Overview of Trans Fats and What to Avoid

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We often hear now that "trans fats are bad", and while it's true that most trans fats in our diets these days have negative health consequences, that's actually an oversimplification. The simplified, but accurate, thing to say is that partially hydrogenated fats are bad for us. "Trans fats" have become shorthand for this type, but there are also other trans fats that may even be good for us. The American Heart Association recommends limiting trans fats to less than 1% of your daily caloric intake.

What are Trans Fats, Actually?

Trans fats are a type of fat.  Trans" refers to the configuration of the molecules. Most fats in our body have the "cis" configuration. "Trans" and "cis" configurations of the same molecule are called "stereoisomers". 

Artificial Trans Fat

Artificial trans fat is a type of fat created when vegetable oil is hardened or solidified through a process of treatment with hydrogen at high pressure and temperature. The process converts the liquid vegetable oil into a solid or semi-solid substance that is often made into margarine or shortening and incorporated into many other processed foods. The vast majority of trans fats in our food are manufactured by adding hydrogen bonds to unsaturated fats. This makes the fat more stable, so it doesn't spoil as quickly. These fats are usually referred to as "partially hydrogenated fats" or "partially hydrogenated oils". "Fully hydrogenated fats" should not contain significant amounts of trans fats, but they have not been studied very much when it comes to safety in terms of our health.

The evidence is reasonably clear that trans fat contributes to heart disease.  In 2015, a Food and Drug Administration ruling removed artificial trans fat as GRAS or "generally recognized as safe" for use in human food. The ruling means food manufacturers have three years to remove all trans fat from their products.

However, studies show that leaving this to food manufacturers may not be a good policy as a study showed that despite being outlawed in Europe, only a slight reduction in trans fats was found in the same foods when compared before and 3 years after the rules' implementation.

A Naturally-Occurring Trans Fat: CLA is a Healthy Trans Fat

The other type of fat that has a trans configuration occurs naturally in the milk and meat from certain animals (ruminants, like cows and bison, especially those who have eaten grass as opposed to grain). These are known as trans fatty acids, or TFAs. This type of fat is called conjugated linoleic acid (CLA), and it appears to be good for us. There is a little evidence that CLA can protect from cancer, for example. However, like many naturally-occurring substances, studies on CLA in the form of a food supplement have been disappointing so far.

Which Foods are High in Trans Fats?

Baked goods and other commercially fried or processed foods may incorporate trans fats. Regular shortening is mostly made of partially hydrogenated oil and therefore has a lot of trans fat in it. In the past, margarine has been primarily made of trans fats, and trans fats were prominently used in manufactured crackers, cookies, breads, pies, muffins, donuts, and the like.

With the new rule, this is changing, but may still be true to a large extent.


Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients) (2005), Institute of Medicine, Food and Nutrition Board, National Academy of Sciences.

Stendar, Steen, Astrup, Arne, Dyerbeg, Jorg. Artificial trans fat in popular foods in 2012 and 2014: a market basket investigation in six European countries. BMJ Open. 2016 Mar 14; 6 (3): e010673.