If you’ve spent any time in the online world of paleo, ketogenic or low-carb eating, you will have noticed that while our community disagrees on some issues, we all hate vegetable oil. It’s unanimous.

So, what’s the problem? Why are we all so vitriolic in our opposition to this ubiquitous ingredient?

Let’s start by explaining what vegetable oil is:

Anyone who has prepared, or eaten a vegetable, will have noticed that they are not, generally speaking, oily. It’s hard to envisage how any meaningful quantity of golden, liquid fat could be squeezed out of a vegetable. This is because these oils are derived from seeds, mainly cottonseed, soybean, rapeseed, sunflower, safflower and corn.

As you can imagine, these seeds aren’t exactly dripping with the stuff either, and the extraction process involves heat (and so oxidation, or ‘reaction with oxygen’) and chemical extraction with hexane, followed by a deodorising process and a chemical colouring – these to create an oil that’s palatable enough for people to accept it as a ‘food’. Let’s just say that nobody is jumping around, barefoot in a vat of olives.

So, what does it actually do to us, that’s so bad?

I’m going to make these points very simple, so that we don’t have to invest in a pile of biochemistry textbooks in order to follow them. At the end of this article, I’ll list some further references for anyone who wants a deeper dive.

1. These fats are unsaturated, which makes them unstable:

We classify fatty acids as saturated, monounsaturated or polyunsaturated.

Fats are chains of carbon and hydrogen atoms. Where the chain is saturated, there’s no spare bond available for other atoms to attach themselves. These fats don’t react with other chemical compounds – they are stable in the presence of heat and oxygen, and solid at room temperature.

Unsaturated fats have at least one (monounsaturated) or more (polyunsaturated) ‘double bonds’ which allow them to react with their environment to a far greater extent. They are vulnerable to heat, light and oxygen, and are liquid at room temperature. These fats easily oxidise, and that makes them harmful to our cells. Those spare bonds ‘steal’ oxygen atoms from other compounds, damaging our DNA, proteins and membranes.

2. Excess polyunsaturated fats can change the way our fat, our ‘adipose tissue’, behaves:

Ruminants, like cows and sheep, have the ability to convert polyunsaturated fats into saturated fatty acids for storage. Monogastric animals like pigs, chickens and humans, can’t do this. If we eat polyunsaturated fatty acids, they will be stored in that form within our adipose tissue, where they will remain for some 18 months.

Surgeons report that the fat itself actually looks different when it contains a high amount of polyunsaturated fatty acids, but it’s not only the appearance that changes – it’s the behaviour:

Our fat is effectively an organ, performing critical metabolic signalling. If it starts to malfunction, those signals go awry, and the whole system breaks down, leading to serious metabolic diseases, like diabetes.

When we eat excessive amounts of linoleic acid, the primary omega 6 fatty acid found in these oils, we make our adipose tissue disproportionately sensitive to the hormone insulin.

Insulin tells cells what to do with incoming sources of energy like fats or sugars. When the fat cells are unresponsive to insulin, or when there is very little insulin knocking around, the fat cells break down their contents for release into the blood as free fatty acids, in a process called lipolysis. This represents a healthy exchange of fat-based energy with the blood.

Conversely, when the fat cells are overly-sensitive to insulin’s message, as in the case of excess dietary linoleic acid, lipolysis is inhibited – imagine the fat cells as a bathtub, and insulin as the plug. The bathtub gets fuller and fuller, in a process of hypertrophy, until the fat within reaches the overflow, at which point it begins to spill out into the blood regardless of the presence of insulin. This time however, it’s accompanied by inflammatory cytokines, released because the fat cell is communicating that it’s too full, inflamed and can’t cope with any more incoming energy – it’s sending out a mayday. At this point we begin to develop a system-wide insensitivity to insulin, or insulin resistance, that is the hallmark of type-2 diabetes.

So, why don’t all obese people have diabetes?

The disease takes hold when the fat cells are enlarged, inflamed, and dysfunctional. If, instead of becoming bigger, a fat cell can simply divide to create two, healthy fat cells – a process called hyperplasia – a person can create ever more fat, without running into the problems of inflamed adipose tissue.

While many obese people are diabetic, or pre-diabetic, the truly obese tend to be healthy from a metabolic standpoint, by virtue of the fact that they have this ability to create almost unlimited numbers of new fat cells. This is largely genetic – caucasians tend to have a greater propensity for hyperplasia than, for example, south asians.

But, there is now evidence in animal models, that shows that linoleic acid, the predominant polyunsaturated omega 6 fatty acid in our diets, inhibits hyperplasia – in other words it prevents this division of fat cells, and forces us more rapidly towards a disease state of fat cell hypertrophy.

In summary, having too much of this polyunsaturated fat within our adipose tissue causes it to expand, making us not only fatter, but once the fat cells reach capacity, sicker.

This seems to particularly affect our visceral fat – the fat around our organs – levels of which are strongly correlated with serious disease. The dysfunction of inflamed adipose tissue may well be the reason why obesity and disease are so tightly linked.

3. The omega 3 to omega 6 ratio.

I realise that I’m making it sound like polyunsaturated fats should be entirely removed from our diet. Absolutely not – they’re found within a variety of perfectly healthy whole foods, and are essential to life: We need both omega 3, and omega 6 polyunsaturated fats, but we need them in a quantity, and a ratio that’s evolutionarily consistent.

Over the past 50 years we have dramatically increased the overall quantity of omega 6 fatty acids, largely by the addition of vegetable oil to our diets, and to the diets of the animals we eat (this especially affects monogastric animals like pigs and chickens).

Back when wild animals ate wild plants, and we ate wild animals, we consumed roughly equal amounts of omega 3 and omega 6, and in relatively small quantities overall. Our largest sources of omega 6 fats would have been seeds and nuts, which we would not have been able to consume in large amounts. We obtained bioavailable omega 3 fats from fish, and from the nose-to-tail consumption of ruminant animals.

Now this picture has changed: We consume vast quantities of polyunsaturated fats, as a direct result of Ancel Keys’ unproven hypothesis that dietary saturated fat is the cause of heart disease. We now know that Keys was so convinced of his theory that he manipulated data to show what he believed to be true. But this information did not come to light until relatively recently, and in the meantime a vast industry has sprung up around the manufacture and marketing of low-fat, and low saturated fat products. These highly-processed, cheaply-made options are worth a fortune annually – industry and governments alike have a vested interest in their propagation.

This swing away from animal fats, towards these cheap plant-based fats has skewed the omega 3 to omega 6 ratio from 1:1 to more like 1:20. This is disastrous, because the fine balance of these fats mediates inflammatory and anti-inflammatory processes within the body. Just like the fats themselves, both processes are essential to life, but when we eat in such a way that we promote inflammation, the result is a host of serious, life-threatening diseases, like cancer, heart disease, asthma, autoimmune disease, depression and Alzheimer’s disease.

4. Vegetable oils cause heart disease – they don’t protect us from it, as we have been led to believe.

In the 1950’s an american biochemist, Ancel Keys, convinced himself, and everyone else, that consuming saturated fat causes high cholesterol, and that high levels of cholesterol cause heart disease. The reality is that our increased consumption of seed oils has likely been a key factor in the reason why, despite a massive reduction in smoking, heart disease remains the biggest killer in the developed world:

As our understanding of the process of heart disease has evolved, we have learned that it’s likely not the amount of cholesterol in our blood, but more the level of inflammation in the arteries, and the health of the cholesterol itself*. Seed oils contribute negatively to both these factors, by virtue of causing inflammation (see point 3, above) and by making the lipoprotein, the containers that carry fats and cholesterol around the blood, more prone to oxidation. The theory here is that when these lipoprotein particles are oxidised, they don’t ‘look right’ to the cells they’re supposed to attach to. They become pariahs in the bloodstream, and the immune system’s macrophages envelope them, drawing them into the artery wall for removal. If this process happens too frequently, they can’t be cleared away quickly enough, and blockages begin to form.

*I’m simplifying here. The process of atherosclerosis is almost certainly multifactorial. In recent years many highly compelling theories have been put forward, and we still do not have a complete grasp of which of these, or what combination of them is at play.

5. Vegetable oils are toxic:

Not only are these oils heated as part of their extraction, but they’re very often heated, and worse, heated repeatedly as part of the way we use them. A good example of this would be the oil used to deep-fry foods in a restaurant. The more that oil is heated, the more it oxidises, and the greater the formation of toxic by-products – free radicals, aldehydes, like HNE and acrolein, and formaldehyde – bad stuff, that has been repeatedly proven to do horrible things to us, and to the animals used in studies. Austrian biochemist, Hermann Esterbauer, credited with discovering the broad category of aldehydes as peroxidation products, identifies extensive evidence that these compounds cause ‘rapid cell death’, interfering with DNA and basic cellular function.

Whenever we stray too far from the foods we have evolved to eat, whether it be replacing good-quality saturated fats, from grass-fed ruminant animals with industrial seed oils, or replacing fresh fruit with refined sugars, or fruit-flavoured soda, problems seem to abound.

So why did we stop using traditional animal fats?

As mentioned above, it all started with Ancel Keys and his ‘Diet Heart’ hypothesis. Keys was so convinced that he had the answer to heart disease, that he was prepared to distort his results to prove it. It wasn’t that he was a bad man, more that he was convinced that he needed to do something unscientific for the greater good. Later, others involved would admit that results were buried because they were disappointing to the team, but back in the 1950’s, following President Eisenhower’s heart attack, the world wanted an answer, and Keys was only too keen to be the one to give it. Saturated fat was demonised.

In 1961, the American Heart Association recommended that saturated fats be replaced with polyunsaturated seed oils. Now, surely the AHA is a noble, altruistic organisation, interested only in the most rigorous science, untainted by commercial influence, seeking only to better the health of the nation by propagating the most up-to-date information?

Sadly not. The American Heart Association is built on generous funding from Proctor And Gamble, the developers of Crisco, who had realised fifty years previously that seed oils, which at the time were used as industrial lubricants, could not only replace tallow in candles, and soaps, but also in cooking fats. They had developed hydrogenated lard replacements, high in trans fats, which we subsequently discovered to be exceptionally harmful to humans. In 1948 P&G contributed some $17 million to the AHA – at the time a vast fortune. More recently funding continues to flow from other industrial benefactors like Bayer.

It’s unfortunate that these instances of the ‘fox having the keys to the henhouse’ seem so pervasive in the world of food products and dietary guidelines, but I don’t believe there was any grand, overarching plot at that time. I think it was really more a question of businesses doing what businesses do, which is to take advantage of the commercial opportunities that present themselves. After all, it wasn’t until later that the harmful nature of these products began to surface.

The food industry continued to promote their ever-expanding ranges of highly-processed alternatives to traditional foods. They did everything they could to paint the ‘old-fashioned’ ways of eating and cooking in a negative light. Far from being seen as wholesome, the foods our grandparents had grown up on were portrayed as disgusting, unhealthy, messy, dirty and out-of-date.

Throughout the 1970’s and 80’s our diets changed out of all recognition, and with this change came two things. The first was money – food companies made a fortune out of these products that were so cheap to produce, had such a long shelf-life and could be made addictive, and hyper-palatable, circumventing our satiety signals. The second, was obesity. When we look back at photographs of people in the 1970’s we’re shocked by how slim they were. If they could have looked forward, they’d have been shocked too.

We are more active than ever. We go to more lengths than ever to control our weight, and to eat in a healthy way, yet obesity continues to rise, and in ever younger children. Despite the best efforts of modern medicine and sanitation, we are sicker than ever – we may keep people alive more easily, but we are not a healthy bunch.

There are many contributions to this in the modern world, and it’s not fair to blame it all on our food. Equally it’s not fair to blame all the problems with our food on seed oils – processed sugars are a strong competitor, and indeed co-factor in the harms these oils do. That said, if we could do one thing to change the health of our population for the better, there’s a strong argument that it should be to exclude processed vegetable oil.

OK, so do we just stop buying bottles of vegetable oil?

It’s not quite that easy. These oils are almost ubiquitous in processed food products. They are used as fillers, and to add ‘mouthfeel’. We find them in sauces, dressings (even those that say ‘made with olive oil’) condiments, confectionary and desserts, bread and other baked goods, marinades, breakfast cereals, packaged sandwiches and of course, anything fried in a restaurant. The list is almost endless. They are so prevalent that it’s safest to assume that anything with an ingredients list contains them, and that that list must be scrutinised to prove that a product does not.

Do we need to fear whole-food sources of unsaturated fat?

It depends. I simply don’t believe that consuming these fats in evolutionarily consistent quantities would cause us any problems.

The issue is that rather than that small handful of nuts or seeds, we have a bag, roasted and salted so that we simply can’t stop eating them. Rather than a few olives, picked from a tree, we drown our salads in olive oil. Instead of allowing our monogastric animals, our pigs and chickens to pasture and forage, we give them commercial feeds that are extremely high in linoleic acid. Their fat is not the fat that it would have been a hundred years ago, and certainly not the fat of a wild animal in pre-agricultural times. So even in the context of whole-food sources, we need to be a little careful not to binge on these things.

Ideally we should be choosing foods that are as close as possible to the foods we have evolved to eat, and in the quantities we would have consumed them:

So, what should we eat instead?

Ideally we want to derive most of our dietary fat from high-quality animal sources, specifically ruminant animals like cows and sheep. The better the diet of these animals, the better the ratio of fatty acids within the fat we are eating – a grass-fed animal will have less polyunsaturated fat, and a better ratio of omega 3 to omega 6 fatty acids.

(In times past, we may have done well with lard, but due to the diet we feed pigs, their fat is now a significant source of omega 6 fats. Lard is naturally more than 50% oleic acid anyway – it’s more unsaturated than saturated. The same applies to chicken fat.)

Dairy products, for those who tolerate them, can provide an excellent source of saturated fats. It is often the case that those who struggle to digest milk and cheese, do fine with butter and ghee.

What about olive oil? Surely that’s healthy? The primary constituent of olive oil is oleic acid, a monounsaturated fat. This is less prone to oxidation than the polyunsaturated fats in vegetable oils, but still not as stable as saturated fats. I believe olive oil is healthy, and a far better option, that holds up well under heat. That said, we need to keep evolutionary consistency in mind, and not go crazy, sloshing it over everything.

N.B if you’re in the United States, be aware that much olive oil is cut with vegetable oil – be very mindful of your source. As a general rule, always try to find olive oil in a dark, or opaque bottle, and store it away from sources of heat. The peppery hit in the back of your throat that comes from an extra virgin olive oil is a good sign that it has not oxidised.

Oily fish provide a good source of fat, specifically omega 3 fatty acids, which can help us to redress that omega 3 to omega 6 imbalance. But should we be supplementing with fish oil? I’d say not, because we quickly begin to include too much polyunsaturated fat overall.

In terms of plant sources of stable, saturated fat, try coconut oil, cacao butter or ethically-sourced unrefined palm oil.

Generally speaking, we can substitute the vegetable oils in our diet for saturated fat in a way that represents an upgrade in flavour as well as health. But there are recipes in which vegetable oil provides a neutral flavour that we struggle to get elsewhere. A good example of this is mayonnaise – we don’t want a strong-tasting oil. The best way around this is to use a mild and light pressing of olive oil, and make the mayo ourselves. Here’s a video demonstrating precisely how to do this yourself:

Find the balance

It’s going to be difficult to completely exclude these things, because we can’t always control the food we eat. If we’re visiting friends, or eating in a restaurant, we’re highly likely to be taking in some processed seed oil. We have to accept that the strictest level of dietary control has other, negative consequences: If we refuse to eat at a friend’s home, we maintain our control, but we lose out on a fundamental pleasure – a crucial factor in happiness and longevity. We need to be careful not to let optimal dietary strategies isolate us, reducing the benefits of social connection.

Sometimes we might choose to take our kids for a burger, and to eat fries. It’s not the perfect choice, and while it’d be a great deal better if they were beef dripping fries, that might not be an option. As always, we need to fine tune our level of control, so that a dietary upgrade doesn’t become a life downgrade. By removing these products from our own kitchens, and from our day-to-day eating, we are making an enormous difference. Excluding them from every possible situation ever, will offer only marginal additional benefit.


Brad Marshall – Fire In A Bottle website
Big Fat Surprise, book by Nina Teicholz
Chris Kresser on seed oils article
Tucker Goodrich and Ivor Cummins podcast
Dr Cate Shannahan on soybean oil article
Paul Saladino and Brad Marshall podcast

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