What is the Difference Between Cis and Trans Fats?

Lesson Transcript
Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Fats are a nutrient that can be beneficial or harmful depending on the fat type consumed. Explore the types of fats, how the molecular structure defines a fat as cis or trans, and the health effects of these fat types. Updated: 01/12/2022

Types of Fats

When you start talking about eating healthy, one topic will often come up: healthy vs unhealthy fats. Olive oil, vegetable oil, and avocado fats all tend to be seen as healthy fats. Butter, beef fat, and cheese fat all tend to be viewed as unhealthy fats. In this case the healthy fats are unsaturated fats, fats with at least one double bond, while the unhealthy fats are saturated fats, fats with no double bonds. Yet there are some fats which are even more unhealthy, and these are trans fats.

Trans fats are actually a type of unsaturated fat, so you would think that they'd at least be more healthy than saturated fats, but they are not. So, if trans fats are unsaturated, how are they different from the other unsaturated fats? This is due to the type of double bond that trans fats have.

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  • 0:04 Types of Fats
  • 0:59 Cis and Trans Fats
  • 2:57 Health Effects
  • 3:57 Lesson Summary
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Cis and Trans

When a double bond forms between two carbon atoms in the middle of a chain, it can either be cis or trans. Cis is the high energy configuration of the double bond; a cis bond forms with the two largest substituents on the same side of the double bond. Trans bonds are low energy configurations where the two largest substituents are on opposite sides of the double bond.

Typically, unsaturated fats are cis fats instead of trans fats. The double bond is in the cis configuration instead of the trans configuration. Very few trans unsaturated fats exist naturally; rather, they are created through a process called hydrogenation, which has been banned in many countries.

So, let's take a look at a fat. Really a fat (or a fatty acid) is just a long carbon chain with a carboxylic acid at the end. Typically, the carbon atoms orient themselves alternating up and down. This orientation makes the most stereochemical (the 3D conformation of the molecule) sense because the large carbon atoms are not bumping into each other and interfering with each other. Think about trying to fit cups into a small cupboard. If you simply place them all in the same direction, you can't fit as many in the same space as if you rotated the direction. This same thing is true with atoms; if the larger atoms alternate, then the molecule can fit together better.

When a double bond forms it can either form in the cis or the trans configuration. In nature, the cis double bond is usually formed.

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