You've probably heard that you should be eating more healthy unsaturated fats and less unhealthy saturated fats. But what does this mean for a fat to be saturated or unsaturated?

Being chains of carbon atoms with hydrogen atoms attached to them, fatty acids can contain carbon-carbon single bonds, or carbon-carbon double bonds. There are important chemical differences between these types of bonds. One of which is that the carbon-carbon double bond has spare electrons it can use to form bonds with other atoms, while retaining the same carbon-carbon bond that it did before, albeit, only a single bond (Figure 1).

Figure 1: A carbon-carbon double bond, as found in the ethene molecule can be saturated by reaction with hydrogen to produce a carbon-carbon single bond, where the extra bonds available for each carbon atom can be saturated by the hydrogen atoms from the hydrogen gas that reacts with the bond. This process of hydrogenation is often used in food manufacturing to solidify vegetable oils, to give them properties similar to solid fats typically derived from animal sources that are saturated fats.

Because double bonds are able to form new bonds with extra atoms and single bonds are not. When a fatty acid only contains single bonds and is unable to form any new bonds with additional atoms, it is said to be saturated. Saturated fatty acids only contain carbon-carbon single bonds throughout their chains. Unsaturated fatty acids by contrast contain double bonds, so they have capacity to make new bonds with additional atoms. This links to a concept in the food industry, hydrogenation. Hydrogen can react with the double bonds in fatty acids to form single bonds, and saturating the fatty acids with hydrogen atoms (Figure 1). This process is used in the food industry to produce solid fats from liquid oils, which helps to make baked goods, which is why such artificial fats are called hydrogenated fats. Although a widely used and useful process in the food industry there is concern that this process can lead to the formation of harmful trans fats.

Unsaturated fatty acids can be further divided into monounsaturated and polyunsaturated fatty acids, depending on the number of double bonds within the molecule, with monounsaturated fatty acids having only a single double bond, while polyunsaturated fatty acids have two or more double bonds (Figure 2). Within these broad classes of saturated and unsaturated fatty acids, there are many different fatty acids which occur in a variety of foods and have various roles in biochemical processes.

Figure 2: From left to right, examples of saturated (stearic), monounsaturated (oleic acid) and polyunsaturated (α-linolenic acid) fatty acids. These all have 18 carbon atoms, but due to the differing number, and position of the carbon-carbon double bonds which they contain cause differences in their chemical properties, biological functions, and occurrence in different food sources.

Generally speaking, due to the double bond, unsaturated fatty acids are more reactive and are less prone to being stored long term as excess bodyfat. They are a major constituent of cell membranes, and are the fatty acids found in healthy oils like olive, sunflower and fish oils. While saturated fatty acids are generally found in animal fat, both meat and dairy, and in other solid fats such as coconut or palm oils. These are fats which have higher melting temperatures and are more usually used for baking, but are generally regarded as less healthy. Furthermore, many of the unsaturated fatty acids, especially the Omega 3 and 6 fatty acids are what are called essential fatty acids and cannot be synthesized, and must be obtained from the diet.