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Differences Between Cis- and Trans-Fatty Acids

By Jan Annigan

Fatty acids are a component of both the fat you eat and the fat in your body. Your body fat stores energy for long-term survival, and, additionally, fat is a structural component of the cell membranes in your body. Fatty acids in foods exist in a natural cis configuration but can be modified to form an artificial configuration known as trans. Appreciating the difference between the two, and their implications for your health, requires an understanding of fatty acid structure.


The basic structure of a fatty acid involves a chain of carbon atoms bonded together with a variable number of hydrogen atoms attached to them. If the carbon chain holds as many hydrogens as chemically possible, that is, if the carbons are saturated with hydrogens, it is considered a saturated fatty acid. Unsaturated fatty acids, on the other hand, contain one or more carbons with fewer hydrogen atoms than the maximum possible. The amount of saturation affects the physical properties of the fatty acid. Saturated fatty acids are rigid and solid at room temperature, while unsaturated fatty acids exhibit a more fluid structure and are liquid at room temperature.

Cis Vs. Trans

To emulate the desirable cooking properties of saturated fats using healthier unsaturated fats, the food industry began the process of partially hydrogenating unsaturated fats. That is, they added hydrogen atoms to vegetable oils to create a product they thought had the best properties of both types of fatty acids. However, hydrogenating an oil creates a different fatty acid structure than a naturally unsaturated fatty acid. Specifically, the cis configuration in a naturally unsaturated fatty acid has hydrogen atoms on the same side of the carbons, resulting in a kink in the fatty acid chain. Hydrogenation of an oil, on the other hand, places hydrogen atoms on opposite sides of the carbon in a so-called trans configuration, causing the hydrocarbon chain to be relatively straight, much like a saturated fatty acid. The different shapes of cis- vs. trans-fatty acids affects not only their physical properties but also your health.

Health Implications

Consuming trans-fatty acids is associated with a higher incidence of cardiovascular disease, although the mechanism of action is not clear, states the Mayo Clinic. One factor may be that your body recognizes trans-fatty acids as saturated fats, and another may be because, although your body has evolved to metabolize both saturated fats and cis-fatty acids, you lack the ability to metabolize trans fats. Whatever the reason, avoiding trans fats in your diet may lead to better health.


Although food labels list the amount of trans fat contained in the product, food manufacturers retain the right to designate any amount less than 0.5 g per serving as 0 g of trans fat. Therefore, the food product might actually contain trans fats that can accumulate to significant levels if you consume more than one serving. To avoid trans fats in your diet, bypass foods that list partially hydrogenated vegetable oils, or better yet, stick with naturally unsaturated fats, which feature the healthier cis configuration.

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