Vegetable cooking oils: the cancer connection

They’re kitchen cupboard staples. But should they be?

Maria Cross MSc
Nov 8 · 10 min read
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When vegetable cooking oils first entered the culinary landscape in the early twentieth century, they quickly became the superfood of their time. Sold as healthy alternatives to saturated fat, these new oils were at the centre of the brave new world of dietary expertise.

It took a while for the first cracks to appear, but by then vegetable cooking oils had been firmly established as the cornerstone of a healthy diet. No amount of scientific enquiry was going to shift that perception in a hurry.

But the closer researchers looked, the worse the view became. There seemed to be an association with a number of serious chronic diseases, such as heart disease, irritable bowel disorder, arthritis. And cancer.

Today, those cancers most associated with high consumption of vegetable cooking oils are those of the breast, prostate and colon.

How can something so seemingly innocuous, so deeply insinuated within our food culture, be in any way involved in the development of such serious disease?

The crops most commonly grown to produce these oils are corn, soya, sunflower and canola. The oils extracted from these crops are used liberally in processed foods and ready meals. You’ll see empty barrels of them deposited at the rear of restaurants. Perhaps they are in your store cupboard.

These oils all have one common denominator: they are rich in omega-6 polyunsaturated fatty acids (PUFAs).

Which means?

There are two classes of PUFA: omega-6 and omega-3. Although functionally distinct and non-interchangeable, these two classes are perpetually engaged in a metabolic balancing act, pushing and pulling as they compete for absorption in the body.

There is nothing intrinsically wrong with omega-6 PUFAs: we need them. They are an essential component of each cell membrane and are involved in brain, reproductive, immune and bone health.

Omega-6 PUFAs are found in high amounts in seeds and nuts, including pine nuts, walnuts, pecans, hazelnuts and Brazils. They are also found in grains (barley wheat, corn etc), and other plant foods. Muscle meat contains modest amounts.

Omega-3 fats are found mainly in oily fish and shellfish, and to a lesser extent pasture-fed meat, offal and eggs (from free-range hens). A few plant foods contain high amounts of omega-3 PUFAs, mainly flaxseeds and chia seeds.

If omega-6 fat is essential to health, it makes no sense that it can also cause cancer. Nuts in particular are natural and full of nutrients; hardly killer foods. That’s why scientists believe that it is not omega-6 per se that is to blame; it’s the balance between the two groups of PUFA that is out of kilter and wreaking havoc on our bodies.


We evolved on, and are genetically adapted to, a diet that provides more or less equal amounts of omega-3 and omega-6. That’s how it was for hundreds of thousands of years.

Now, with the industrialisation of our diets, and the vast quantities of vegetable cooking oils that go into them, the ratio between omega-6 and omega-3 has shifted enormously and we consume up to 25 times more omega-6 than omega-3. This diet reorientation has been described as “a total new phenomenon in human evolution.”

Our food supply is now overflowing with omega-6 oils, with very little omega-3 to provide the necessary balance.

Our diets may have evolved, but our genes have not. Our ancient genome has an average mutation rate of 0.5% per million years, meaning it still resides for the greater part in the Paleolithic era”. There can only be consequences, and indeed there are: experimental data supports the theory that it is this skewed balance between the two PUFAs that influences the development of a tumour.

(Huerta-Yépez et al 2016)

The heavy weighting in favour of omega-6 means that omega-3 is depleted: as omega-6 levels in the body rise, omega-3 levels fall. This dysfunctional see-saw effect increases the risk of breast, prostate and colon cancer in many animal and human studies and the ratio between the two classes of PUFA has been suggested as a predictor of cancer progression.

Equally, there is compelling evidence that omega-3 PUFAs offer protection against the development of those cancers most associated with excessive omega-6 consumption.

(Bourre 2007)

Breast cancer

Most studies looking into the effects of PUFAs on cancer progression, especially cancer of the breast, have been conducted on animals. The development of the human mammary gland is similar to that of rodents, which is why they are often the subject of breast cancer research.

Supplementing the diets of rats with fish oil — an exceptionally rich source of omega-3 PUFAs — has been shown to reduce the rates of breast cancer by inhibiting tumour growth and reducing metastases.

Across the world, breast cancer is the most common cancer in the world among women. The global prevalence is not, however, an even distribution. Neither is it fixed.

Asian women who eat a diet high in fish typically experience comparatively low levels of breast cancer — until they emigrate to the West. Within just one generation, rates of breast cancer rise to equal those of westerners. This observation suggests that it is diet and not genes that are involved in the development of the disease.

Prostate cancer

The most commonly diagnosed cancer among men in both the US and the UK is prostate cancer. Like breast cancer, it appears that the balance of omega-3 to omega-6 PUFAs is in some way involved in the development of the disease, though there have been relatively few studies from which to draw any definite conclusions.

Even so, increasing dietary intake of omega-3 PUFAs has been shown to decrease risk of developing aggressive prostate cancer in men with a genetic disposition to the condition. Conversely, a high fish intake appears in studies to have a protective role against the disease.

Colorectal cancer

Colorectal cancer is one of the most common cancers in the West, especially among men. In the same way that breast cancer rates among Asian women increase when they move to the West, rates of colorectal cancer increase significantly among Japanese people after they migrate to the US and adopt the diet there. Not that anyone needs to move: incidence of colorectal cancer has “dramatically increased” recently in Japan as a westernized diet has become the norm.

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What’s the connection?

Association is not the same as causation: what these studies suggest is that one particular dietary factor — omega-6 vegetable oils — is strongly associated with one particular health outcome — cancer.

What we really want to know is this: does excessive intake of these oils directly cause cancer?

Possibly. Various mechanisms have been proposed in an effort to establish a causal factor, but the main focus is on inflammation.

Inflammation is now known to underpin virtually all major chronic diseases: cardiovascular disorders, diabetes, neurodegenerative diseases, inflammatory bowel disease, macular degeneration, rheumatoid arthritis, asthma, autoimmune diseases and of course cancer.

(Greene et al)

Inflammation is not in itself a bad thing — the body creates inflammation as part of the healing process to fight infection or injury. It’s an integral part of the body’s immune response.

Again, it’s all about balance. Excessive and prolonged inflammation, the kind that continues to flare long after it should have subsided, leads to disease. That’s where omega-6 falls under suspicion.

Both omega-6 and omega-3 PUFAs are metabolised in the body to hormone-like substances called eicosanoids. Generally speaking, omega-6 eicosanoids are pro-inflammatory and omega-3 eicosanoids are anti-inflammatory.

(Bhardwaj et al 2016)

The pro-inflammatory eicosanoids produced by omega-6 PUFAs contribute to mammary tumour formation in both women and rodents, and to tumours in other sites, including the colon and lung.

The omega-3 PUFAs, on the other hand, are well known for their anti-inflammatory properties. They work by directly blocking pro-inflammatory omega-6 eicosanoids.

No wonder that omega-3 is now seen as having a potential role in cancer prevention and treatment: they inhibit the growth of tumours that are associated with inflammation.

(Simopoulos 2002)

We now consume vast quantities of these oils in our daily diets. Since the 1960s they have come to replace traditional lard, beef dripping (tallow) and butter in our cooking and in the food we buy.

The 2017 U.S. Department of Agriculture report, US Trends in Food Availability, reveals that between 1970 and 2014, overall saturated fat consumption decreased by 27%. At the same time, salad and cooking oil consumption rose by a staggering 248%.

Alternative fats

We humans are unique in the animal kingdom in that we cook our food, and we have been doing so for at least a million years, according to cave evidence. Some foods just demand to be cooked in fat. “Fat gives food flavor”, as chef and TV personality Julia Child once famously put it. It’s filling, and instantly lights up your brain’s reward centres.

Until the food industry turned up and then teamed up with the newly-formed diet advice industry, the only fat we had at our disposal, and which had served us well since those pioneering cave days, was animal fat, which is predominantly saturated and monounsaturated.

Dietary fats are either polyunsaturated, monounsaturated or saturated. It’s the polyunsaturated fats you need to worry about.

There isn’t space here to discuss the detrimental effects on health of the refining process of these PUFA oils, from bleaching, degumming and deodorising, but you can read about the chemical carnage that is created by this process in my article “Why vegetable cooking oils are as bad as sugar”. And if you are still afraid of saturated fat, you’ll find out why you shouldn’t be.

What to do now

In a nutshell, if you want to cook your food in fat — and who doesn’t — just avoid cooking with PUFAs.

If you prefer not to cook with animal fat, choose virgin coconut oil. It is almost entirely saturated, and full of health promoting properties. Your other option is (extra virgin) olive oil.

I said earlier that the only fat we used in the past is animal fat. That’s not strictly true: in some countries, oil from crushed olives has been used as a cooking medium for thousands of years. Olive oil contains predominantly monounsaturated fat, so is fine to use.

For everyone else, butter, lard and tallow are excellent options.

When it comes to food you haven’t prepared yourself, your options are less clear. You’ll be hard-pressed to find ready-meals or any processed food that does not contain soya, corn, canola, sunflower, sesame or safflower oils.

Meat eaters should aim for meat from pasture-fed livestock — intensively farmed animals are fed corn and soya, which increases the omega-6 fat content in muscle meat. Free-range, pasture-fed animals have a higher ratio of omega-3 to omega-6.

The same goes for eggs — the ratio between omega-3 and omega-6 PUFAs depends on what the laying hens eat.

Eat fish, lots of fish. Oily fish — mackerel, salmon, sardines, herring, anchovies, fresh tuna — are full of omega-3 fatty acids. So too is shellfish.

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Don’t worry about occasional lapses. After all, omega-6 PUFAs are essential and found naturally in real food, and it would be dangerous to write them off as an unqualified carcinogen. Remember, it’s that balance thing.


During the Middle Ages, the superfood du jour was sugar. It was originally a high-end luxury item, destined for the privileged few. Gary Taubes, in his book The Case Against Sugar, describes how this new product, being exotic and foreign, was considered a salve for all sorts of maladies and was prescribed by medics. Medieval crusaders brought it home after their global marauding. Christopher Columbus took sugar to the New World in the fifteenth century.

By the sixteenth century, the Caribbean had become the hub of sugar cane production, using slave labour to produce as much as possible for as little as possible and for a wider market. Prices continued to drop, making it more affordable.

It took a very long time to figure out what fools we were. Sugar has finally — though only recently — been outed as the food felon it truly is. Let’s stop history repeating itself with vegetable oils.

Maria Cross MSc

Written by

Nutritionist and nutrition science writer, specialising in diet and mental health. Subscribe to AllYouCanEat.org.uk for free brain food guide. @MariaXCross

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