Trans fats have broadly been recognized as an unhealthy form of fat that is present in the diets of many people, especially in the U.S. Though it is true that almost any nutrient in excess can be harmful, when it comes to fat, trans fats are associated with the worst health risks. Dietary fat can be grouped into four categories based on characteristics of their molecular structure that influence how they affect the body, in order of most to least healthy: polyunsaturated, monounsaturated, saturated, and trans (1). Extensive research has established clear recommendations to avoid the latter two types and/or replace with the first two types when possible (1-3). However, it is important to note that fats are a necessary nutrient for normal cellular functioning (1).
The advantages of trans fats are that they are inexpensive to produce and store well, making them attractive for food preparation at both the commercial and individual level (2). Trans fats (a.k.a. partially hydrogenated oils) can be produced artificially from liquid oil, which typically contains mostly unsaturated fats, by changing its molecular structure and turning it into a solid (2-4).
Specifically, unsaturated fats are hydrogenated – the fatty acid chains of unsaturated fats contain one or more cis double bonds, causing “kinks” that make it more difficult for the substance to pack in an orderly manner into a solid. Hydrogenation causes some of those double bonds to become single bonds and some to become trans double bonds, both of which straighten out the molecule’s structure and increase the substance’s melting point.
Research in the past few decades led to the discovery of the many health risks of trans fats. In particular, these fats increase LDL cholesterol and decrease HDL cholesterol, which are respectively considered the “bad” and “good” kind (1-3). They are known to increase the risk of heart disease and stroke, and are also associated with a higher risk of type 2 diabetes (2,3). Consuming 5 grams of trans fats each day increases your risk of ischemic heart disease by 25% (5). Monkeys that were fed a diet with trans fats over six years gained weight, had increased fat within the abdomen, and showed signs of poorer glucose regulation compared to the control group who received the equivalent diet with unsaturated fats (4).
Fortunately, restrictions on this type of fat are tightening worldwide. Denmark, for example, has strongly restricted the use of trans fats in all food products, and a comparison across twenty countries found that products from the same fast-food chains had much less trans fat in Denmark (1,5). Several jurisdictions in the U.S. have also implemented restrictions, while the FDA requires trans fats to be identified in ingredient and nutritional food labels nationwide.
There are also easy steps that U.S. residents can take to monitor their fat intake, such as looking for the amount of trans fat in a nutritional label or for “partially hydrogenated oils” in the ingredients label (2-4). Furthermore, most fried foods and many baked treats contain high levels of trans fat (2,3). Solid vegetable shortening and margarine should be avoided as much as possible (2-4); note that butter, while solid, is made from milk fat in a different process and contains high levels of saturated fat but relatively small amounts of trans fat. Unsaturated fats are relatively healthier, though they should still be consumed in moderation, and include canola, safflower, and olive oil (non-tropical plant oils) (1-3).
References
- AHA. “Dietary Fats.” American Heart Association. Reviewed November 1, 2021. https://www.heart.org/en/healthy-living/healthy-eating/eat-smart/fats/dietary-fats
- AHA. “Trans Fats.” American Heart Association. Reviewed March 23, 2017. https://www.heart.org/en/healthy-living/healthy-eating/eat-smart/fats/trans-fat
- Bridges M. “Facts about trans fats.” MedlinePlus. Updated May 26, 2020. https://medlineplus.gov/ency/patientinstructions/000786.htm
- Stender S, Dyerberg J, Astrup A. High Levels of Industrially Produced Trans Fat in Popular Fast Foods. NEJM. 2006;354(15):1650-1652. doi: 10.1056/nejmc052959
- Kavanagh K, Jones KL, Sawyer J, Kelley K, Carr JJ, Wagner JD, Rudel LL. Trans Fat Diet Induces Abdominal Obesity and Changes in Insulin Sensitivity in Monkeys. 2012;15(7):1675-1684. doi: 10.1038/oby.2007.200