All About Trans Fats

What are trans fats? 

To start, let’s talk about hydrogenation. In this process, hydrogen atoms are added to a liquid oil, breaking the double bonds present in the oil by saturating them with hydrogen, creating a solid saturated fat.

Trans fats are a type of fat created through partial hydrogenation. In this process, only some of the double bonds are broken, and the remaining double bonds shift into what is known as a trans configuration. The end result is a partially-hydrogenated oil (PHO), which has a higher melting point, longer shelf life, and imparts a more satisfying flavor and texture [1]. Because of these properties, PHOs were used in fried and processed foods beginning in the 1940s!

Trans fats are also sometimes created in the production of nonhydrogenated oils, due to high temperatures, and fully hydrogenated oils, due to incomplete hydrogenation; however, the amount generated is much lower [2,3].

Myth: The most common source of trans fats today is processed foods.

This is false! While trans fats were abundant in processed foods in the past, this is not the case now. As we will discuss below, the discovery of the harmful effects trans fats have on our health led to a public health initiative to eliminate unapproved industrial trans fats in the U.S. and limit overall intake of trans fats.

Timeline

November 1999: FDA proposes that trans fat content be included on nutrition labels [4]. This was based on research study findings showing that trans fats were associated with increased LDL cholesterol and risk for coronary heart disease.

July 2003: FDA issues final rule requiring declaration of trans fat content on nutrition labels for conventional foods and dietary supplements [4]. This rule was based on recommendations from the Dietary Guidelines for Americans, American Heart Association, and Institute of Medicine to limit trans fat consumption, and went into effect in January 2006.

November 2013: FDA tentatively determines that PHOs, the primary dietary source of industrially-produced trans fats, are not Generally Recognized as Safe (GRAS) for any use in human food [2]. This was based on citizen petitions, opinions of expert panels, recommendations from the Institute of Medicine, and CDC predictions that elimination of PHOs could prevent thousands of coronary events and deaths annually. Additionally, trans fat and PHO consumption was found to be associated with insulin resistance, increased diabetes risk, potential impaired growth of fetuses and infants, and lowering of HDL cholesterol.

June 2015: FDA finally determines that PHOs are not GRAS for any use in human food. Food manufacturers were given until 2018 (later extended to 2021) to find suitable replacement ingredients for PHOs in their products and modify labeling of affected products [5]. This declaration makes it so that manufacturers are not allowed to sell PHOs, either directly or as ingredients in other food items, without FDA approval for use as a food additive.

January 2021: All PHOs are removed from the U.S. food supply.

Are there other sources of trans fats? 

Trans fats are naturally present in food items from ruminant animals including milk, butter, cheese, and various meat products. Through biohydrogenation, unsaturated fats are converted into saturated fats by bacteria in these animals’ rumen (first stomach) [6]. Some trans fats are created in the process, and absorbed into the animal’s milk and tissues.

Trans fats can also be produced through different cooking methods, particularly deep-frying [3].

Should we be concerned about these trans fats? 

A 2022 review found that ruminant trans fats may contribute to greater increases in LDL cholesterol and total cholesterol levels, but less of a decrease in HDL cholesterol levels, when compared to industrial trans fats [6]. It is important to note, however, that many of the studies included in this review had short intervention periods, small sample sizes, and utilized ruminant trans fat amounts that were higher than the daily average intake in the adult population. Looking at animal and in vitro studies, there are some instances in which ruminant trans fats have a protective effect and other instances in which they act similarly to industrial trans fats, inducing inflammation and cell death [7]. Simply put, the effects of ruminant trans fats are still not well understood. That being said, the amount of trans fats present in ruminant food products is much lower than the amount present in PHOs (8% vs. 60%), and the average daily intake of ruminant trans fats within the adult population is also low (0.5% of daily energy or 1.2 g/day), so there is not too much cause for concern [6].

In regards to deep-frying, most people do not reach the specific temperatures and times that are required to produce significant amounts of trans fats when frying at home. A 2025 review found that trans fats begin to form at 150C (302F) but are not significant until 200-240C (392-464F), and frying between 30-60 minutes is optimal [3].

Conclusion

While trans fats are still present in our diet today, the FDA has enforced strict guidelines to eliminate unapproved industrial trans fats from food items, and to limit the amount of trans fats present in our diet.

Currently, the World Health Organization (WHO) recommends limiting trans fat intake to less than 1% of daily energy or 2.2 g/day, avoiding industrial trans fats as much as possible [8]. When purchasing products, look for PHOs in the ingredients list (if buying foods not produced in the US) rather than looking for trans fats on the nutrition label, as all products with less than 0.5 grams of trans fats per serving can still be listed as having 0 grams of trans fat [1].

References

1. Food and Drug Administration [FDA]. (2003). Small Entity Compliance Guide: Trans Fatty Acids in Nutrition Labeling, Nutrient Content Claims, and Health Claims. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/small-entity-compliance-guide-trans-fatty-acids-nutrition-labeling-nutrient-content-claims-and
2. Tentative Determination Regarding Partially Hydrogenated Oils; Request for Comments and for Scientific Data and Information, 78 F.R. 67169 (2013). https://www.federalregister.gov/documents/2013/11/08/2013-26854/tentative-determination-regarding-partially-hydrogenated-oils-request-for-comments-and-for
3. Mavlanov, U., Czaja, T. P., Nuriddinov, S., Dalimova, D., Dragsted, L. O., Engelsen, S. B., & Khakimov, B. (2025). The effects of industrial processing and home cooking practices on trans-fatty acid profiles of vegetable oils. Food chemistry, 469, 142571. https://doi.org/10.1016/j.foodchem.2024.142571
4. Food Labeling: Trans Fatty Acids in Nutrition Labeling, Nutrient Content Claims, and Health Claims, 68 F.R. 41434 (2003). https://www.federalregister.gov/ documents/2003/07/11/03-17525/food-labeling-trans-fatty-acids-in-nutrition-labeling-nutrient-content-claims-and-health-claims
5. Final Determination Regarding Partially Hydrogenated Oils, 83 F.R. 23358 (2018). https://www.federalregister.gov/documents/2018/05/21/2018-10714/final-determination-regarding-partially-hydrogenated-oils
6. Verneque, B. J. F., Machado, A. M., de Abreu Silva, L., Lopes, A. C. S., & Duarte, C. K. (2022). Ruminant and industrial trans-fatty acids consumption and cardiometabolic risk markers: A systematic review. Critical reviews in food science and nutrition, 62(8), 2050–2060. https://doi.org/10.1080/10408398.2020.1836471
7. Oteng, A. B., & Kersten, S. (2020). Mechanisms of Action of trans Fatty Acids. Advances in nutrition (Bethesda, Md.), 11(3), 697–708. https://doi.org/10.1093/advances/nmz125
8. World Health Organization [WHO]. (2024). Trans fat. https://www.who.int/news-room/fact-sheets/detail/trans-fat