This is Part 2 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 1undefined.
As detailed in my previous post, vegetable oil consumption is one of the few diet and lifestyle trends that has increased consistently in line with increasing rates of chronic disease. While saturated fat, sodium, red meat, carbohydrate, and sugar consumption have all decreased in the last couple of decades, along with rates of smoking and physical inactivity, our consumption of vegetable oil has skyrocketed.
“Vegetable oils” in this context refer to oils extracted from seeds, grains, and legumes, and include soybean oil, corn oil, sunflower oil, safflower oil, canola oil, peanut oil, rice bran oil, grape seed oil, and cottonseed oil.
Vegetable oils, while nearly nonexistent a hundred years ago, now account for 20% of Americans’ calories and have made their way into nearly every packaged food and restaurant meal we eat, from oat milk, tortilla chips, margarine, and mayonnaise to Subway’s breads, Domino’s pizza crust, and Chipotle’s rice .
Meanwhile, as vegetable oil consumption has grown, rates of obesity, cancer, and diabetes–among other chronic illnesses–have surged to unprecedented levels. Six in ten adults in the US have a chronic disease and four in ten adults have two or more .
While the association between vegetable oil consumption and chronic disease may lead us in the right direction, it only proves a correlation and is not necessarily causative. To determine whether vegetable oil causes disease, we look to scientific studies.
While there are only a handful of well designed studies on the subject, and most only look at short-term effects, the studies we do have show that consuming vegetable oil has devastating short-term consequences on our health.
As we explore later in this post, after decades or an entire lifetime of consuming vegetable oil, especially in the form of deep fried food, the long-term consequences may be even worse.
In the Sydney Diet-Heart Study, researchers separated study participants into two groups. Both groups consumed the same amount of fat and oil, but the first group’s fat came primarily from vegetable oil sources like safflower oil and margarine while the second group’s fat came from sources like olive oil and butter. Everything else about their diets and lifestyles remained unchanged.
Both groups were monitored and evaluated regularly for the next seven years. The result? The group consuming more vegetable oil had a 62% higher rate of death during the seven-year study compared to the group eating less vegetable oil.
To put that into perspective, of the commonly cited diet and lifestyle risk factors, only severe obesity and heavy smoking are more dangerous:
Consuming vegetable oil increases your risk of death more than physical inactivity and heavy drinking, and for all the attention that red meat and sodium get, eating vegetable oil is 12 to 20 times more deadly.
If heavy smoking (a pack of cigarettes per day) increases risk of death by 80% and increasing vegetable oil consumption (by 12% of calories) increases risk of death by 62%, we can use some back-of-the-napkin math to infer that every 5% increase in daily calories from vegetable oil is as dangerous as smoking 7 cigarettes per day.
Another way of looking at it: each additional teaspoon of vegetable oil you consume could increase your risk of death as much as smoking 2 cigarettes.
When I attended a cooking class at a Thai restaurant a few years back , I learned that they use four tablespoons (12 teaspoons) of vegetable oil in one curry dish alone. That means that eating an oily restaurant meal could have the same detrimental effect on your health as chain smoking a pack of 20+ cigarettes, in one sitting.
Sadly, most restaurants don’t use traditional cooking fats like coconut oil and ghee because they are too expensive.
Is a cigarette or two every once in a while going to kill you? No, and neither is the occasional small amount of vegetable oil. But will chain smoking kill you? Eventually, yes. And according to the data, so too will regularly eating vegetable oil, usually from heart disease or cancer.
In another study, the Minnesota Coronary Experiment, participants who increased their consumption of corn oil and margarine had 86% more heart attacks, and for those aged 65 or older, a higher risk of death after four years:
In the MARGARIN Study, 282 participants were separated into different groups and told to eat different types of margarine, either margarine made with more fat from vegetable oil or margarine made with less fat from vegetable oil. After two years, the number of strokes, heart attacks and cardiovascular deaths was seven times higher in the group eating the vegetable oil rich margarine than in the group eating the margarine made with less vegetable oil:
An older and smaller study, the Rose Corn Oil Trial, tested replacing existing dietary fats with corn oil. The result was a 92% increase in cardiac events (e.g. heart attacks) and a 364% increased risk of death in the group consuming corn oil . The two-year trial had only 54 participants, so while the results are dramatic, the data from this trial should be taken with a grain of salt.
In the Los Angeles Veterans Administration Study, the group of participants who increased fat from vegetable oil–while keeping total fat the same–were 82% more likely to die from cancer compared to the control group that didn’t increase fat from vegetable oil. Despite randomization, the control group consuming less fat from vegetable oil had twice as many heavy smokers, but still experienced significantly fewer cancer deaths .
In all of the studies we’ve talked about so far, researchers only followed up with study participants for 2–8 years, not nearly enough time to see all the long-term consequences of vegetable oil consumption play out. Unfortunately (or maybe fortunately), we don’t have long-term randomized human trials that look at increased vegetable oil consumption over 20–50 years.
In addition, in all of the above studies, the oils in question were consumed fresh or with light cooking, but vegetable oil appears to be most dangerous and cause the most weight gain when repeatedly heated, as is the case with deep fried foods.
It’s important to consider the effects of deep fried oil consumption on health outcomes because over a third of American adults consume foods, usually deep fried, from fast food restaurants every day . In addition, packaged snacks, which make up a significant portion of Americans’ calories, are also typically deep fried in vegetable oils.
For data on the long-term consequences of a typical vegetable oil rich diet that includes deep fried foods, we’ll need to look at randomized trials in other mammals. Animals such as mice live only a few years, so the effects of consuming vegetable oil, heated versus unheated, over an entire lifespan can be tested in a much shorter time period than in humans.
In one study, rats were divided into different groups receiving diets identical in fat, protein, and carbohydrate calories but differing in the source of the fats. The rats in the group receiving fat from safflower oil had a 12.3% increase in total body weight compared to the rats eating traditional fats .
To put a 12.3% body weight increase into perspective, the average American would gain 23 pounds on a diet that includes more vegetable oil, even with total fat and total calories remaining the same.
In a randomized trial on rabbits, three groups of rabbits were given access to identical foods, with only one difference: the first group of rabbits was fed unheated vegetable oil, the second group was fed vegetable oil that had been heated once, and the third group was fed vegetable oil that had been repeatedly heated multiple times. Everything else about their diets was kept the same .
The outcome? Compared to the group of rabbits eating unheated oil, the group eating single heated oil gained 6% more weight, and the group eating repeatedly heated oil gained 45% more weight!
Amazingly, the group eating repeatedly heated oil was actually consuming slightly fewer calories than the other groups, and still managed to gain significantly more weight.
If you’ve ever wondered why your diet isn’t working, or why calorie restriction is so hard, consuming vegetable oils, especially in the form of deep fried foods, may be part of the answer.
A 2020 study in mice showed that consumption of soybean oil leads not only to weight gain, but also to gene dysregulation that could cause higher rates of neurological conditions like autism, Alzheimer’s disease, anxiety, and depression . The same study found that in soybean oil-fed mice, levels of oxytocin (the “love” hormone) in the hypothalamus went down. The author of the study concludes:
“If there’s one message I want people to take away, it’s this: reduce consumption of soybean oil” .
In another mouse study, feeding mice the equivalent of 2 tablespoons of canola oil per day is associated with worsened memory, learning ability and weight gain, along with “considerable neuronal damage” and increased formation of beta-amyloid plaques, the signature of Alzheimer’s disease . The study’s summary states:
“[M]ice received either regular chow or a chow diet supplemented with canola oil for 6 months. At this time point we found that chronic exposure to the canola-rich diet resulted in a significant increase in body weight and impairments in their working memory[.]”
In a study on baby pigs, 1.2% of calories from the primary fatty acid in vegetable oil leads to healthy brain development, but increasing intake to 10.7% of calories compromises neurodevelopment . Americans now consume nearly twice that amount, and most baby formulas include vegetable oil as one of the first ingredients.
In a study on our primate cousins, monkeys were fed a “Western diet” that included safflower oil. The most depressed monkeys had the highest levels of fatty acid from safflower oil in their blood. In other words, the monkeys that ate more safflower oil were more depressed .
In one study, when mice with implanted tumors were fed a diet high in the fatty acid found in vegetable oil, they experienced a rate of metastasis (spread of cancer) four times higher than mice fed the fats found in olive and avocado oil .
In a second study, when mice were fed soybean oil that had been previously heated in a deep fryer, they had four times as much metastatic growth as mice that consumed unheated soybean oil. 
Tying these two mouse studies together: if deep fried vegetable oil causes four times as much cancer growth as unheated vegetable oil, and if unheated vegetable oil causes four times as much cancer growth as unheated olive oil, then deep fried vegetable oil may be sixteen times more carcinogenic than unheated olive oil.
Another study in mice showed that an increased consumption of vegetable oil (corn oil), but not other fats, stimulates the progression of prostate cancer . The same association is seen in humans, but has not been tested in a randomized trial.
There are likely many factors that contribute to today’s chronic disease and obesity epidemic. Sugar, pesticides, plastics, factory farms, refined carbohydrates, gluten, hyper-palatable junk food, iPhones, and television have all been accused of causing obesity and disease. We’re a long way off from being in agreement about the true causes of today’s largest health problems, but the more I study the issue, the more I’m convinced that vegetable oils play a much larger role in the diseases of modernity than most people realize.
In a world without vegetable oil, we may be twenty pounds lighter, have half the amount of heart disease and cancer, live longer, think clearer, feel better, and be happier and less depressed.
In the long list of things we’ve gotten wrong and need to improve in order to get ourselves out of this chronic disease epidemic, I think reducing vegetable oil consumption is the lead domino.
This is Part 2 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 (current post)
Part 3: Why is Vegetable Oil Unhealthy?
A note on the studies included:
There are two primary ways of undertaking studies to find out what happens when we eat one thing versus another: observational studies and randomized trials.
Observational studies are the easier of the two options as they only require handing out a questionnaire to people about their diet and lifestyle habits, and then following those people for a number of years to find out who dies from what. Observational studies lead to conclusions like, “People who eat nuts have lower rates of heart disease.”
The problem is, we don’t know whether nuts are actually the thing preventing heart disease, or if people who eat a lot of nuts happen to have healthier lifestyle habits in general, and therefore have less disease. The results of observational studies show correlation, not causation; they make catchy headlines, but are not causative evidence.
To determine causation, we look to randomized trials, the gold standard of causative evidence. In randomized trials, two groups of randomly selected people are each assigned a different intervention. One group may eat a diet higher in vegetable oil, while the other group is assigned a diet lower in vegetable oil. Everything else about their diets and lifestyles remain unchanged.
Typically these randomized trials are also “blinded” in that neither group knows which intervention they’re receiving. For example, participants aren’t aware of whether their scrambled eggs are cooked in olive oil or soybean oil.
Because of the numerous problems with observational studies, only the results of well designed randomized trials without significant confounding factors were considered for this post.
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