Many people perceive vegetable oils as healthy.
Maybe it's because they have the word "vegetable" in them.
I mean... vegetables are good for you, right? So vegetable oil must be too...
Even the mainstream nutrition organizations recommend that we eat them, because according to them, unsaturated fats are much healthier than saturated fats.
However, many studies have now demonstrated that these oils can cause serious harm (1).
The composition of the fatty acids in them is different than anything we were ever exposed to throughout evolution.
This is leading to physiological changes within our bodies and contributing to multiple diseases.
Here are 6 reasons why vegetable oils can be downright harmful.
In this article, I'm referring to processed seed oils like soybean oil, sunflower oil, corn oil, canola oil, cottonseed oil, safflower oil and a few others.
Even though they aren't really vegetables, these oils are commonly referred to as "vegetable oils." These oils contain very large amounts of biologically active fats called Omega-6 polyunsaturated fatty acids, which are harmful in excess.
This does NOT apply to healthy plant oils like olive oil or coconut oil, which are extremely good for you.
Humans have been evolving for a very long time, but industrial food processing is brand new. We didn't start producing vegetable oils until about a hundred years ago.
Between the years 1909 and 1999, the consumption of soybean oil increased more than a thousandfold and now supplies about 7% of calories in the U.S. diet (2).
This processing method is really disgusting and involves pressing, heating, various industrial chemicals and highly toxic solvents. Other vegetable oils are processed in a similar manner.
It baffles me that anyone would think this stuff is fit for human consumption.
If you choose healthier brands that have been cold pressed (lower yield and therefore more expensive) then the processing method will be much less disgusting, but there is still the problem of excess Omega-6 fats.
Bottom Line: Humans were never exposed to these oils until very recently on an evolutionary scale, because we didn't have the technology to process them.
There are two types of fatty acids that are termed "essential" - because the body can't produce them.
These are the Omega-3 and Omega-6 fatty acids.
It is absolutely essential for the human body to get these fatty acids from the diet, but it must get them in a certain balance.
While humans were evolving, our Omega-6:Omega-3 ratio may have been around 4:1 to 1:2. Today, our ratio is as high as 16:1 on average, with great variation between individuals (3).
These fatty acids aren't just inert structural molecules or fuel for the cell's mitochondria, they serve vital functions related to processes known to affect various systems like the immune system (4).
When the balance of Omega-6s and Omega-3s in the cell is off, things can start to go terribly wrong.
Another problem is the relative unsaturation of these fatty acids. Polyunsaturated fats have two or more double bounds, while monounsaturated fats have one and saturated fats have no double bonds.
The more double bonds in a fatty acid, the more reactive it is. Polyunsaturated fats tend to react with oxygen, which can cause chain reactions, damaging other structures and perhaps even vital structures like DNA (5, 6).
These fatty acids tend to sit in the cell membranes, increasing harmful oxidative chain reactions.
According to this graph, our body fat stores of Linoleic Acid (the most common Omega-6 fat) have increased 3-fold in the past 50 years.
Photo from: Stephan Guyenet.
That's right, excessive consumption of vegetable oils leads to actual structural changes within our fat stores and our cell membranes.
I don't know about you, but I find that to be a pretty scary thought.
Bottom Line: Omega-6 and Omega-3 fatty acids are biologically active and humans need to eat them in a certain balance to function optimally. Excess Omega-6s in our cell membranes are prone to harmful chain reactions.
Omega-3 and Omega-6 fatty acids are used to make substances called eicosanoids in the body.
These are modified fatty acids that sit in the cell membranes.
There, they play a crucial role in bodily functions like cellular messaging, immunity and inflammation.
If you've ever taken aspirin or ibuprofen and noticed relief from headache or some kind of pain, then that's because these drugs inhibit the eicosanoid pathways and reduce inflammation.
Whereas acute inflammation is good and helps your body heal from damage (such as when you step on a lego), having chronic, systemic inflammation all over your body is very bad.
Generally speaking, eicosanoids made from Omega-6s are pro-inflammatory, while those made from Omega-3s are anti-inflammatory (7).
These different fatty acids compete with each other. The more Omega-6 you have, the more Omega-3 you need. The less Omega-6 you have, the less Omega-3 you need (8).
Having high Omega-6 AND low Omega-3 is a recipe for disaster, but this is the case for people eating a Western diet.
Put simply, a diet that is high in Omega-6 but low in Omega-3 contributes to inflammation. A diet that has balanced amounts of both Omega-6 and Omega-3 reduces inflammation (9).
It is now believed that increased inflammation can contribute to various serious diseases, including cardiovascular disease, arthritis, depression and even cancer.
Bottom Line: Eicosanoids, signaling molecules made from Omega-6 and Omega-3 fats, are crucial in regulating inflammation in the body. The more Omega-6s you eat, the more systemic inflammation you will have.
Trans fats are unsaturated fats that are modified to be solid at room temperature.
They are so bad that even the governments around the world have started taking action, setting laws that command food manufacturers to reduce the trans fat content of their foods.
However, a little known fact is that vegetable oils often contain massive amounts of trans fats.
In one study that looked at soybean and canola oils found on store shelves in the U.S., about 0.56% to 4.2% of the fatty acids in them were toxic trans fats (13).
If you want to reduce your exposure to trans fats (you should) then it's not enough to avoid common trans fat sources like cookies and processed baked goods, you also need to avoid vegetable oils.
Bottom Line: Trans fats are highly toxic and associated with multiple diseases. Soybean and canola oils commonly sold in the U.S. contain very large amounts of trans fats.
Cardiovascular disease is the most common cause of death in the world (14).
Now the attention is increasingly being turned to vegetable oils.
Multiple randomized controlled trials have examined the effects that vegetable oils can have on cardiovascular disease.
Only one study found a protective effect, but this study had a number of flaws (24).
If you look at observational studies, you find a very strong correlation.
This graph is from one study where the Omega-6 content of blood was plotted against the risk of death from cardiovascular disease (25):
You can see the U.S. sitting there at the top right, with the most Omega-6 AND the greatest risk of death from cardiovascular disease.
Even though this study only shows a correlation, it makes perfect sense given that inflammation is a known contributor to these diseases.
I'd like to point out that there are some studies showing that polyunsaturated fats reduce the risk of cardiovascular disease. But the problem is that they don't make the distinction between Omega-3s and Omega-6s, which is absolutely crucial.
When they do, they see that Omega-6s actually increase the risk, while Omega-3s have a protective effect (26).
Bottom Line: There is evidence from both randomized controlled trials and observational studies that vegetable oils can increase the risk of cardiovascular disease.
Because polyunsaturated fats are so tightly involved in the function of the body on a molecular level, it makes sense that they could affect other diseases as well.
Many of these association aren't well studied in humans (yet), but there are both observational studies and animal studies linking vegetable oils to other serious diseases:
- In one study, increased Omega-6 in breast milk was associated with asthma and eczema in young children (27).
- Studies in both animals and humans have linked increased Omega-6 intake to cancer (28, 29).
- One study shows a very strong correlation between vegetable oil consumption and homicide rates (30).
- The Omega-6:Omega-3 ratio in blood has been found to be strongly associated with the risk of severe depression (31).
This is just the tip of the iceberg. Inflammation, and therefore vegetable oil consumption, is associated with a wide range of serious diseases and it is beyond the scope of this article to cover all of them.
I am personally convinced that vegetable oils (along with added sugars and refined wheat) are key players in the epidemics of chronic, Western diseases, which are currently the biggest health problems in the world.
If you want to be healthy, feel good and lower your risk of serious diseases, then you should consider avoiding vegetable oils.