Fats in the diet are highly controversial.
For some reason, dietary guidelines still recommend that we consume vegetable oils instead of saturated fats like butter.
Vegetable oils are claimed to lower cholesterol levels, which should help prevent heart disease, the world's biggest killer.
However... many studies have raised serious concerns about these oils (3).
Despite lowering LDL cholesterol, they can have negative effects on other aspects of health and metabolism.
Here are 11 reasons why you may want to avoid vegetable oils.
You've probably heard of Omega-3 and Omega-6 fatty acids before.
These fatty acids are polyunsaturated, meaning that they have many double bonds in their chemical structure.
They are often termed the essential fatty acids, because the body lacks the enzymes to produce them.
These fatty acids play important roles in many biochemical pathways, including those related to inflammation, immunity and blood clotting.
They often have related but opposing roles. For example, both of them are used to produce signalling molecules called eicosanoids.
Throughout evolution, we consumed balanced amounts of both Omega-3 and Omega-6 fatty acids. The problem today, is that this balance is drastically skewed towards Omega-6.
Not only are people eating way too much Omega-6, but their Omega-3 intake is also incredibly low, which is a recipe for disaster.
Whereas back in the day our Omega-6:Omega-3 ratio may have been about 1:1-3:1, these days it is about 16:1... which is way outside of evolutionary norms (9).
Vegetable oils are the biggest source of Omega-6 fatty acids in the diet... by far.
They are particularly high in the Omega-6 fatty acid linoleic acid. This fatty acid causes a host of problems when consumed in excessive amounts... especially when Omega-3 intake is low (which is usually the case).
Bottom Line: Vegetable oils are very high in an Omega-6 fatty acid called linoleic acid, which can contribute to all sorts of problems in large amounts.
Fats are more than just sources of energy.
Some of them have potent biological activity, and some remain in the body where they are used for structural and/or functional purposes.
What this means, is that our excessive consumption of vegetable oils is leading to actual structural changes within our body's tissues.
The linoleic acid content of breast milk has also increased significantly (18).
I don't know about you, but I think that's pretty scary.
Bottom Line: Studies have shown that the linoleic acid content of human fat cells and cell membranes has increased drastically in the past few decades.
Again, polyunsaturated fats like linoleic acid have two or more double bonds in their chemical structure.
This makes them sensitive to damage by free radicals, highly reactive molecules that are constantly being formed in the body (19).
This is actually what antioxidants are for, they help neutralize the free radicals.
When free radicals in the body outnumber the antioxidants, this leads to a condition known as oxidative stress.
Not surprisingly, because polyunsaturated fats are more susceptible to damage by free radicals, studies have shown that a high intake of linoleic acid can contribute to oxidative stress (20).
In one controlled trial, people were fed a diet high in Omega-6 linoleic acid, mostly from sunflower oil (21).
After 4 weeks, blood markers of oxidative stress had increased significantly. Another thing they noted was that blood markers of Nitric Oxide (NO) levels had gone down.
Nitric oxide is a signalling molecule produced by the endothelium, the thin layer of cells that lines the vascular system. It helps dilate blood vessels and keep blood pressure down.
Reduced nitric oxide is the beginning stage of endothelial dysfunction, where the lining of the vascular system stops working as it is supposed to (22).
Another study in test tubes showed that linoleic acid stimulated a pro-inflammatory state in endothelial cells (23).
Endothelial dysfunction is actually one of the earliest steps in the pathway towards heart disease and other serious vascular problems (24).
Bottom Line: Linoleic acid from vegetable oils increases oxidative stress in the body, contributing to a state called endothelial dysfunction. This is a stepping stone towards heart disease.
One of the main reasons vegetable oils are (mistakenly) considered healthy, is that consuming them can lower Total and LDL cholesterol levels.
As most people know, LDL is often referred to as the "bad" cholesterol.
However... it's important to keep in mind that this is just a risk factor, not an actual disease.
What really matters is how vegetable oils affect hard end points like heart disease itself, as well as other diseases and the risk of death.
Bottom Line: It is true that vegetable oils can lower Total and LDL cholesterol levels. However, they can also lower HDL, the "good" cholesterol.
What people refer to as "LDL cholesterol" isn't really cholesterol.
LDL stands for Low Density Lipoprotein... the protein that carries cholesterol in the bloodstream.
One of the crucial steps in the heart disease process, is Low Density Lipoprotein becoming oxidized, forming what are called oxidized LDL particles, or ox-LDL (30).
These are the LDL particles that build up inside the walls of the arteries (31).
Bottom Line: Vegetable oils increase the susceptibility of LDL lipoproteins to oxidation, a crucial step in the development of heart disease.
Heart disease is the world's most common cause of death.
The evidence regarding vegetable oils and heart disease is fairly mixed, and their use is highly controversial.
The best way to determine how they affect heart disease, is to look at the randomized controlled trials where large groups of people are fed vegetable oils for many years.
Fortunately, many such studies have been performed.
It is often claimed that "polyunsaturated fats" prevent heart disease, but it is a huge mistake to lump all polyunsaturated fats together, because this category includes both Omega-3 and Omega-6 fatty acids.
In one review that compared studies where people are fed mixed Omega-3s and Omega-6s, there was a protective effect.
But when they looked at the studies where people were only advised to eat more Omega-6 (from vegetable oils), they found that the risk of heart disease was increased by 16%. The effect was not statistically significant, but very close (46).
However... observational studies can not prove causation, they are mostly useful for generating hypotheses to be studied further.
When we have conflicting information from observational studies and controlled trials, we must go with what the controlled trials say... because these are the only types of studies that can demonstrate causation.
If we look at the best available evidence, consuming vegetable oils appears more likely to cause heart disease rather than prevent it.
Bottom Line: The evidence regarding vegetable oils and heart disease is mixed, but several high quality studies have found them to increase heart disease risk.
As mentioned above, one problem with the fatty acids in vegetable oils, is that they tend to react with oxygen.
This doesn't just happen inside the body, it also happens when these oils are heated. That's why using vegetable oils for cooking is a terrible idea.
Some of these harmful compounds vaporize and may contribute to lung cancer when inhaled. Just being present in a kitchen where vegetable oils are being used may raise your risk of lung cancer (51, 52).
Bottom Line: Vegetable oils are high in polyunsaturated fats, which easily damage during cooking and may even vaporize and form compounds that can contribute to lung cancer when inhaled.
There is some evidence that vegetable oils can raise the risk of cancer.
Because vegetable oils contain highly reactive fatty acids that sit in cell membranes, they contribute to oxidative damage.
When fatty acids in membranes get oxidized, they can cause chain reactions.
If you think of the cell membrane as a cloud, these oxidative chain reactions are like little streaks of lightning passing through.
These reactions can harm important molecules in the cell. Not just fatty acids in the cell membrane, but also other structures like proteins and DNA.
They can also form various carcinogenic compounds within the cells (53).
By damaging DNA, these oils can raise the risk of harmful damage that contributes to increased cancer risk over time.
In one 8-year controlled trial, the group that replaced saturated fats with vegetable oils was almost twice as likely to die from cancer. The difference wasn't quite statistically significant, but very close (54).
This is supported by a plethora of studies in test animals, showing that vegetable oils drive cancer in these animals... especially breast cancer, the most common type of cancer in women (60, 61, 62).
Bottom Line: Several lines of evidence suggest that vegetable oil consumption can raise the risk of cancer, which makes perfect sense given the fact that they make the cells more susceptible to oxidative damage.
One place where polyunsaturated fats gather is in the brain.
In fact... the brain is about 80% fat, and a large portion of it is Omega-3 and Omega-6 fatty acids, about 15-30% of the brain's dry weight (63).
If Omega-6 fats from vegetable oils compete for the same enzymes and same spots in cell membranes as Omega-3 fats, then it makes sense that they should affect the function of the brain as well.
Interestingly, studies have found very strong correlations between vegetable oil consumption and violent behavior, including homicide.
The graph below shows data from one study, looking at Omega-6 intake and homicide rates in 5 countries (64).
Of course, correlation does not equal causation, so there's no guarantee that vegetable oils caused the increased rates of homicide, but the statistical correlation is striking.
Bottom Line: Polyunsaturated fats are concentrated in the brain, and many believe that our high vegetable oil consumption is leading to mental health problems, including violent behavior.
One thing that most nutrition folks agree on, is that whole foods are best.
Whole, unprocessed foods tend to be significantly more nutritious and healthier than their processed counterparts.
But most vegetable oils are highly refined... the most common way to extract them from their seeds is via harsh chemical processes that involve bleaching, deodorizing and the toxic solvent hexane.
Because of this, pretty much all of the vitamins and phytonutrients are removed from these oils.
Therefore, they can most definitely be classified as "empty" calories.
Bottom Line: Most vegetable oils are highly processed and refined products, which are completely lacking in essential nutrients.
Unless you've been living under a rock, you've probably heard of trans fats before.
These are unsaturated fats that have been chemically modified to be solid at room temperature.
They are usually found in highly processed foods. They are so toxic that the governments around the world have set laws in order to remove them from foods.
However... what most people don't know, is that vegetable oils contain significant amounts of trans fats.
Surprisingly, the trans fat content is rarely listed on the label.
There are many healthy fats that humans have been eating for hundreds of years without any problems (since before all the "modern" diseases became common).
Unfortunately, the same can NOT be said about vegetable oils. These are refined and processed fats that have been shown to cause harm in numerous studies.