If you are looking to eat healthier, and are watching your diet, stay away from processed vegetable oils, such as soybean oil, corn oil, and any type of processed seed oil, says Dr. Catherine Shanahan, MD, a metabolic expert. She warns that most of the vegetable oil we eat today is processed in a way that is making it harder for people to burn fat.

Because processed vegetable oil is in almost everything you can buy, from cookies to salad dressings, Americans now get a huge amount of these unhealthy oils in our diet, Dr. Shanahan explains. Now, Americans get 80 percent of their fat calories from processed vegetable oil, or about 650 calories a day, as opposed to generations ago when it was less than 100 calories a day. So what are processed oils, and why are they so terrible for us?

Processed vegetable oils include the oil from:

  • soybean oil
  • sunflower oil
  • safflower oil
  • corn oil
  • canola oil
  • cottonseed oil
  • rice bran oil
  • grapeseed oil

It's not just in cooking but also in store-bought packaged foods:

  • salad dressing
  • marinara sauce
  • health bars
  • dairy-free spreads
  • breakfast cereals
  • bread

High in polyunsaturated fatty acids, of PUFAs

These vegetable oils all contain polyunsaturated fatty acids, or PUFAs, which are unhealthy in large amounts. (Note that oils of all types, even olive oil, contain polyunsaturated fatty acids, so this is where the amounts we eat and the source of the fat we consume matter.)

"Polyunsaturated fatty acids basically weaponize our body fat so when we try to burn our body fat between meals, such as when we go work out, these PUFAs short circuit our ability to burn fat. It turns us into sugar addicts," Dr.Shanahan explains. "That's because the one fuel in your bloodstream is sugar and when it runs out, you should be able to burn fat, but you can't mobilize fat from these oils as easily, and when your body can't burn one type of fuel, it turns to another." This is why Dr. Shanahan says it's so hard for some people to lose weight.


PUFAs are making us gain weight
Back in our ancestors’ day, their diets were equally comprised of omega-3 fatty acids, found in nuts and fish, and omega-6 fatty acids, from seeds and plants. That was a healthy balance, and if we could have maintained that ratio, our bodies likely would not be holding onto fat the way they do today, Dr. Shanahan explains. Now we eat more PUFAs, or omega-6 fatty acids, at a ratio of 20 to one.

“These PUFAs zap our mitochondria and short circuit our ability to burn fat,” explains Dr. Shanahan. This is why it’s so hard for some people who have become metabolically challenged to lose body fat. “When your body can’t use one form of fuel – such as our locked fat cells – it is forced to use another, so we find ourselves craving sugar. The body tries to protect itself from having to ever burn off our body fat. So it’s as if your body is at war with the largest organ in the body, which is our adipose tissue."

In the US, soybean oil is consumed more than any other kind of oil; Americans consumed 11.4 million metric tons of soybean oil in 2021, or about 25 times the amount of olive oil.  “Our ancestors once ate about one 20th of that amount, or 30 to 35 calories, or about 4 grams of it.

Even those oils that carry labels such as "heart-healthy" are highly refined and contain a high amount of polyunsaturated omega-6 fats. Most vegetable oils are a blend of soybean, palm, and other oils. You may be surprised to learn about their detrimental effect on human health.

Factory processing involves chemical solvents

What is so harmful about processed vegetable oil? The vegetable oil we eat today is processed in a way that uses heat, chemicals, and even cleaning solvents that end up in our bodies and plays a role in hindering our ability to burn fat, she explains.

Oils that are refined and processed are treated with any number of chemicals, such as:

  • acid
  • bleach
  • purified with an alkali
  • hexane

According to Happily Unprocessed, your vegetable oil has likely been filtered, deodorized and even bleached. A chemical called Hexane is also often used to extract edible oils from seeds and vegetables.

Hexane is classified by the Environmental Protection Agency as "a special-use solvent, and as a cleaning agent" and is often used in glue. The government site adds: "Acute (short-term) inhalation exposure of humans to high levels of hexane causes mild central nervous system effects, including dizziness, giddiness, slight nausea, and headache, according to the EPA.

Dr. Shanahan explains that all this processing turns vegetable oil into an ingredient that affects your body differently than fat from whole foods such as olive and avocado, and when it gets into your fat, the body is trying to protect itself by keeping these toxins in deep storage.

But this is also why it's so hard for some people who eat a lot of vegetable fat to lose body fat. She explains that the way our food is processed, has changed in the past 100 years, and the methods used today can have a detrimental impact on our health and can affect how our fat cells react.

Because of the way plants like soybeans and corn and other raw materials are heated, processed, and then chemicals are added to these raw materials to extract the vegetable oil from the plant, those same toxins get into our bodies and are stored as fat.

How to stop the cycle and burn body fat

"Back 100 years ago, when our ancestors ate fat it came from animal sources like butter, lard, and dairy, which are not healthy sources, but they ate much less. And when it came from plants it was pressed out of the olives or avocados or nuts," Dr. Shanahan explains. "Today, store-bought vegetable oil is extracted in much harsher methods, which takes out all the nutrients and puts in toxins," she adds. She has teamed up with a company, Zero Acre Farms, which has raised $37 million to develop a healthier plant-based oil.

"Olive oil can be cold-pressed, so that's the healthiest oil you can have, so long as it's the really good stuff that has not been diluted or mixed with cheaper oils," Dr. Shanahan explains. "But only about 3 percent of the globe can grow olives," she adds, so we need to find another alternative that is healthy and inexpensive, and that is what Zero Acre is working on.

They expect to announce a "better for you" oil that is being developed under stealth conditions with funding from all-star investors, chefs, climate advocates, and others, including Robert Downey Jr.’s FootPrint Coalition Ventures, and Chef Dan Barber, among others. Until then, Dr. Shanahan says, "your best bet is to stay away from processed oils  and anything that contains soybean oil, corn oil, safflower oil, and any seed oil, high in polyunsaturated fats."

Instead, choose whole foods and healthier fats:

  • Nuts
  • Avocado
  • Canola oil
  • Olive oil
  • Peanut oil or nut butter

To get the health benefits, you need to replace unhealthy fats with healthy fats, according to Medicine Plus. Examples include:

  • Eat nuts instead of cookies or chips
  • Add avocado to salads and sandwiches
  • Replace butter and solid fats with olive oil
For shopping, if you are buying oil look for dressings or cooking, look for 100 percent olive oil in a dark container that signifies it will go bad if left in the heat or direct sunlight.

How to choose a healthy oil

So where does that leave us? We need to be mindful of the type of fat or oil we eat, Dr. Shanahan explains, and the total fat in our diet. No more than 25 to 30 percent of our total calorie intake should come from fat, according to the USDA, Since fat has 9 calories per gram, or more than twice as much as protein or carbs (which have 4 calories per gram), eating any fat, even healthier vegetable-derived fats, is something to limit since too much fat can lead to weight gain.

The heart healthiest oils are those that come from whole foods, like olives, avocados, and nuts. The worst fats for your heart health are considered saturated fat and trans fat, which are solid at room temperature, like butter and lard.

But just because an oil is liquid at room temperature doesn't mean it's healthier. Vegetable oils have become popular because they are inexpensive, easy to mass-produce, and are only considered heart healthier than animal fat as they relate to heart disease.

So while saturated fat has been linked to high cholesterol, clogging of the arteries, and other risk factors for heart disease, highly processed vegetable oil intake has been associated with metabolic syndrome, obesity, and type 2 diabetes risk, so neither is particularly healthy.

As vegetable oils have gained in popularity, Americans are simply eating too many of them to be healthy. Vegetable oil has 124 calories per tablespoon, so even using two in cooking will increase your daily calories by about 250 calories.

Whole foods are healthier

The best fats and oils to eat are the kind that you could squeeze out of the nut, olive, or avocado in your own kitchen, says Dr. Shanahan, and even then limit your intake if you're watching calories.

"If we can cold-press a food, such as olives, and get oil out of it, or if we can eat whole food, such as avocados, our bodies can use the healthy fat in these whole foods, and those fats are not dangerous to our bodies and actually have health benefits."

Stay away from junk food

Vegetable oils are in packaged foods like chips, cookies, and other processed items at the store, including salad dressings, pasta sauces, mayonnaise, and more.

"These are the reason that junk food is so bad for us – beyond the caloric content of the foods," she adds. "Why is junk food so damaging? Other than the empty calories, devoid of all nutrients? Beyond those, which are just fattening, these PUFAs will create these fat cells that will kill you." These foods make it virtually impossible for our bodies to unlock the fat cells and burn fat for fuel.

So what can you do to eat in a way that allows your fat cells to unlock and burn as fuel? "You can't go on just any diet and expect to lose weight," Dr. Shanahan says. "A lot of people are so damaged metabolically that it's impossible to change their metabolic state since they have fat that is essentially locked in and at that point, it's almost impossible to actually eat in a way to burn fat. Their body fat is resisting being burned, and it makes them crave sugar all the time."

Most people assume that they can starve themselves and start losing fat, she says, but that's not practical or healthy. "Instead of trying to starve yourself," Dr.Shanahan advises, "you have to go about losing fat carefully. You have to get in touch with your hunger. So if you have crafted your meals in a way that gives you enough energy but without any processed oils, your body will start to respond. The key is to eat whole foods.

"Stay away from junk food at all costs," she says. "And we have to be clear about what junk food is." It's anything that has processed oil in it. These all cause oxidative stress on the body which accelerates the aging process.

In order to combat that, she says, "your body actually needs more antioxidants, more vitamin E and vitamin C and Glutathione. These all help to fight oxidation, or aging on a cellular level. So when you eat junk food, it rapidly accelerates the aging of the body.

Healthy alternatives

  • Use vegetable broth for cooking
  • Use avocado or avocado oil in spreads and dressings
  • Use olives and 100 percent olive oil in sauces or recipes

So what should we eat? "The perfect dinner would probably be sushi, especially avocado roll," says Dr. Shanahan. "Avocado oil is a very balanced source of fatty acids. The tricky part is getting enough omega 3s. Tofu happens to be a great source of protein and also low in fat."

When you buy oil: If you buy olive oil just make sure to buy the highest grade to make sure you are getting 100 percent pure virgin olive oil, which should tell you that it has not been diluted and has been minimally processed.

A good rule of thumb: If you can eat it as food, then the oil from it is probably less refined. Think about soy: You can't eat it as it comes off the plant. It's inedible when unrefined. That is the difference between the traditional oil (of generations ago) and the factory oil of today. The factory oil is so ultra-processed as to make it toxic, and so our bodies sweep it into our fat cells to lock it up.

Bottom Line: Vegetable Oil is So Highly Processed It's Not Healthy

Vegetable oil is so highly processed these days that the chemicals in our oils get locked into our fat cells and make it nearly impossible to burn fat for fuel. Back when our grandparents were alive, they didn't have these same chemical processes and the oils they ate were healthier and less damaging to our metabolic health.

Whole foods like olives and avocado contain healthy fat that is available without heating and chemical processing, so focus on a diet rich in whole foods. But as for the question of how to get this out of our systems, the answer is to do a PUFA detox and stay away from processed vegetable oils.

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