The Lipid Lowdown: Saturated, Unsaturated, and Trans Fats

// Written by Jung Wu //

Fat may be one of the most controversial elements of food and eating. There are many questions surrounding this simple molecule. Is it good for me?  Is it bad for me? Does eating fat make me fat? What’s the difference between all the different types of fat? Even among professionals and studies there is disagreement about fats—some studies point towards the health benefits of certain fats while others suggest the opposite. Nutritionists and doctors disagree on what type, how much, and who should consume fats. While I can’t provide answers to everything, hopefully this will give you a better basic understanding of the different types of fats, starting from the basics of what they are.

What are fats?

First off, fats are lipids, one of the four macromolecules essential to life (the other three being proteins, carbohydrates, and nucleic acids). They’re made up of long hydrocarbon chains, which are just hydrogen and carbon atoms bonded together. Fats have nonpolar covalent bonds that are hydrophobic and dislike water. Their hydrophobicity make them insoluble in water, which has polar bonds. This is the reason why oil and water don’t mix. All fats are also made up of a combination of glycerol and fatty acids, which are also types of lipids. Fatty acids are the hydrocarbon chains themselves; they have a carboxylic acid “head” (comprised of a carbon atom double-bonded to an oxygen atom on one side and single-bonded to oxygen and hydrogen on another) followed by a chain of hydrogens and carbons.

fatty acid

A fatty acid chain. The black spheres represent carbon and the white spheres represent hydrogen. The carbon with the red oxygens is the carboxylic acid. Image by Wikimedia Commons

Types of Fats

Fats can be classified into two main types: saturated and unsaturated. Monounsaturated fats have one double bond, while polyunsaturated fats have more than one double bond. Trans fats are actually a type of unsaturated fat with a particular molecular structure. Saturated and unsaturated fats both occur commonly in nature while natural trans fats are very rare, with the majority of trans fats being artificially created. Most oils and fats are a mixture of saturated and unsaturated fats. Usually, the more solid a fat is at room temperature, the more saturated fat it contains, while liquid oils are usually composed of more unsaturated fats. This is due to the differences in molecular structure of saturated and unsaturated fats, which we’ll get to in a moment.

First we’ll review some basic chemistry here to explain what double bonds mean in a fat: carbon has four valence and thus can form four single bonds. Alternatively, it can use two valence electrons to form a double bond in any combination of single and double bonds (that add up to four valence electrons, of course). When all carbons in a fatty acid chain are in single bonds with hydrogen, the fat is known as saturated. This is because the carbons have the maximum number of single bonds possible, hence the term “saturated”. Unsaturated fats have at least one double bond in the fatty acid chain. They are unsaturated because the double bond indicates that a carbon could still potentially bond to another hydrogen by breaking the double bond and forming single bonds with hydrogen.

Saturated Fats

Saturated fats are usually solid at room temperature due to their molecular structure, which is made up of a nice, even chains of single-bonded carbons. The straight lines are able to stack together easily, allowing the saturated fat molecules to arrange in a denser, solid structure. Lard, butter, and coconut oil are all examples of saturated fats. Since their structure is more stable, saturated fats are able to be stored for longer periods of time before oxidizing and going rancid. Saturated fats are also known to have adverse health effects when consumed excessively; they are usually known as the “bad fats” that raise bad cholesterol levels and result in heart disease. However, recent studies have suggested that there might not be a conclusive link between saturated fats and adverse health effects.

Unsaturated Fats

Unsaturated fats are often liquid at room temperature. Unlike saturated fats, which have nice, straight chains, unsaturated fat molecules have “kinks” in their fatty acid chains due to the double bond (or multiple double bonds). These “kinks” don’t allow unsaturated fat molecules to stack together in a nice, stable structure like saturated fat molecules. As a result, unsaturated fat molecules are not as tightly packed together and slide around each other as a liquid. The exceptions to this rule are trans fats.

Trans Fats

Trans fats are unsaturated fats with at least one double bond, but unlike “normal” unsaturated fats, trans fats still have a nice, linear structure. This is because in a trans fat molecule, the two carbons in a double bond are twisted so that the hydrogens line up linearly. This twisting around a double bond forms a configuration known as trans and also occurs in other organic molecules. “Normal” unsaturated fats have a cis structure, with the hydrogens on the same side of the fatty acid chain, resulting in the kinks. Since trans fats are more linear, they are also able to stack together and remain solid at room temperature.

A cis-unsaturated fat versus a trans-unsaturated fat. The cis-unsaturated fat has a kink while the trans-unsaturated fat is linear. Image by Wikimedia Commons

A cis-unsaturated fat versus a trans-unsaturated fat. The cis-unsaturated fat has a kink while the trans-unsaturated fat is linear. Image by Serious Eats


As mentioned previously, trans fats are usually artificially created (there are very few natural exceptions), and this is done through a process called hydrogenation. Unsaturated fats are hydrogenated to increase their stability so they have a longer shelf-life. Essentially, hydrogenation adds hydrogens to a fatty acid. Fully hydrogenated fats have enough hydrogen added to them to become saturated fats—therefore fully hydrogenated fats cannot be trans fats (which are unsaturated). Trans fats occur as a by-product of partial hydrogenation, which is when an unsaturated fat has double bonds removed but has not been fully turned into a fully saturated fat. Any ingredient list that includes “partially hydrogenated oil” contains trans fat.


So there you have it: a basic run-down of the different types of fats. While there is much more information surrounding fats, especially in relation to health, hopefully this guide explained some of the very basics. To summarize:

1. Fats are lipids, one of the four macromolecules necessary for life.

2. Fats can be either unsaturated or saturated (with further divisions of categories in each).

3. Saturated fats are linear and can form a more compact, stable structure and is thus solid at room temperature.

4. Unsaturated fats have “kinks” in their structure and their molecules cannot compact together as well. They are often liquid at room temperature.

5. Trans fats are artificially created through partial hydrogenation, which results in the twisting of an unsaturated fat molecule. This twisting gives trans fats a linear shape similar to saturated fats. Thus, they are able to stack together stably and remain solid at room temperature.


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