Why is Vegetable Oil Unhealthy?

This is Part 3 in a series of posts investigating how vegetable oil impacts our health and our planet. If you’re new to the series, feel free to visit Part 1 or 2.

In the previous post in this series, we talked about the studies (specifically, the randomized trials) showing the deleterious effects of consuming vegetable oil. The obvious next question is: why is vegetable oil unhealthy? Why does its consumption result in higher rates of heart attack, cancer, and disease? And why do mammals get fatter and more depressed when consuming vegetable oil compared to more traditional fats?

We can explore why vegetable oils (seed oils) are unhealthy from several angles:

  • Common sense
  • Historical precedent
  • Omega-6 in vegetable oils
  • The effect of cooking with vegetable oils

Common Sense

Vegetable oils, also known as seed oils, are everywhere, from canola oil in oat milk, bread, and cereal, to grape seed oil in salad dressings, sunflower oil in so-called healthy snacks, and rice bran oil in high-end restaurant food. As a result of vegetable oil’s sneaky ubiquity, the average American consumes 5–10 tablespoons of vegetable oil per day, usually unknowingly [1].

It would be impossible for us to consume tablespoons of vegetable oil without modern industrial processing that allows manufacturers to squeeze minuscule  amounts of oil out of corn kernels, sunflower seeds, grape seeds, and rice bran.

In order to consume the amount of vegetable oil in the average American diet, you would have to eat an impossibly large amount of real food.

You would have to eat 98 ears of corn in order to consume five tablespoons of corn oil, which is about the amount of oil found in a typical Mexican restaurant meal with tortilla chips.

You would have to eat 2,800 sunflower seeds to get the amount of sunflower oil found in a bag of Kettle Brand potato chips, about five tablespoons. That’s the equivalent of ten bags of sunflower seeds.

You would have to eat 5 cups of cooked soybeans, about 2,500 calories worth, to get the equivalent of five tablespoons of soybean oil. The typical Thai restaurant curry uses about four tablespoons of soybean oil. That means that for the average 2,000 calorie diet, you would have to eat an exclusively all-soybeans diet in order to consume the amount of soybean oil found in a Thai curry.

Some higher quality restaurants opt for grape seed or rice bran oil. You would have to eat 125 grapes, first pulverizing the seeds with your teeth, to consume just one tablespoon of grape seed oil, and 625 grapes to consume five tablespoons of oil. For every tablespoon of rice bran oil you eat in a restaurant meal, you would have to eat 8 cups of brown rice to get the same amount of oil from rice bran. That means you would have to chow down nearly 40 cups of brown rice just to get the amount of vegetable oil the average American eats every day.

With the amount of processing required to make vegetable oil an edible substance, does calling it a health food pass the common sense test? Another common sense consideration is that there is a lack of historical precedent for vegetable oils in the human diet...

No historical precedent

Whether we go back 100,000 years or 100 years, to our hunter gatherer ancestors or to our great grandparents, there is no historical precedent for widespread vegetable oil consumption.

As we’ve detailed previously, the rise of vegetable oils has gone hand-in-hand with the rise of chronic disease. Chronic disease was almost nonexistent before vegetable oils came along, and now it’s widespread in America, with the rest of the world following in our oily footsteps.

While minimally consumed a hundred years ago, vegetable oils now account for 20% of our daily calories, representing the greatest increase in sources of calories since the globalization era began [1].

Americans consume significantly more vegetable oil today than a hundred years ago. The most popular vegetable oil in the US is soybean oil, which has seen a meteoric rise in consumption since the 1950s [2].

Globally, consumption of vegetable oil has increased 150x since 1909, has doubled in the last 20 years, and is expected to grow 30% in the next four years [3]. And there’s no end in sight. Vegetable oil is the fastest growing sub-sector of global agriculture [4].

Global vegetable oil consumption has increased 150x since 1909, has doubled in the last 20 years, and is expected to grow 30% in the next four years.

More vegetable oil is produced globally than poultry, beef, cheese, and butter combined [5]. In fact, vegetable oil is now the most consumed food in the world, after rice and wheat [6].

More vegetable oil is produced globally than poultry, beef, cheese, and butter combined.

Even wheat and dairy, controversial as they might be, have been a part of human diets for at least 10,000 years, giving us some time to adapt. Vegetable oils on the other hand–like trans fats, high fructose corn syrup, and artificial sweeteners–have only been a part of our diet for a matter of decades. Usually when we start consuming a processed food in unprecedented amounts for the first time in human history, it doesn’t end well for our health and we realize much later that it wasn’t such a good idea. But what is it about vegetable oil that makes it unhealthy? That brings us to omega-6…

Omega-6 in Vegetable Oils

To best answer the question about why vegetable oil is unhealthy, we first have to look at what vegetable oil actually is. Vegetable oil is made up of fat molecules, and fat molecules are made primarily of fatty acids. There are different types of fatty acids and each one plays a different role in our body.

The primary fatty acid in most vegetable oils is linoleic acid, a type of omega-6 fat. The omega-6 content of vegetable oils is what makes them so problematic.

Omega-6 fats, while necessary in extremely small amounts, contribute to general inflammation when eaten in excess. While chronic inflammation is cited as a source of many of the diseases we face today [1], it’s just the tip of the iceberg. The unstable, reactive properties of dietary omega-6 create a host of other downstream effects that have been causally linked to poor health and chronic disease, including heart disease, the leading cause of death in the world [2].

As we’ll discuss below, consuming too much omega-6 is nearly impossible to avoid in today’s food system and has also been implicated in weight gain, diabetes, osteoarthritis, cancer, cognitive decline, dementia, irritable bowel syndrome, macular degeneration, asthma, and autoimmune disease.

The Growth of Omega-6 in our Diets

The shift to diets high in omega-6–largely from vegetable oils–is the most striking nutrient change of the last hundred years.

In 1909, the average American consumed about 9.5 grams of omega-6 per day, representing 2.79% of our daily calories. Ninety years later, in 1999, our omega-6 intake was 24.6 grams per day [1, 2]. Today, because of our increased consumption of omega-6-rich vegetable oils, we’re consuming closer to 40 grams of omega-6 per day [3, 4]. That means that over 10% of our calories now come from a single type of fatty acid: omega-6.

The average American consumes 40 grams of omega-6 per day, up from ~10 grams in 1909.

Unsurprisingly, as our consumption of omega-6 has increased, so too has the amount of omega-6 (linoleic acid) in our fat cells. Whereas in 1960, our fat cells contained only 7–9% omega-6, they contained over 23% in 2008, and based on the fact that we’re eating about 60% more vegetable oil than we were ten years ago, our fat cells are probably closer to 30% omega-6 today [5]:

In 1960, omega-6 linoleic acid (LA) comprised 7–9% of our fat cells. Only fifty years later in 2008, our fat cells were comprised of over 23% omega-6. For reference, our chimpanzee cousins have about the same amount of omega-6 in their cells that we did in our healthier years of the 1960s.

As we’ll see in the Omega-6 & Disease section below, a high amount of omega-6 in our cells is associated with higher risk of heart disease and other chronic diseases.

Our increasing consumption of omega-6, and the resulting increase of omega-6 in our cells, is almost entirely caused by our growing consumption of vegetable oils. Vegetable oils like soybean, sunflower, safflower, and grape seed oil are two-thirds to three-fourths omega-6 by weight. Soybean oil alone accounts for about 45% of our dietary omega-6 intake in the US [6].

Just as it would be impossible to consume a significant amount of vegetable oil when eating whole foods, it would be just as difficult to consume significant amounts of of omega-6 without consuming vegetable oil. Whole foods, even the foods that are highest in omega-6 fat, contain very little omega-6 compared to vegetable oil.

Whole foods, even the foods that are highest in omega-6 fat, contain very little omega-6 compared to vegetable oil.

You may be wondering: if omega-6 is so bad, why is it in healthy whole foods at all? Omega-6 serves a purpose; we need it for normal human growth and development. There’s nothing inherently unhealthy about omega-6; like with sugar, calories, and bok choy, the dose makes the poison. While an extremely small amount of omega-6 is needed to thrive, too much omega-6 leads to weight gain, disease, and accelerated aging. Currently, the amount we’re eating far exceeds the amount we need.

As long as humans get at least 0.3–1% of calories from omega-6, there is no risk of deficiency [7, 8]. For a 2,000 calorie diet, that’s about 1 gram of omega-6 per day, the amount you’d find in a few cashews, a half avocado, or a couple of eggs. Nearly every food we eat contains trace amounts of omega-6, so there is almost no way of being deficient unless you eat a zero-fat diet or are a lab rat with an intentionally mutated gene that prevents your body from utilizing omega-6. Neither scenario is realistic.

The only recorded cases of omega-6 deficiency, not surprisingly, were in hospitals that fed patients fat-free purified-nutrient diets, void of all fats including omega-6.

In other words, there’s no reason to seek out omega-6. Consuming the minimum amount needed to thrive is an inevitable result of eating any diet based on real food. Since all life is made of cells, and all cells contain lipid (fat) membranes, all whole foods–whether plant or animal–contain fats, and for most foods, 2% or more of those fats are omega-6. Under-consuming omega-6 is nearly impossible, while over-consuming omega-6 is nearly impossible to avoid in today’s world.

Omega-6 to Omega-3 Ratio

You’ve probably heard of omega-3, the healthy type of fat found in salmon, fish oil, and flaxseeds. Omega-3 is especially important in today’s world because it “competes” with omega-6 in the body. In other words, the more omega-6 you eat, the less omega-3 your body is able to utilize. Generally speaking, omega-3 is anti-inflammatory, while omega-6 is pro-inflammatory [9].

In addition to promoting inflammation, over-consuming omega-6 prevents omega-3 from doing its job, which includes functions such as improving heart health, supporting mental health, reducing waist size, decreasing liver fat, supporting infant brain development, preventing dementia, promoting bone health, and preventing asthma [10].

Therefore, the ratio of omega-6 to omega-3 we consume is important. Human beings evolved on a diet of about 1:1 omega-6 to omega-3. However, today that ratio in the US is between 10:1 to 20:1, and as high as 50:1 in some parts of the world, such as urban India [11, 12]. (That means that we’re eating 10, 20, or 50 grams of omega-6 fat for every one gram of omega-3 fat we eat.)

In the US, consumption of omega-6 fats has increased considerably, while consumption of omega-3 fats has not, resulting in an increasing omega-6 to omega-3 ratio [13].

Based on today’s levels of omega-6 consumption, we’d have to eat salmon and sardines at every meal to get enough omega-3 and achieve anything close to a 1:1 ratio. Instead of embracing an all-sardines diet, it’s more practical to significantly decrease our consumption of omega-6 and only slightly increase our consumption of omega-3. On a diet without vegetable oil, eating seafood a couple times per week is sufficient for proper omega-6 to omega-3 balance.

If omega-3 has numerous important functions in the body and omega-6 partly prevents omega-3 from performing these functions, then we should see a decline in those important functions as a result of significantly over-consuming omega-6. And we do…

Omega-6 & Disease

Poor heart health and mental health, along with increasing rates of obesity, dementia, autoimmune disease, and asthma are all trends in America today, the exact complications and negative health outcomes that omega-3 helps prevent when in balance with omega-6.

An entire book could be written on the diseases caused by excess omega-6, and the biological pathways that are to blame. Instead of boring you with 700 pages of research paper methods and results, I’ll let the researchers’ summaries speak for themselves when it comes to the impact of omega-6 consumption on chronic disease:

Alzheimer’s Disease:

“This review supports growing evidence of a positive association between the dietary omega-6/omega-3 ratio and the risk of Alzheimer’s disease.” [14]

Autoimmune Disease:

“[T]he increased consumption of omega-6 or a vegetable source of oils and decreased omega-3 intake may increase in vivo [in a living human] the production of free radicals and higher proinflammatory cytokines. Our ongoing studies reveal that proinflammatory vegetable oil could increase autoimmune disease by increasing the free radical formation by decreasing the antioxidant enzyme mRNA levels, thereby further decreasing immune function, particularly the production of anti-inflammatory cytokines such as IL-2 and TGF beta mRNA levels.” [15]


“Soybean oil consumption is increasing worldwide and parallels a rise in obesity. Rich in unsaturated fats, especially linoleic acid [omega-6], soybean oil is assumed to be healthy, and yet it induces obesity, diabetes, insulin resistance, and fatty liver[.]” [16]
“Recent findings that […] linoleic acid [omega-6] is positively related to insulin resistance, suggest that diet may influence the development of insulin resistance in obesity, insulin-dependent diabetes mellitus (IDDM), hypertension, and coronary artery disease[.]” [17]

Mental Health:

“To our knowledge, our data provide the first prospective evidence that the omega-6/3 PUFA ratio is associated with an increased risk for mood disorders in young people[.]” [18]

Heart Disease:

“Dietary changes over the past few decades in the intake of n-6 [omega-6] and n-3 PUFA [polyunsaturated fatty acids] show striking increases in the (n-6) to (n-3) ratio (~15 : 1), which are associated with greater metabolism of the n-6 PUFA compared with n-3 PUFA. Coinciding with this increase in the ratio of (n-6) : (n-3) PUFA are increases in chronic inflammatory diseases such as nonalcoholic fatty liver disease (NAFLD), cardiovascular disease, obesity, inflammatory bowel disease (IBD), rheumatoid arthritis, and Alzheimer’s disease (AD). By increasing the ratio of (n-3) : (n-6) PUFA in the Western diet, reductions may be achieved in the incidence of these chronic inflammatory diseases.” [19]
“In summary, numerous lines of evidence show that the omega-6 polyunsaturated fat linoleic acid promotes oxidative stress, oxidised LDL, chronic low-grade inflammation and atherosclerosis, and is likely a major dietary culprit for causing CHD [congenital heart disease], especially when consumed in the form of industrial seed oils commonly referred to as ‘vegetable oils’.” [20]

In the case of heart health, there is compelling evidence that omega-3s protect whereas omega-6 promotes heart disease, and that “high LDL” is only dangerous in the context of a high omega-6 diet. It’s been found that heart attack patients have significantly more linoleic acid (omega-6) in their arteries than healthy patients, and the more severe the heart attack, the more plaque buildup caused by omega-6 found in the arterial walls [21].

When comparing the diets of different regions, we find that the more omega-6 a region eats, the higher rates of death from heart disease:

The more omega-6 a region eats, the higher rates of death from cardiac events [22].

While the above data shows correlation, not causation, and it could be argued that the data points are cherry picked, we already showed in the last post that vegetable oil consumption is causally linked to heart disease. In three different randomized trials, the group of participants eating more omega-6 from vegetable oil were at significantly increased risk of cardiac events, such as heart attack and stroke.


“High amounts of omega-6 fatty acids have been linked with increased prostate cancer risk, whereas omega-3 fatty acids have been shown to inhibit PCa [prostate cancer] growth.” [23]
“The lower omega-6/omega-3 ratio in women with breast cancer was associated with decreased risk. A ratio of 2–3/1 suppressed inflammation in patients with rheumatoid arthritis, and a ratio of 5/1 had a beneficial effect on patients with asthma, whereas a ratio of 10/1 had adverse consequences.” [24]
“Higher omega-6 fat intake was associated with risks of squamous cell carcinoma, basal cell carcinoma, and melanoma.” [25]

Researchers studying cancer in mice even suggest that dietary polyunsaturated fats like omega-6 are required in order to induce tumors in mammals. In other words, even when trying to do so, researchers may be unable to intentionally form tumors in lab animals without feeding them fats like omega-6. The same phenomenon is found with liver damage: omega-6 consumption is required for the development of alcohol-induced liver injury, and the severity of the liver damage is correlated with the amount of omega-6 in the diet. Furthermore, replacing omega-6 fats with more traditional fats, like those from coconut, reverses alcohol-induced liver damage, even when maintaining the same level of alcohol consumption [26].


“Recent studies in humans show that in addition to absolute amounts of omega-6 and omega-3 fatty acid intake, the omega-6/omega-3 ratio plays an important role in increasing the development of obesity via both AA [arachidonic acid] eicosanoid metabolites and hyperactivity of the cannabinoid system, which can be reversed with increased intake of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). A balanced omega-6/omega-3 ratio is important for health and in the prevention and management of obesity.” [27]
“In summary, dietary LA [omega-6 linoleic acid] increased tissue AA [arachidonic acid], and subsequently elevated 2-AG + 1-AG and AEA resulting in the development of diet-induced obesity. The adipogenic effect of LA can be prevented by consuming sufficient EPA and DHA [omega-3] to reduce the AA-PL pool and normalize endocannabinoid tone.” [28]

The role of omega-6 in obesity is especially troubling. You may have noticed references to “cannabinoids” in the two paragraphs above, a term you hear more often in cannabis shops than in scientific research papers on omega-6 consumption.

Consuming the omega-6 found in vegetable oil results in the production of compounds like 2-AG and anandamide. As Tucker Goodrich points out in his thorough post on omega-6, those compounds act on certain receptors in our brain the same way that THC from marijuana does, generating signals to increase appetite, thus leading to weight gain. In fact, the more obese someone is, the higher their 2-AG and anandamide levels [29]. In the same way that smoking a joint causes “the munchies,” omega-6 derivatives bind to receptors in our brain that cause cravings, usually for salty, sweet, oil-rich junk food [30]. It’s a vicious cycle where the more vegetable oil-rich foods we eat, the more we crave junk food, which is usually high in vegetable oils, causing us to continue craving it.

Why do we get fat? The classic answer, “calories in, calories out” may be partly true, but it’s missing the point of the question. As diet researcher Gary Taubes explains, it’s like asking a rich person how they got rich and hearing in response, “I made more money than I spent.” Well of course that’s how they got rich, but what specifically resulted in them making more money than they spent? Why do certain people take in more calories than they “spend,” while others are in balance? Omega-6 from vegetable oil consumption may be an answer. Diets high in vegetable oils may cause us to continually crave more food, especially junk food.

Macular Degeneration (AMD):

“Higher consumption of linoleic acid was also associated with a higher risk for AMD. Higher intake of omega-3 fatty acids was associated with a lower risk for AMD among individuals consuming diets low in linoleic acid, an omega-6 fatty acid[.]” [31]

A quick note on skin cancer and AMD, the leading cause of blindness in the world…

We literally are what we eat. The cells in our bodies are made entirely of what we put in our mouths. When we eat vegetable oils, their omega-6 fats are incorporated into our cell membranes, and because omega-6 fats are more unstable than other types of fats, our cells become more fragile and prone to damage on diets high in omega-6. Cellular damage is most noticeable when those cells are on the outside, rather than the inside, of our bodies, as is the case with cells in our eyes and skin.

Excess omega-6 consumption from vegetable oils plays a role in accelerated skin aging and the eye disease AMD. Again to Tucker Goodrich who emphasizes that AMD may be the only condition where it is becoming widely and irrefutably accepted that excess omega-6 is the culprit [32, 33]. Our eyeballs are made largely from fats, and when those fats are comprised of too much omega-6, they become damaged more easily in the presence of UV light, which may be the same pathway that causes skin cancer. Too much omega-6 from vegetable oils means more damage-prone skin and eyes [34].

The above quoted summaries from researchers only represent the tip of the iceberg. There are literally thousands of research papers on omega-6’s role in disease. And humans aren’t the only animals that are impacted by omega-6…

Omega-6 in Nature

Across a number of animal species, omega-6 plays a large role in longevity. The more polyunsaturated fatty acids, like omega-6, that animals have in their cell membranes, the higher the “peroxidation index” of their cells and the shorter their lifespan:

The more polyunsaturated fatty acids, like omega-6, that animals have in their cell membranes, the higher the “peroxidation index” of their cells and the shorter their lifespan.

It has been suggested that no other variable in biology accounts for differences in lifespans across species as well as the fat composition of cell membranes, specifically the amount of omega-6 and other polyunsaturated fats that are contained in cells (“peroxidation index”).

While humans have a maximum lifespan of over 100 years, our average healthy life expectancy in the US has actually been decreasing for the last decade as our consumption of vegetable oil has increased, even with advancements in medical science and pharmaceutical drugs.

The fortunate news is that we can change the fat content in our cells (and thus our life expectancy) by changing our diets. When mice are put on calorie restricted diets, the fat content of their cells changes to resemble that of longer living species, and the mice live similarly long lives as a result [35]. As Paul Jaminet points out in Perfect Health Diet, this is promising for human longevity because it means consuming less omega-6 fat can be an impactful step in extending our years of healthy life.

We find a fascinating example of the acute effect of omega-6 on lifespan in honeybees. Researchers suggest that the differences in longevity between queen and worker bees may be explained by the amount of polyunsaturated fat like omega-6 in their cells. Female honeybees can either be long-living queens with a longevity measured in years, or short-lived workers with a lifespan of only weeks, depending on what they are fed in their first week of life in the hive. Bees that are fed more omega-6 rich pollen become short-living worker bees, while bees that are never allowed to consume pollen, and instead eat low diets low in omega-6, become queens. Long-living queens have the lowest levels of omega-6 in their cell membranes, while short-living worker bees have a significantly increased content of omega-6 in their cells [36].

Earlier we asked why healthy whole foods contain omega-6 if it’s so unhealthy and decreases life expectancy. We answered the question through the lens of a human diet, exploring our biological requirements for the nutrient. However, the real answer to why plants and animals contain omega-6 has more to do with them than with us.

Unlike saturated fats like coconut oil or cocoa butter, polyunsaturated fats like omega-6 are liquid even at extremely low temperatures, allowing the omega-6 oils to remain fluid in the cells of plants and animals that find themselves in subzero temperatures or snowy terrains. For that reason, plants and seeds grown in Northern latitudes, such as sunflower in Russia and canola in Canada, tend to have significantly higher omega-6 oils in their cells. Similarly, plants that grow near the equator, such as coconut and cacao tend to have more saturated fats in their cells, and plants that grow in more mild regions like avocado in Mexico and olive in Italy tend to have more monounsaturated fats, which are liquid at most temperatures, but would still solidify in more Northern latitudes.

In other words, it appears that plants are composed of polyunsaturated omega-6 fats as an evolutionary last resort. In order for plant cells to function normally, the lipids (oils) that make up their cell membranes must remain liquid at all temperatures typical of their natural environments. Since all fats and oils are liquid at high temperatures, plant fats optimize for the extreme colds in their environments. Sunflower must be prepared for the cold of Russia, avocado for a cool Mexican night, and coconut for the occasional lukewarm temperature of the tropics.

As a result, sunflower seeds contain about five times as much omega-6 as avocados, and avocados contain about five times as much omega-6 as coconuts.

Plants evolve to contain the most stable fats possible, which means minimizing omega-6 unless absolutely necessary for their fats to remain liquid at cold temperatures. Evolution prefers fats that don’t easily react, such as stearic acid found in cocoa butter. However, stearic acid has an extremely high melting point, meaning it’s solid at most cool temperatures. Thus, plants in more mild climates like olives in Italy evolve to contain the slightly less stable oleic acid. While oleic acid is slightly less stable than stearic acid, it is significantly more stable than linoleic acid (omega-6), as we’ll see in the next section.

While mice, honeybees, and plants can’t ponder the role of omega-6 in their diets and cells, we can. As humans, we have the ability to make wise food choices. The more omega-6 from vegetable oils we consume, the shorter we live and the more disease we have. The less omega-6 we consume (especially in balance with omega-3), the healthier we are. The dietary choices we make affect not only our health, but also the health and development of our children.

There is only one food that we can say with absolute certainty is intended for human consumption, that has been consumed by nearly every human to have ever lived: breast milk.

The breast milk of hunter gatherers gives us insight into the types of fat made by nature exclusively for the purpose of nourishing humans. For hundreds of thousands of years, the fat in human breast milk was less than ~5% linoleic acid (omega-6). Today, the average woman’s breast milk in Western countries is 15-20% linoleic acid, and as high as 30% according to some research, thanks to an increased consumption of vegetable oil [37, 38, 38]. We don’t have human studies on the impact on infant health as a result of today’s increased omega-6 content in breast milk, but the data in other mammals should raise red flags. In baby pigs, while 1.2% of calories from omega-6 leads to healthy brain development, increasing intake to 10.7% of calories compromises neurodevelopment [39].

As the amount of omega-6 in our diets, food supply, and breast milk have all increased, our bodies’ cells have become more and more saturated with omega-6, and we’ve increasingly begun to resemble walking bottles of soybean oil. As a result, our health has suffered. While the inflammatory properties of omega-6 and its growing imbalance with omega-3s are partly to blame, there’s more to the story.

The worst consequences of vegetable oil consumption are found when omega-6 is exposed to light, heat, and air, in a process called oxidation, a hallmark cause of disease and aging.

Omega-6 & Oxidative Stress

To understand how vegetable oils cause oxidation, we must first understand what oxidation is and why it can be harmful.

You’re probably already familiar with oxidation without actually knowing it. In mainstream health, there are terms like “free radicals” and “antioxidants” that describe the downstream effects of oxidation, but mainstream health rarely gives those terms any context.

Exposure to air turns an iron surface to rust, through the process of oxidation. (Image source: Pexels).

If you’ve seen iron turn to rust or bronze statues turn green, you’ve witnessed the results of oxidation. Oxidation is the process of a molecule losing an electron and having an unpaired electron as a result. When a molecule has an unpaired electron, it becomes a “free radical.” Free radicals wreak havoc in our bodies; they steal electrons from other cells, causing cellular damage and contributing to a number of diseases [40].

According to researchers, “Evidence is accumulating that most of the degenerative diseases that afflict humanity have their origin in deleterious free radical reactions. These diseases include atherosclerosis, cancer, inflammatory joint disease, asthma, diabetes, senile dementia and degenerative eye disease. The process of biological aging might also have a free radical basis.” [41]

In other words, the cause of most chronic disease is free radical formation, which may also be a leading contributor to biological aging. We can’t fully prevent aging (yet), but we can certainly slow it down by reducing exposure to free radicals.

The commonly cited external sources of free radicals are pollution, cigarette smoke, and radiation, all of which cause oxidation. Free radicals are also caused by normal cell metabolism, but our bodies are typically able to produce enough antioxidants, which as their name implies help to counteract the negative effects of oxidation and free radicals, to prevent significant damage. Antioxidants can “donate” an electron to a free radical without becoming unstable themselves. When our bodies are overwhelmed with oxidation and free radicals, and not able to produce enough antioxidants, the result is “oxidative stress.” What happens when we experience oxidative stress?

Let’s quote Healthline here, as they sum up the results of oxidative stress quite well:

When there are more free radicals present than can be kept in balance by antioxidants, the free radicals can start doing damage to fatty tissue, DNA, and proteins in your body. Proteins, fats, and DNA make up a large part of your body, so that damage can lead to a vast number of diseases over time. These include:
Diabetes, atherosclerosis (or the hardening of the blood vessels), inflammatory conditions, high blood pressure (which is also known as hypertension), heart disease, neurodegenerative diseases, such as Parkinson’s and Alzheimer’s, and cancer.
Oxidative stress also contributes to aging. [42]

So it sounds like we should really avoid things that cause oxidation, which kicks off the whole process leading to disease and accelerated aging. Those diseases and disorders caused by oxidative stress listed above look eerily similar to the list of consequences of consuming too much omega-6 from vegetable oils from earlier in this post, and that similarity is no coincidence.

In addition to pollution, cigarette smoke, and radiation, oxidation is also caused by alcohol consumption, excess sugar, physical inactivity, chronic psychological stress, environmental toxins, and you guessed it: vegetable oils.

Compared to other types of fats, such as oleic acid found in olives and avocados, or stearic acid found in cocoa butter, the polyunsaturated omega-6 fat found in vegetable oils oxidizes especially easily. This is not controversial. Even Harvard, notoriously conservative in their views on diet, state:

“Of potentially greater concern is the formation of oxidation products of polyunsaturated fatty acids […] for oils with higher levels of more readily oxidized polyunsaturated fat such as corn, soybean, sunflower, and safflower oils” [43].

The fatty acids (linoleic acid and linolenic acid) found in most vegetable oils oxidize extremely easily compared to the fatty acids found in other fatty foods.

The peroxidation index, which describes how different fats contribute to oxidative stress, is zero or close to zero for the healthy fats found in cocoa butter and olive oil, but is extremely high in the fats found in vegetable oil, namely linoleic acid and linolenic acid [44]. As we’ll discuss in the next section, some vegetable oils such as canola oil are high in both linoleic acid and linolenic acid.

Like an iron gate left to rust, vegetable oils quickly oxidize in the presence of light, oxygen, and heat. Even a sealed store-bought bottle of vegetable oil, or vegetable oils in cold food products such as oat milk and mayonnaise, are exposed to intense heat, light, and oxygen before ever making it into the bottle, carton, or jar. As I explain in my response to Oatly, where I describe the process of producing rapeseed (canola) oil, vegetable oils are produced through a high-heat process that results in oxidation. It’s not possible to extract significant amounts of oil from a soybean, corn kernel, or grape seed without the use of harsh processing.

Let’s quickly summarize:

Vegetable Oil → Oxidation → Free Radicals → Cellular Damage → Disease

External factors such as cigarette smoke and vegetable oil consumption can cause molecules in our body to lose electrons, a process known as oxidation. These molecules with unpaired electrons are called free radicals, which can cause cellular damage by stealing electrons from other cells. To some extent, our bodies can prevent free radical damage, but become overwhelmed when there are too many free radicals and not enough antioxidants to counteract their harmful effects, an imbalance known as oxidative stress. The results of oxidative stress are disease and accelerated aging.

The Oils Lowest in Omega-6

At this point, it should be clear that we want to avoid omega-6 fats in our cooking oil. So where does that leave us? Which oils and fats are lowest in omega-6? Glad you asked:

Oils from tropical plants such as cacao, coconut, and palm are lowest in omega-6, while oils from cold-weather plants such as sunflower and soybean are highest in omega-6 and best avoided.

In addition to the plant oils in the chart above, ruminant fats such as ghee, butter, and tallow are also very low in omega-6. Fat from chicken (schmaltz) and fat from pigs (lard), on the other hand, tend to be higher in omega-6 as a result of feedlot diets of soy and corn.

The oils that are lowest in omega-6 also happen to be the oils that are most plausible to consume naturally from real food, without eating 5 cups of soybeans, 98 ears of corn, 625 grapes, or 2,800 sunflower seeds.

For example, a medium-sized coconut contains about 10 tablespoons of coconut oil. That means that consuming one tablespoon of coconut oil only requires a small chunk of coconut meat. Similarly, a tablespoon of olive oil or avocado oil can be pressed from only a handful of olives or a half an avocado. Consuming oils from naturally oily foods passes the common sense test. It’s easy to imagine how humans could consume a few tablespoons of coconut oil or olive oil without high-heat pressing, industrial processing, and chemical solvents.

A Note on Canola Oil:

Canola oil is a special case where there is a considerable amount of polyunsaturated linolenic acid, a type of omega-3 fat. However, that omega-3 comes in the form of ALA (alpha-linolenic acid), as opposed to the more bioavailable DHA or EPA omega-3 that’s found in seafood. Our bodies must convert ALA to DHA or EPA before it can be utilized, and this conversion process is inefficient in humans; less than 10% of ALA is actually converted into the bioavailable forms DHA and EPA [45, 46].

Supporters of canola oil claim that the omega-3 content makes it healthful. While we discussed why that’s not true in the context of canola oil in oat milk, it’s especially untrue in the case of canola oil for cooking. The delicate omega-3’s in canola (rapeseeds) are subjected to a number of high-heat high-friction oil extraction processing methods. For that reason, and because the oils are separated from their protective seeds, they begin to oxidize well before making it into a bottle. In the case of canola oil for cooking food, the already-oxidized oil becomes significantly more oxidized when heated, especially when repeatedly heated. Fresh canola oil is bad, cooked canola oil is worse, and deep fried canola oil is worst.

It’s important to get enough omega-3, and it’s important to have a balanced ratio of omega-6 to omega-3, but that doesn’t mean we should seek out canola oil (or any vegetable oil for that matter) as a source of omega-3, both because its form of omega-3 is far less bioavailable than the omega-3 found in seafood and because the omega-3 found in vegetable oil is already oxidized as a result of separating the oil from its seed.

There’s evidence that the oxidation of omega-3’s like linolenic acid may cause organ damage, inflammation, carcinogenesis (cancer), and advanced atherosclerosis (heart disease) [47].

In addition to the oxidized and unstable fatty acids found in vegetable oils, and the resulting oxidative stress they causes in our bodies, there are other compounds that form when vegetable oil is heated. Heating vegetable oils high in omega-6 takes a bad problem and makes it worse.

Cooking with Vegetable Oils

Common sense, lack of historical precedent, and the detrimental consequences of excess omega-6, the primary fat found in most vegetable oils, helps explain why vegetable oil consumption causes negative health outcomes. But why does heating vegetable oil make matters worse, as we saw in multiple studies in the previous post in this series?

The answer again lies in omega-6. When polyunsaturated fats such as omega-6 are heated, they result in accelerated oxidation and the formation of toxic byproducts, such as trans fats and aldehydes. The longer they are heated, the more damage they do. Not surprisingly then, repeatedly heated vegetable oils, like those found in fried foods–from french fries to tortilla chips–are especially harmful.

Accelerated Oxidation

For most refined vegetable oils, the level of oxidation (peroxide value) doubles or quadruples after a few minutes of heating, about the amount of time it takes to sauté a quick meal in a pan sprayed with oil.

Common vegetable oils experience significantly more oxidation after only minutes of heating. The above chart shows peroxide values of 25 mL oil at 225°C for 25 minutes [1].

Interestingly, canola oil is the most oxidized oil in its “fresh” state, likely due to its linolenic acid content and harsh production process, as discussed in the previous section, but it does not immediately oxidize further when heated for a short amount of time. This means that canola oil is one of the worst choices for use in cold products such as oat milk, but if you had a gun to your head and had to eat a meal sautéed in refined vegetable oil, canola oil would be the least bad option. That changes quickly though if canola oil is heated for more than a few minutes…

In the below chart, we see the effects of heating oil over a longer period of time, in this case to fry back-to-back batches of frozen chicken nuggets in either soybean oil, canola oil, or palm oil.

When heated continuously the oil lowest in omega-6 (palm oil) barely oxidizes at all beyond its fresh state, while the oils high in omega-6 (canola oil and soybean oil) oxidize significantly [2].

As expected, when heated continuously the oil low in omega-6 (palm oil) barely oxidizes at all beyond its fresh state, while the oils high in omega-6 (canola oil and soybean oil) oxidize significantly, to about 65 meq/kg after 101 chicken nugget batches. It is recommended that the peroxide value of edible food should not exceed 30 meq/kg [3]. Typical fast food restaurants serve 100+ orders of fried food in a matter of a few hours, and replace their fryer oil over the course of days or weeks, not hours. Let that sink in.

A 2018 study on restaurant oil quality found that more than 80% of randomly selected restaurant cooking oil was oxidized beyond the recommended limit [4].

Trans Fats in Vegetable Oils

Trans fats are formed when vegetable oils are heated to high temperatures, and each time an oil is reused for frying, its trans fat content increases [5]. At many restaurants, the same vegetable oil is reused hundreds of times for frying before being replaced.

We’ve all heard of the dangers of trans fats. Once touted as a healthy and “clean” alternative to animal fats, trans fats are now known to cause a host of diseases, including heart disease, obesity, cancer, and diabetes [6]. There is no safe level of trans fat consumption. In fact, for every 2% increase in calories from trans fats, your risk of heart disease nearly doubles [7]. Vegetable oils like canola oil are found to have up to 3.6% trans fat content, but because that results in less than 0.5 grams per 14 gram serving, the trans fat content is allowed to be listed as “0 grams” on bottles of vegetable oil or foods containing them [8].

While the overt use of artificial trans fats was banned in 2018 (after decades of widespread consumption), labeling loopholes mean trans fats still exist covertly in most refined vegetable oils, as a byproduct of the high-heat process required to extract unstable omega-6 oil from seeds and grains. In fact, vegetable oils are one of the very few foods that still contain any artificial trans fats at all [9]. While in widespread use, trans fats are estimated to have caused 30,000–100,000 premature deaths per year [10].

Aldehydes in Vegetable Oils

Another harmful byproduct in vegetable oil (as well as cigarettes) is a group of compounds called aldehydes, which include chemicals like formaldehyde.

According to The Telegraph, in a 2015 paper, “scientists found that heating up vegetable oils led to the release of high concentrations of aldehydes, which have been linked to illnesses including cancer, heart disease and dementia.”

The research showed that a typical meal of fish and chips in the UK, fried in vegetable oil, contained as much as 100 to 200 times more toxic aldehydes than the safe daily limit set by the World Health Organization. In contrast, heating up fats low in omega-6 like coconut oil produced much lower levels of aldehydes:

Heating up vegetable oils high in omega-6 leads to the formation of aldehydes (data source: The Telegraph).

After only 30 minutes of continuous heating, the concentrations of toxic aldehyde in corn oil and sunflower oil had increased 1,000%. Most restaurants repeatedly heat the same vegetable oil for dozens or even hundreds of hours, only replacing it every few days or weeks.

The research team concluded that the most obvious solution to the generation of harmful chemicals in food is to avoid consuming food fried in vegetable oils as much as possible.

A 2019 research paper by a different group of scientists compared the aldehydes in food fried in vegetable oil to the aldehydes ingested from smoking cigarettes and concluded that our current level of fried vegetable oil consumption is equivalent to smoking 25 cigarettes per day [11].

The average American currently consumes at least 5 tablespoons (15 teaspoons) of vegetable oil daily, so according to this research paper, each teaspoon of vegetable oil is equivalent to smoking up to 1.7 cigarettes, similar to the estimate of 2 cigarettes per teaspoon of vegetable oil that we estimated in the previous post.

Harmful Additives in Vegetable Oils

Plants don’t want to have cellular damage or oxidative stress any more than we do, so plants with high levels of omega-6 in their cells evolve natural antioxidants to defend themselves from the harmful effects of omega-6 oxidation in their cells. Namely, we find a lot of antioxidants like tocopherols (Vitamin E) in the high omega-6 seeds of plants such as soy, canola, and sunflower. While these antioxidants partially protect the plants themselves from the harmful effects of oxidation, they only marginally protect us.

The production process required to extract oil from the small seeds of sunflower, canola, and soy exposes the delicate seeds to heat, light, and air, causing the natural antioxidants to be “used up” in an attempt to protect the seeds from the oxidation caused by the harsh oil extraction process. There are few antioxidants left in the oils that end up in frying pans and food products.

As a result, oil manufacturers typically include synthetic antioxidants in cooking oils to make up for the loss of natural antioxidants and to give the oils a longer shelf life before going rancid. TBHQ (tertiary butylhydroquinone) is the most popular synthetic antioxidant.

TBHQ has been linked to numerous health problems. The Centers for Science in the Public Interest (CSPI) categorizes TBHQ as one of only a handful of ingredients to avoid at all costs, along with more well-known dangers such as the artificial sweetener aspartame and partially hydrogenated oils (trans fat) [12]. Other studies find that TBHQ consumption causes tumor growth, liver enlargement, neurotoxic effects, convulsions, and paralysis in laboratory animals, along with the development of food allergies, immune disruption, and “vision disturbances” in humans [13, 14, 15].

Unfortunately, TBHQ is ubiquitous in vegetable oils. Let’s take a quick peek at the cooking oil ingredients listed by two of the largest fast food chains in the US:

McDonald’s: “Vegetable Oil: Canola Oil, Corn Oil, Soybean Oil, Hydrogenated Soybean Oil with TBHQ and Citric Acid added to preserve freshness. Dimethylpolysiloxane added as an antifoaming agent.”

Carl’s Jr./Hardees: “Vegetable Oil (Soybean Oil, Hydrogenated Soybean Oil with TBHQ and Citric Acid to protect flavor, Dimethylpolysiloxane (as an antifoaming agent))”

Even if you avoid fast food all together, TBHQ is included in the oil used by most restaurants, even the high end ones. One of the more popular frying oils for smaller restaurants is Mel-Fry, from Ventura Foods.

In the Mel-Fry Essential “premium oil” the ingredients are listed as: “Cottonseed Oil, Canola Oil, TBHQ and Citric Acid added to protect flavor, Dimethylpolysiloxane, an anti-foaming agent.”

While McDonald’s and Carl’s Jr. proactively list TBHQ on their ingredient lists, most restaurants and food manufacturers do not. According to the FDA, if an ingredient is used for a functional purpose in an ingredient of the food, but is not used as a functional purpose in the food itself, it need not be included in the food’s ingredient list [16]. And it’s a judgment call. If  food manufacturers decide that TBHQ helped prevent oxidation in the oil but doesn’t serve a functional purpose in their chips or crackers, they can choose not to list TBHQ on the ingredient list. That’s part of the reason that few people know about synthetic ingredients like TBHQ; it is only listed on the ingredients of the frying oil, and not on the ingredients of the actual food cooked in the oil.

In 1999, the World Health Organization  determined that the average intake of TBHQ in the US was around 0.62 mg/kg of body weight, or about 90% of the acceptable daily intake. Today, we consume nearly twice as much vegetable oil (the primary source of TBHQ in our diets) as in 1999, likely putting our average consumption of TBHQ closer to 1.2 mg/kg of body weight. That results in 180% of the acceptable daily intake [17, 18].

Closing Thoughts

Just as the previous post in this series begged the question for this post’s topic (why are vegetable oils unhealthy?), this post begs another question: How did we get here? How did we get to a point where vegetable oil  is the fastest growing sub-sector of global agriculture and the most consumed food in the world after rice and wheat? If most vegetable oils are so bad for us, why are we producing so much of them, and why aren’t they treated like cigarettes and alcohol, or more in the spotlight like sugar and red meat? The answers are part of a fascinating story that will be the subject of a future post in this series. Sign up for my email list and I’ll let you know when new posts are published.


This is Part 3 in a series of posts investigating how vegetable oil impacts our health and our planet:

Part 1: What’s Driving Chronic Disease?

Part 2: Death by Vegetable Oil: What the Studies Say

Part 3: Why is Vegetable Oil Unhealthy? (current post)

Part 4: The Environmental Impact of Vegetable Oils

Part 5: How Seed Oils Cause Weight Gain (Zero Acre blog)

Part 6: Seed Oils as a Driver of Heart Disease (Zero Acre blog)

Part 7: Coming soon, sign up to stay informed

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