What Is Hydrogenation and Why Is it Bad for Fat?

Stick margarine may still contain trans fats formed by partial hydrogenation.
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Hydrogenation is the process by which hydrogen is forced into heated vegetable oil by 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.

Partial Hydrogenation

Partially hydrogenated oils contain trans fats that give them a softer 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 their texture.


Partially hydrogenated oils have fallen out of favor due to the trans fats that are created by the hydrogenation process. The sad part is that 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 it turns out that trans fats are even worse than saturated fats.

The trans fats found in partially hydrogenated fats raise your LDL cholesterol levels (the bad kind) and lower your HDL cholesterol levels (the good kind) at the same time. So eating trans fats raises your risk of developing heart disease and stroke. It’s also linked to developing type 2 diabetes.

They're bad enough that trans-fats are required to be listed on Nutrition Facts labels, and their use has been banned in some places.

Full Hydrogenation

Fully hydrogenating the oils makes them solid, similar to the saturated fats found in meat.

  Fully hydrogenated oils are probably better for you than partially hydrogenated oils, but I wouldn't say they're good for your health -- maybe less dangerous is a better way to put it -- because they don't contain the trans fats. 

Fully hydrogenated oils are much more like stearic acid, which is a less harmful form of saturated fat.

Stearic acid doesn't raise LDL cholesterol levels, and it's relatively stable, so it's good for kitchen use.

The problem is that 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 the texture and soften it up a bit. The problem is that research isn't clear on how these interesterified fats will impact cholesterol levels and cardiovascular disease risk.

I think it's important to note that both forms are high in calories (all fat has nine calories per gram), and if you find either of them in a grocery store, it's probably going to be in heavily processed foods that aren't good for you anyway.


American Heart Association. "Frequently Asked Questions About "Bad" Fats." http://www.heart.org/HEARTORG/General/Frequently-Asked-Questions-About-Bad-Fats_UCM_306349_Article.jsp.

American Heart Association. "Trans Fats." http://www.heart.org/HEARTORG/GettingHealthy/FatsAndOils/Fats101/Trans-Fats_UCM_301120_Article.jsp.

Harvard School of Public Health. "Shining the Spotlight On Trans-Fats." http://www.hsph.harvard.edu/nutritionsource/nutrition-news/transfats/.

Hayes KC1, Pronczuk A. "Replacing trans fat: the argument for palm oil with a cautionary note on interesterification." J Am Coll Nutr. 2010 Jun;29(3 Suppl):253S-284S. http://www.ncbi.nlm.nih.gov/pubmed/20823487.

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