Are Vegetable and Seed Oils Bad for Your Health?

Health Line

25-07-2025

While these oils can be a healthy source of fat, some varieties are better choices than others. That said, the often-mentioned adverse effects of omega-6 fats may not be as clear as previously believed.
The consumption of vegetable oils has increased dramatically in the past century.
Most mainstream health professionals consider them healthy, but vegetable oils may cause health problems.
Their health effects vary depending on what fatty acids they contain, what plants they are extracted from, and how they are processed.
This article examines the evidence to determine whether vegetable and seed oils harm your health.
What are they and how are they made?
Edible oils extracted from plants are commonly known as vegetable oils.
In addition to their use in cooking and baking, they're found in processed foods, including salad dressings, margarine, mayonnaise, and cookies.
Common vegetable oils include safflower oil, soybean oil, and canola oil.
Refined vegetable oils were not available until the 20th century, when the technology to extract them became available.
These are extracted from plants using either a chemical solvent or an oil mill. Then they are often purified, refined, and sometimes chemically altered.
Health-conscious consumers prefer oils made by crushing or pressing plants or seeds rather than those produced using chemicals.
Consumption has increased drastically
In the past century, vegetable oil consumption has increased at the expense of other fats like butter.
They are often labeled 'heart-healthy' and recommended as an alternative to sources of saturated fat, such as butter, lard, and tallow.
Vegetable oils are considered heart-healthy because studies consistently link polyunsaturated fat to a reduced risk of heart problems compared with saturated fat.
Despite their potential health benefits, some scientists worry about how much of these oils people consume.
As explained in the next chapter, these concerns mainly apply to oils that contain a high amount of omega-6 fats.
What do we know about omega-6?
It's important to note that not all plant oils are bad for your health. For example, avocado oil and olive oil are both excellent choices.
Both omega-6 and omega-3 fatty acids are essential fatty acids, meaning that you need some of them in your diet because your body can't produce them.
Throughout evolution, humans got omega-6 and omega-3 in a certain ratio. While this ratio differed between populations, it's estimated to have been about 1:1.
However, in the past century or so, this ratio in the Western diet has shifted dramatically and may be as high as 20:1.
Older research has hypothesized that too much omega-6 relative to omega-3 may contribute to chronic inflammation. On the other hand, a more recent study from 2025 did not show a connection between higher omega-6 consumption and inflammation.
Chronic inflammation is an underlying factor in some of the most common Western diseases, such as heart disease, cancer, diabetes, and arthritis.
Observational studies have also associated a high intake of omega-6 fat with an increased risk of heart disease.
However, these associations don't necessarily imply a causal relationship. In fact, the evidence for omega-6 being detrimental to human health is mixed.
Some studies investigating the effects of omega-6 fat consumption generally do not support the idea that these fats increase inflammation.
For instance, eating a lot of linoleic acid, which is the most common omega-6 fat, doesn't appear to affect blood levels of inflammatory markers.
Consider limiting plant oils high in omega-6, as well as processed foods that contain these plant oils, which include:
soybean oil
corn oil
cottonseed oil
sunflower oil
peanut oil
sesame oil
rice bran oil
Scientists do not fully understand the effects of omega-6 fats on the body, and more human studies are needed.
However, if you are concerned, it is more important to limit processed foods high in omega-6 fats, such as baked treats, chips, deep-fried fast foods, or takeout meals, as these have a more direct and proven effect on health.
Current studies remain inconclusive about the actual impact of avoiding vegetable oils when cooking at home on health.
These oils are easily oxidized
Saturated, monounsaturated, or polyunsaturated fats differ by the number of double bonds their chemical structures contain:
Saturated fats: no double bonds
Monounsaturated fats: one double bond
Polyunsaturated fats: 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 in the atmosphere and start deteriorating.
The fat you eat isn't only stored as fat tissue or burned for energy ⁠; it is also incorporated into cell membranes.
If your body contains a lot of polyunsaturated fatty acids, your cell membranes are more sensitive to oxidation.
In short, you have a very high level of fragile fatty acids that can easily be degraded to form harmful compounds called free radicals, which can cause cell damage.
For this reason, it may be best to eat polyunsaturated fats in moderation. Vary your diet by eating a mix of healthy saturated, monounsaturated, and polyunsaturated fats.
The USDA 2020-2025 Dietary Guidelines for Americans recommend aiming for 20-35% of total daily calories from fats, with no more than 10% of these calories coming from saturated fat.
They are sometimes high in trans fats
Commercial vegetable oils may also contain trans fats, which form when the oils are hydrogenated.
Food producers use hydrogenation to harden vegetable oils, making them solid like butter at room temperature.
Margarines are no longer full of trans fats since the FDA ban on trans fats in 2015, which went into effect in 2018, with an extended deadline of 2020 for some food manufacturers. However, some margarines may still contain a small amount of trans fat, so it's important to read the nutrition label.
However, non-hydrogenated vegetable oils may also contain some trans fats. However, it's important to note that naturally occurring trans fats found in non-hydrogenated oils are less detrimental to health than industrial trans fats.
Also, natural trans fats are found in very small amounts compared to hydrogenated oils. One 2015 study found extremely low amounts of trans fats (less than a quarter of 1%) in vegetable cooking oils.
A high intake of trans fats is associated with all sorts of chronic diseases, including heart disease, obesity, cancer, and diabetes.
If a product lists hydrogenated oil or partially hydrogenated oil as an ingredient, it likely contains trans fats. For optimal health, avoid these products, although with the trans fat ban, they are likely to be present only in very small amounts now.
Vegetable oils and heart disease
The reason is that vegetable oils are generally low in saturated fat and high in polyunsaturated fat.
The benefits of reduced saturated fat intake are controversial.
However, studies show that reducing saturated fat intake for at least 2 years reduces the risk of heart problems by 21%, but it has no significant effects on the risk of death from heart disease.
Furthermore, omega-3 polyunsaturated fatty acids appear to have a greater benefit than omega-6 polyunsaturated fatty acids.
The omega-6 linoleic acid has been associated with a lower risk of heart disease. However, it is important to be vigilant about avoiding highly refined vegetable oils and processed foods that contain these oils.
Our food environment makes it very easy for us to get more than enough omega-6-rich vegetable oils. We can control how we prepare food at home, so in an effort to practice moderation, it may be a good idea to choose other oils that are lower in omega-6 content, like olive oil or avocado oil.
There is a lot of evidence on the benefits of olive oil on heart health.