Are Vegetable and Seed Oils Bad For Your Health? A Critical Look

Written by Kris Gunnars, BSc on April 27, 2013

The consumption of processed seed and vegetable oils has increased dramatically in the past century.

Commonly mistaken as health foods, quite a few studies show that these oils can cause harm.

These are oils that are extracted from seeds like Soybean, Cottonseed, Sunflower and a few others.

They were never available to humans until the 20th century, because we simply didn't have the technology to extract them.

The way these oils are manufactured is very disgusting (see video) and it is mind-baffling that someone ever thought they would be suitable for human consumption.

It involves a harsh extraction process that includes bleaching, deodorizing and the highly toxic solvent hexane.

These oils have made their way to all sorts of processed foods, including "healthy" salad dressings, butter replicates, mayonnaise, cookies and more.

Bottom Line: The processing method for industrial seed- and vegetable oils involves factories, many machines and chemicals like hexane.

The main problem with most of these oils is that they are way too high in Omega-6 polyunsaturated fatty acids.

Both Omega-3 and Omega-6 fatty acids are so-called essential fatty acids, meaning that we need some of them in our diet because the body can't produce them.

Throughout evolution, we got Omega-3 and Omega-6 in a certain ratio.

Our Omega-6:Omega-3 ratio used to be about 1:1. However, in the past century or so, this ratio in the Western diet has shifted drastically, all the way up to 16:1 (1).

When the Omega-6:Omega-3 ratio shifts too high in favor of Omega-6, bad things start to happen in the body.

The excess Omega-6 fatty acids build up in our cell membranes and contribute to inflammation (2).

Inflammation is an underlying factor in some of the most common western diseases and include cardiovascular disease, cancer, diabetes, arthritis and many, many others.

Bottom Line: Seed oils are high in Omega-6 fatty acids. Eating an excess of Omega-6 can lead to increased inflammation in the body and potentially contribute to disease.

In the past century, consumption of these oils has increased at the expense of other healthy fats like butter.

They were labelled as "heart-healthy" and the governments all around the world encouraged us to eat more of them.

This graph shows how consumption of polyunsaturated fats (mainly Omega-6) has increased in the U.S. to levels previously never known to humans.

Increase of Consumption of Polyunsaturated Fats in USA Graph

This graph here shows how the Omega-6 content in our body fat stores has increased.

Linoleic Acid in US Body Fat, 1961-2008 Graph

Photo Source: Stephan Guyenet.

That's right, the industrial seed and vegetable oils aren't just burned for energy like any other macronutrient.

These oils (that are very sensitive to chemical reactions) are also stored and incorporated into cells.

I don't know about you, but I find that to be a very scary thought. These oils are leading to actual physiological changes in our bodies.

Bottom Line: Consumption of Omega-6 fatty acids has increased drastically in the last century or so and their amount in our bodies has increased 3-fold.

When we refer to saturated, monounsaturated or polyunsaturated fats, we're referring to the number of double bonds in the fatty acid molecules.

  • Saturated fats contain no double bonds.
  • Monounsaturated fats contain one double bond.
  • Polyunsaturated fats contain two or more double bonds.

The problem with polyunsaturated fats is that all these double bonds make them susceptible to oxidation. The fatty acids react with oxygen and this damages them.

If we have a lot of these fatty acids in our bodies (remember: they get stored) - then our cell membranes are more sensitive to oxidation.

Basically, we've got our bodies loaded with very fragile fatty acids that can easily be degraded to form harmful compounds.

Given that polyunsaturated fats can easily go rancid at room temperature, it is quite likely that a large part of the vegetable oils on the market are in fact already damaged, before we even consume them.

These oils that we find in the stores may also contain trans fats. One source looked at vegetable oils in the U.S. market and discovered that the trans fat content varies between 0.56% and 4.2% (3).

That's right, these oils contain trans fats... which are incredibly harmful. Trans fats cause all sorts of horrible diseases, including cardiovascular disease, obesity, cancer and diabetes (4, 5, 6).

Bottom Line: Oils that are high in polyunsaturated fats are very susceptible to oxidation, both on the shelf and inside our bodies. They may also be high in trans fats.

Back in the day, people thought saturated fat raised cholesterol and caused heart disease.

A lot of money has been wasted on trying to prove that hypothesis, to no avail.

The attention is now increasingly turning to trans fats and seed- and vegetable oils.

At least 8 randomized controlled trials have examined replacing saturated fat with high-omega-6 oils (hat tip to Paul Jaminet).

  • In three of these studies, the seed oil group had a drastically increased risk of death (7, 8, 9).
  • In four studies, there was no statistically significant effect (10, 11, 12, 13).
  • One of the studies showed an improvement, but it had a number of flaws that make its results questionable (15).

When you plot the consumption of these oils to the prevalence of certain diseases in the population, you see some interesting correlations.

The amount of Omega-6 in the blood is highly correlated with death from cardiovascular disease (16):

Correlation Between Omega-6 in Blood and Death From Cardiovascular Disease Graph

This study only shows a correlation and can not prove that the Omega-6 fats are causing these cardiovascular deaths, but it's definitely something to think about.

Bottom Line: Several randomized controlled trials show that Omega-6 fatty acids increase death from heart disease, while other studies show no statistically significant effect. Observational studies show a strong association.

There are observational studies showing a drastic correlation between consumption of these fats and violence (17), indicating that they may have a detrimental effect on mental health.

Correlation Between Linoleic Acid Consumption and Violence Graph

This data can only prove a correlation, not that Omega-6 fats caused the increased violence. But the association is very strong and it is consistent not only between countries, but also within countries over time.

Bottom Line: Data from observational studies suggests that consumption of these oils is strongly associated with violent behavior and homicide.

Having more Omega-6 fats in mothers milk is associated with altered immune function in young children (18).

In rats, a high consumption of these oils can cause fatty liver and severe liver damage (19).

Several other animal studies also show an increase in cancer when rats eat a diet high in Omega-6 fats (20).

There are many other potentially harmful effects of eating these nasty oils that are beyond the scope of this article.

I am personally convinced that these oils are key players in the epidemics of many chronic Western diseases.

Bottom Line: Eating a high amount of Omega-6 fatty acids is associated with many harmful effects and they may be key players in many Western diseases.

It's important to note that not all plant oils are bad. For example, coconut oil and olive oil are both excellent.

The plant oils that you should avoid due to their high Omega-6 content include:

  • Soybean oil.
  • Canola oil.
  • Corn oil.
  • Safflower oil.
  • Cottonseed oil.
  • Sunflower oil.
  • Peanut oil.
  • Sesame oil.
  • Rapeseed oil.
  • Rice Bran oil.

Also avoid all margarines and fake butters.

Saturated fats are best for cooking, because they are the most stable in high heat. These include butter, coconut oil, lard and tallow.

Olive oil is also an excellent choice for cooking.

If you want to avoid these unhealthy oils, you must read labels!

Many nutrition professionals still peddle these oils as health foods and recommend that we replace saturated fats with not-so-"heart-healthy" vegetable oils.

This is one example of where blindly following mainstream recommendations can lead to detrimental effects on health.

An evidence-based nutrition article from our experts at Authority Nutrition.

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