You've probably heard of trans fats, and you probably haven't heard of cis fats. Trans fats are always talked about negatively, but what exactly are trans fats?
Unsaturated fatty acids contain carbon-carbon double bonds. Double bonds, unlike single bonds, are unable to freely rotate. Because of this, the two remaining bonds of each carbon atom in the double bond are fixed in their orientation. This means where there are different groups coming from the carbon atoms either side of the double bond, the molecule has cis/trans isomerism (Figure 1). Isomerism is where two molecules have the same composition of atoms, but they have a different arrangement, and this can contribute to different chemical properties, depending on the types of reactions taking place. This is especially in biochemistry, where precise isomers have precise biochemical properties.
Figure 1: But-2-ene, a simple ethane, a molecule which contains a carbon-carbon double bond, is able to form both cis (left) and trans (right) conformations, depending on the orientation of the carbon atoms either side of the double bond. The same phenomenon occurs in unsaturated fatty acids, which are large carboxylic acids that also contain carbon-carbon double bonds. These two forms are not interchangeable without chemical reactions that can convert them, and as they are not super imposable they exhibit different chemistry.
Healthy unsaturated fatty acids typically have the cis conformation across their double bonds (Figure 2). Examples of this include oleic acid, which is the fatty acid found in olive oil. By contrast, unsaturated fatty acids with the trans conformation are usually unhealthy. This relates to the difference in chemical properties between the two, which causes differences in the way these are metabolized by the body, and how they interact with other biochemical processes. While cis conformation unsaturated fatty acids appear to improve the lipid profile (the composition of lipids found in the blood), trans conformation fatty acids worsen it to a greater extent than even saturated fatty acids.
Figure 2: Oleic (left) and elaidic (right) acids, which are cis/trans isomers of one another, differing only in orientation across the single double bond. While oleic acid is one of the healthiest dietary fats, being a prominent constituent of olive oil, eliadic acid causes unhealthy changes in blood lipid chemistry. This shows the significance that orientation across the double bond has for a compound's behavior.
Trans fats are associated with artificial foods and are a controversial food additive. They are often produced when fats are hydrogenated, a process where hydrogen is passed through vegetable oils, which saturates double bonds, causing the formation of saturated fatty acids. This makes fats more solid, so they can be used in spreads and shortening used for baked goods. Unfortunately, sometimes the process does not complete fully, and rather than saturating the double bonds, some are instead converted to trans fatty acids, this was especially a problem in the early 20th century, but as trans fats became identified as a source of public health concern, processes used to hydrogenate fats were modified in order to ensure complete hydrogenation, and limit the formation of trans fats. In jurisdictions such as Canada, such artificial trans fats are not legal to use as a food additive.
Despite their association with mass produced unhealthy food, trans fats are found in a few natural sources. Fats from ruminant animals (cattle and sheep) contain trans fats, as these are produced by the gut bacteria of these animals. These trans fats are absorbed by the animals can for part of the animals’ lipid chemistry, including being deposited in tissue and secreted in their milk. Consequently, both dairy and meat contain small amounts of trans fats, notably rumenic acid, which is produced in the rumen of these animals (Figure 3). Trans fats can also be found in human breast milk, via a similar process, as what the mother eats can enter her milk, so how much trans fats the breast milk contains reflects the dietary consumption by the mother.
Figure 3: Rumenic acid is an unsaturated fatty acid that contains both a cis and a trans orientation carbon-carbon double bond. Rumenic acid is produced in the rumen of ruminant animals (including sheep and cattle), and is found in their meat and milk. It is a conjugated linoleic acid (CLA), a type of fatty acid that has been promoted for alleged health benefits, and is the main CLA found in dairy products. It is also an ω-7 trans fatty acid, as the last double bond is found 7 bonds away from the terminal double bond of the fatty acid chain.
Rumenic acid (Figure 3), which is also what is called a conjugated linoleic acid (CLA), also contains a cis carbon-carbon double bond. This means it is both a cis and trans fat, as the presence of trans orientation double bonds does not affect the prevalence of cis double bonds, nor vice versa. Unlike trans fats produced by the incomplete hydrogenation of unsaturated fatty acids, CLAs don't appear to have many of the deleterious health effects of other trans fatty acids. But the evidence of their health benefits is unclear. It is possible that the main danger posed by the trans fats is that they form straight fatty acid chains, unlike the kinked chains produced by cis fatty acids, thus the presence of a cis double bond, and thus the kinked chain means that this trans fatty acid doesn’t have the same deleterious properties of other trans fatty acids.