What Is Hydrogenation and Why Is it Bad for Fat?

Differences Between Partial and Fully Hydrogenated Fats

Stick margarine may still contain trans fats formed by partial hydrogenation.
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You've probably been hearing the hype about hydrogenated fats. What does it mean if a fat is fully hydrogenated or partially hydrogenated? Is this the same thing as saturated and polyunsaturated fats? Where do trans fats fit in?

What do 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 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. Both partially and fully hydrogenated fats are considered saturated fats.

You may ask why would a food manufacturer want to alter fat in this way. This will become clearer as we define these terms below, but one of the primary reasons is to alter the consistency of the product. Another reason is to increase the shelf life.

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.

Heating and reusing partially hydrogenated fats may also be a problem, with studies suggesting that ​the amount of trans fats increases when partially hydrogenated fats are heated to 180 to 220 F. It did not matter what method of heating 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. 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.

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 we 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.

A 2017 study brought further concerns over interesterified fats. Though this study was done on mice and not humans, it found that the mother's intake of interesterified fats influenced lipid metabolism in the liver of her adult offspring. Not only did intake of interesterified fats in utero affect the mice metabolically, but adult mice offspring had blood sugar levels that ran around 20 percent higher than mice whose mothers were fed soybean oil instead.

Calories in Hydrogenated Fats

It's important to note that both partially and fully hydrogenated fats 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 very good for you anyway.

Saturated vs. Unsaturated Fats

If you are reading food labels it can be confusing. For example, partially hydrogenated fats and polyunsaturated oils can be easily mixed up. Polyunsaturated fats, in contrast to partially hydrogenated fats which are a form of "saturated fats," can actually be good for you (within limits.) Learn more about the differences between saturated and unsaturated fats.

Bottom Line on Hydrogenated Fats

So, are hydrogenated fats bad for you? Partially hydrogenated fats are concerning because of the creation of trans fats, and trans fats are unhealthy in a good diet. If you find either partially or fully hydrogenated fats in products in the grocery store, it's also likely that they will be found in heavily processed foods—foods that are usually unhealthy anyway. To make this more practical, check out some of the foods containing saturated fats which are best avoided.

Many nutrition specialists now recommend shopping in the periphery of the grocery store. 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 store. To make this more fun, try grocery shopping without walking through the aisles, with the exception of picking up some mono or polyunsaturated oils such as olive oil or canola oil.

Sources:

American Heart Association. Trans Fats. http://www.heart.org/HEARTORG/HealthyLiving/FatsAndOils/Fats101/Trans-Fats_UCM_301120_Article.jsp#.Wep0YVtSyUl

Bhardwaj, S., Passi, S., Misra, A. et al. Effect of Heating/Reheating of Fats/Oils, as Used by Asian Indians, on Trans Fatty Acid Formation. Food Chemistry. 2016. 212:663-70.

de Velasco, P., Chicaybam, G., Ramos-Filho, D. et al. Maternal Intake of Trans-Unsaturated or Interesterified Fatty Acids During Pregnancy and Lactation Modifies Mitochondrial Bioenergetics in the Liver of Adult Offspring in Mice British Journal of Nutrition. 2017. 118(1):41-52.

Harvard School of Public Health. Shining a Light on Trans Fats. https://www.hsph.harvard.edu/nutritionsource/transfats/

Hayes, K., and A. Pronczuk. Replacing Trans Fat: The Argument for Palm Oil with a Cautionary Note on Interesterification. Journal of the American College of Nutrition. 2010. 29(3 Suppl):253S-284S.

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