to say any food is inherently fattening is a bit of a misnomer. Really what matters is how much of it you’re eating. If you ate 5000 calories of apples a day, you would get fat (and probably die). If you ate a single Big Mac each day, you would lose weight (and probably die). It’s just conservation of energy.

However, some foods are certainly calorie dense. The whole reason people think Big Macs are fattening is because they have a lot of calories in them without satiating you. So, it’s easier to eat 5000 calories of Big Macs than it is to eat 5000 calories of apples. In this way, seed oils have the same danger. They are fat, and so pack a lot of energy in a small package.

However, the same is also true for proposed replacements. Beef tallow and butter have the same calorie density as rapeseed oil, so it’s not clear what the criticism is. Seed oils aren’t uniquely positioned to cause obesity. Like any food, if you keep an eye on your calorie intake, including them in your diet won’t cause you to get fat. This is how Mediterranean people have low obesity rates despite consuming large amounts of oil.

If I had to guess what’s actually causing the obesity crisis, I’d say it has something to do with fast food being easy to access, the rise of white collar work, and portion sizes getting insane (especially in the US. Every meal I’ve had there has done me in like the chocolate cake kid in Matilda). Maybe seed oils contribute, but then we should limit them because of basic calorie restriction, not because they’re particularly prone to chub us up.

Cancer

Similar to Heart disease, some people suspect that seed oils cause cancer because of oxidising LDL. Well, that’s the scientific concern. I reckon your average person might think seed oils cause cancer because they’re processed, and if something is processed it means it was made in a lab by Bill Gates specifically to give us cancer. Something something, New World Order.

Now, I’ve already discussed lipid peroxidation, so I won’t repeat all that. As I’ve said, I’m not super concerned with mechanisms. I’d rather we investigate this concern the same way we usually do - by looking at outcome data (which isn’t particularly conclusive). This meta-analysis found that it’s “unlikely that a high intake of linoleic acid substantially raises the risk of breast, colorectal, or prostate cancer”. A separate one, specifically on breast cancer, found higher consumption of vegetable oils didn’t increase cancer risk. There’s also some (albeit, weaker) evidence that vegetable oils protect against prostate cancer.

However, this meta-analysis of 47 randomised control trials found that high intake of seed oils may slightly increase the risk of cancer, which is score one for RFK Jr. Having said that, that same meta-analysis does specifically use the phrase “may very slightly increase cancer risk, offset by small protective effects on cardiovascular diseases” and that “total Polyunsaturated fatty acid doses were very high in some trials” - so, maybe not a slam dunk.

The evidence on seed oils relationship with cancer is less conclusive than heart disease, however it doesn’t seem like it sways us either way with much force. If they do increase cancer risk, the risk is small enough that it can be outweighed by other factors, like protecting against heart disease (or hell, just not going through the pain of removing seed oils from your diet your whole life).

One smart next step is to investigate the effect seed oils have on all cause mortality. If the cancer effects are big enough to move the needle, we’d expect to see them here. However, replacing saturated and trans fat with linoleic acid is routinely associated with lower mortality risk. - “Intake of n-6 polyunsaturated fat, especially linoleic acid, was inversely associated with mortality due to most major causes, while marine n-3 polyunsaturated fat intake was associated with a modestly lower total mortality”.

This observational study documenting 129,328 deaths found that “butter consumption was positively associated with cancer mortality. Substituting corn oil, canola oil, or olive oil for equal amounts of butter and margarine was related to lower all-cause mortality and mortality from certain causes, including CVD, diabetes, cancer, respiratory disease, and Alzheimer’s disease.”

Bottom Line

The scaremongering about seed oils is not well backed by evidence. Many of the claims made by the likes of RFK Jr. lack real motivation (besides maybe cancer, but the effects seem so small that they’ll be largely outweighed by the cardiovascular benefits of replacing saturated fat). I imagine the fear over seed oils will pass like all the other fads, and in 5 years nutritional quacks will be saying bread causes glaucoma or something instead.

If you want to improve your health, there are other interventions that probably require less effort than removing seed oils, but have way more upside. They might be boring, but they’re boring because they work, and we all know they do.

Get better sleep by waking/going to bed at a regular time, getting 8 hours, and not eating late.
Eat plenty of wholegrains.
Do both cardio, and some form of resistance training.
Eat lots of fruits and vegetables.
Reduce your red meat intake. (Although, I would recommend replacing it with plant protein instead of chicken/eggs because of the impact on animals.)

If you can tick those off your list, you will run circles around the seed oil averse, liver eating, shirtless influencers every time.