Why Hydrogenated Fat Is Bad

Margarine on a dish

Verywell / Alexandra Shytsman

You've probably heard that hydrogenated fats are "bad." Is this really true? What does it mean if a fat is fully hydrogenated or partially hydrogenated? Are they both bad? Is this the same thing as saturated and ​polyunsaturated fats? Where do trans fats fit in?

Here's what you need to know about fat, both the good and the bad, in order to make wise dietary choices for yourself and your family.

Understanding Fat Hydrogenation

Hydrogenation is the process by which hydrogen is forced into heated vegetable oil using a catalyst such as nickel. Forcing hydrogen into the oil changes the chemical structure from a liquid into a more solid shape. Oil can be partially hydrogenated or fully hydrogenated.

Both partially and fully hydrogenated fats are considered saturated fats. Unsaturated fats can actually be good for you (within limits).

You may ask why would a food manufacturer want to alter fat in this way. One of the primary reasons is to alter the consistency of the product. Another reason is to increase shelf life.

What Is Partial Hydrogenation?

Partially hydrogenated oils contain trans fats that give them a soft, buttery consistency. Food manufacturers may use partially hydrogenated oil in processed foods, baked goods, and stick margarine because it lasts longer than regular oil and gives pastries a more appealing texture.

Partially hydrogenated vegetable oils were originally thought to be a healthier alternative to saturated fats, some of which are associated with cardiovascular disease risk. But the hydrogenation process creates trans fats, and it turns out that trans fats are even worse than saturated fats.

The trans fats found in partially hydrogenated fats raise LDL cholesterol levels (the bad kind) and lower HDL cholesterol levels (the good kind) at the same time. Eating trans fats increases the risk of developing heart disease and stroke and is linked to type 2 diabetes.

Heating and reheating partially hydrogenated fats may also be a problem, with a 2016 study suggesting that the amount of trans fats increases when partially hydrogenated fats are heated to 180 to 220 degrees F. It did not matter what method of cooking was used.

Trans fats are considered bad enough that they are required to be listed on Nutrition Facts labels, and their use has been banned in some places (artificial trans fats are banned throughout the the United States).

If you live in a region where foods aren't required to be labeled with trans fats, look for the words "hydrogenated" or "partially hydrogenated" on the label.

What Is Full Hydrogenation?

Fully hydrogenating oils makes them solid, similar to the saturated fats found in meat. Fully hydrogenated oils are probably better for you than partially hydrogenated oils because they don't contain trans fats. Still, it's hard to say they're good for your health—less dangerous is a better way to put it.

Fully hydrogenated oils are mostly stearic acid, which is a less harmful form of saturated fat. Stearic acid has a neutral affect on LDL cholesterol levels, and it's relatively stable, so it's good for kitchen use.

Fully hydrogenated oils are solid and waxy, so they're difficult to use. They can be blended with polyunsaturated oils like soy and sunflower oils through a process called interesterification to improve texture and soften them a bit. But research isn't clear on how these interesterified fats impact cholesterol levels and cardiovascular disease risk.

Both partially and fully hydrogenated fats are high in calories. All fat has nine calories per gram.

A Word From Verywell

Partially hydrogenated fats are concerning because the hydrogenation process creates trans fats, and trans fats are unhealthy in a balanced diet. Partially or fully hydrogenated fats are often found in heavily processed foods—foods that are usually unhealthy for other reasons.

Foods that do not have labels, such as fruits and vegetables, plus foods that are not highly processed, such as fresh meats and dairy products, are usually found in the outer areas of the grocery store. Try shopping without walking through the aisles, with the exception of picking up some monounsaturated or polyunsaturated oils such as olive oil or canola oil.

7 Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Iqbal MP. Trans fatty acids - A risk factor for cardiovascular disease. Pak J Med Sci. 2014;30(1):194-197. doi:10.12669/pjms.301.4525

  3. American Heart Association. Trans fats.

  4. Islam MA, Amin MN, Siddiqui SA, Hossain MP, Sultana F, Kabir MR. Trans fatty acids and lipid profile: A serious risk factor to cardiovascular disease, cancer and diabetes. Diabetes Metab Syndr. 2019;13(2):1643-1647. doi:10.1016/j.dsx.2019.03.033

  5. Bhardwaj S, Passi S, Misra A, et al. Effect of heating/reheating of fats/oils, as used by Asian Indians, on trans fatty acid formationFood Chem. 2016;212:663-670. doi:10.1016/j.foodchem.2016.06.021

  6. Astrup A, Magkos F, Bier DM, et al. Saturated fats and health: A reassessment and proposal for food-based recommendations: JACC state-of-the-art review. J Am Coll Cardiol. 2020;76(7):844-857. doi:10.1016/j.jacc.2020.05.077

  7. Hayes KC, Pronczuk A. Replacing trans fat: The argument for palm oil with a cautionary note on interesterification. J Am Coll Nutr. 2010;29(3 Suppl):253S-284S. doi:10.1080/07315724.2010.10719842

Additional Reading

By Shereen Lehman, MS
Shereen Lehman, MS, is a former writer for Verywell Fit and Reuters Health. She's a healthcare journalist who writes about healthy eating and offers evidence-based advice for regular people.