• Stir frying noticeably increases the amount of trans fatty acids, significantly more than any other method.
  • Deep frying in high temperatures and in oils used multiple times, also produces some trans fats.
  • Other cooking methods either don’t raise trans fats contents or only raise them a negligent amount.
  • Although the trans fats generated through stir frying and deep frying in reheated oils are still within the recommended safe limits, it is important to note that frequent consumption of these foods adds up during the day and may exceed the maximum limit.


Does frying in oil produce trans fats?

Most of the harmful trans fatty acids (TFAs) in our diet come from partially hydrogenated oils, (only small percentage from ruminants) which involves high temperatures and high pressure. But does heating up vegetable oils during cooking also generate those dangerous substances? What are the cooking methods that generate the highest amounts of trans fats?

It has been broadly proven that TFAs cause many negative health effects, such as increased risk of cardiovascular disease and various cancers. Therefore, trans fat foods should be avoided.

Some countries, such as Denmark, have introduced strict limits on the contents of TFAs in foods and on the usage of partially hydrogenated oils (PHOs).

These measures successfully resulted in a drastic reduction of the consumption of TFAs, below the recommended upper levels.

Nevertheless, the question remains whether home food preparation techniques, which use high temperatures, can generate these harmful substances.

How are trans fatty acids formed?

TFAs are formed in the process of lipid oxidation. High temperatures accelerate the rate of this oxidation and, therefore, speed up the formation of TFAs.

Unsaturated fatty acids, such as vegetable oils, contain carbon double bonds in “cis” formation that are unstable.

This means that they are more prone to spoilage. When heated up, the atoms get agitated and a hydrogen atom is removed from the “cis” configuration, forming a lipid radical.

To create a more stable configuration, a hydrogen atom is then added to the other side of the damaged chain, forming what we know as “trans” fatty acids. (read more..)

This is the most common process of partial hydrogenation and it is how the majority of the dietary trans fats are formed. Process of partial hydrogenation has been common in production of margarine and shortenings from 1920s till about two decades ago. (1)

Can you generate trans fats while cooking?

Partial hydrogenation process, generates high amounts of industrial trans fatty acids (iTFAs), as a result of heating unsaturated fats. These damaged fatty acids are similar in chemical structure to saturated fatty acids, but their effects on health differ. Can the same happen during cooking at home or in restaurants?

NOTE: Studies show that cooking in high heat increases the risk of cardiovascular disease and type 2 diabetes.

Several studies were performed on different types of oils and cooking methods, such as baking, stir frying, pan frying, deep frying and roasting.

Trans fats formed during stir frying – study 1
The following are the main conclusions of a study on trans fat generation, using various methods of cooking with corn oil.

They found evidence that stir frying at a temperature of 170 degrees Celsius (338 Fahrenheit) increases the trans fatty acid contents, when compared to raw oil before cooking.

Other cooking methods did not show an increase in Trans fats.

The study concluded that the formation of trans fats in stir-frying was simultaneously due to high temperatures and stirring, causing an increase rate of oxidation of fatty acids. (2)
Trans fats generated during cooking
Trans fats formed during deep frying – study 2
Another study on deep frying, showed that frying potato chips in a reheated oil for 10 times at the temperature of 180 degrees Celsius (356 Fahrenheit), generated more TFAs than heating oils to the same temperature for 4 hours without chips.

When potatoes were fried in an oil, which was reheated 10 times, the total fat content in fried potatoes was 9%.

The TFA contents were, however, within the U.S. health recommendations of <0.5g per serving. 100g of fried potatoes contained only 0.1g of TFAs, which represented about 1% of total fat contained in fried potatoes. (3)

Trans fats formed during cocoa bean roasting – study 3
Another study showed a negligible increase of TFA contents in cocoa butter by roasting cocoa beans at up to 135 degrees Celsius (275 Fahrenheit) for 15 minutes. (4)


You will find a summary of the most common nutrition myths and evidence-based nutrition facts here.


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