Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.
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.
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.
Since a cis bond changes the shape of the molecule, it acts very differently than a saturated fat. A trans bond does not change the shape of the molecule, so the physical properties of a trans fat are very similar to a saturated fat. For example, saturated fats and trans fats are both typically solid at room temperature, while unsaturated cis fats are typically liquid, since the melting point of cis fats is lower than the melting point of trans fats.
When trans fats were first created, scientists were excited. Here, they had found an unsaturated (healthy) fat that could act like a saturated fat in baking. But it was soon realized that the trans fats acted like saturated fats when it came to health, so the theory of this being a healthy alternative went out the window. However, trans fats continued to be used because they are very stable, with a longer shelf life without going rancid, and inexpensive, because they are often made from cheap vegetable oils.
As time continued, it became more and more apparent that trans fats have additional health risks, above and beyond that of saturated fats. Both trans fats and saturated fats increase your LDL (bad cholesterol), but trans fats have a double whammy because they also decrease your HDL (good cholesterol). Since low LDL levels and high HDL levels are necessary for good heart health, decreasing HDL while increasing LDL is particularly bad for your heart.
Unsaturated fats can be cis fats or trans fats. Cis fats are the typical form of unsaturated fat found in nature, while trans fats are made through hydrogenation. Trans fats have a similar shape to saturated fats, so many of the physical properties are the same. Cis fats have a different shape, so the physical properties are different. Trans fats are even more unhealthy than saturated fats in that they raise your LDL levels and lower your HDL levels. Due to these negative health effects, the process to make trans fats has been banned in many countries.
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