Trans Fats: the Bad Fat in Town
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Trans Fats: the Bad Fat in Town

a tub of margarine Trans fatty acids (trans fats) have gained quite a reputation. In fact, they’ve taken over from saturated fats as the "baddest fat in town." But do they deserve their bad rap? The answer is yes. That’s because their effect on cholesterol levels is worse than that of saturated fats. Identifying foods high in trans fats and reducing or even eliminating those foods from your diet is an important way to keep your heart healthy. In fact, it takes such a small amount of these so-called trans fats to negatively impact cardiovascular health that the American Heart Association recommends trans fats make up less than 1% of total calorie intake.

What Are Trans Fats?

The term “trans fat” refers to vegetable oils (made up of mostly unsaturated fats) that have had hydrogen atoms added to them. This process is known as hydrogenation or partial hydrogenation .

Partial hydrogenation of vegetable oils is done to make fats more solid at room temperature, to increase their shelf life by keeping their flavor stable over longer periods of time, and to guard against spoilage. Margarines, shortening, the oils used to cook fast food French fries, and commercial baked goods are all examples of foods that contain trans fatty acids.

The Harm of Trans Fats

Trans fats not only raise LDL ("bad") cholesterol levels (as saturated fats also do), but they also lower levels of HDL cholesterol. Trans fat also increases blood triglyceride levels. Several studies have examined the effects of trans fats on cholesterol levels and heart disease risk.

Trans Fats and Cholesterol Levels

A study in the New England Journal of Medicine compared the effects on cholesterol levels of two different diets. One diet derived 20% of calories from fat sources containing varying amounts of trans fats (either soybean oil, semiliquid margarine, soft margarine, shortening, or stick margarine), and the other diet was enriched with butter. The researchers found that the use of soybean oil or semiliquid margarine (which contain the lowest amounts of trans fats) resulted in the most favorable effects on total and LDL cholesterol levels and ratios of total cholesterol to HDL cholesterol. The use of stick margarine (which contains the highest levels of trans fats) had the least favorable effects.

The researchers also found that trans fats lowered HDL cholesterol levels to the same degree that saturated fats raised HDL cholesterol levels. So, while both saturated fats and trans fats raise LDL levels to a similar degree, trans fats do the added damage of lowering levels of protective HDL cholesterol. Stick margarine lowered HDL cholesterol levels to a greater degree than sources containing less trans fats.

Trans Fats and Heart Disease Risk

A study in the American Heart Association’s journal Circulation found intake of trans fatty acids to be directly related to the risk of heart attack . In the study, people who ate the greatest amount of trans fatty acids were 2.44 times as likely to have a heart attack as people who ate the least trans fats.

Another study in the New England Journal of Medicine looked at the relationship between specific types of fats and the risk of coronary heart disease (CHD) in 80,082 women. Researchers estimated that replacing just 2% of total calories from trans fats with calories from unhydrogenated, unsaturated fats (such as olive or canola oil) would reduce the risk of CHD by 53%.

Other Effects of Trans Fats

Recent studies also suggest that trans fats are associated with inflammation, which is a risk factor for CHD, heart failure, sudden death from heart failure, and diabetes. A few studies have examined the relationship between trans fat intake and the risk of developing type 2 diabetes, but the results have been inconsistent. More research is needed.

Another study showed that eating trans fat causes weight gain in the abdomen, more so than other types of fat. All fats have the same calories, but the chemical makeup of trans fat is such that it deposits fat in the abdomen. Increased girth in the abdomen is associated with higher risk of most diseases.

For now, it is clear that all it takes is a small amount of trans fat, just 1%-3% of total calorie intake, to negatively impact health—for someone consuming 2,000 calories a day this translates into two to seven grams of trans fat.

Food Labels and Trans Fats

Because of the strong evidence showing that trans fats increase the risk of heart disease, the US Food and Drug Administration (FDA) requires food manufacturers to list trans fat amounts on the nutrition facts panel of the food label .

Under this rule, trans fat is listed as a separate line item under saturated fat. However, if a food contains less than 0.5 grams of trans fat per serving, food manufacturers may list it as containing 0 grams of trans fat. So, a food may contain trans fats in small amounts, but still be considered “trans-fat free.”

Therefore, the only way to be sure that a food is completely free of trans fat is to carefully inspect the ingredient list for the phrases “partially hydrogenated” or “shortening." These terms indicate that the food contains trans fats. This also makes it important to pay attention to serving sizes, since eating just two servings of a food could boost the trans fat content to a higher level.

Which Foods Contain Trans Fats?

In addition to reading the Nutrition Facts panel, here are some ways for identifying foods that contain trans fats:

  • Partially hydrogenated oils is listed as an ingredient
  • Certain groups of foods tend to be high in trans fats, including:
    • Deep fried foods, such as doughnuts and French fries
    • Foods that sit on the shelf, but still stay fresh (eg, cookies, chips, crackers, baked goods)
    • Frozen foods, especially frozen meals and pizzas
    • Foods that are solid at room temperature when they should be liquid (eg, margarine that is solid, but made from oil)
  • Meat and dairy products (small amounts of trans fat are found naturally in these foods)

Since the FDA's rule went into effect, many food companies have eliminated trans fats from their products. But because food labels are only required on packaged foods, foods served at restaurants and bakeries are still likely to contain partially hydrogenated vegetable oils or shortening. Some cities in the US, though, have banned restaurants from using trans fats. For example, restaurants in New York City are now trans-fat-free. Some large fast food companies, like McDonald's and Kentucky Fried Chicken, have also made the switch.


Margarine (particularly stick margarine) is another food high in trans fats. When it comes to margarine, the more solid it is, the more trans fat it contains. The following table compares the amount of total fat, saturated fat, and trans fat in butter and margarine.

Fat Per Serving of Butter or Margarine (grams/serving)
ProductTotal FatSaturated FatTrans Fat
Margarine, stick 1123
Margarine, tub710
Adapted from: US Food and Drug Administration

Lowering How Much Trans Fat You Eat

Use the following strategies to cut down on your trans fat intake:

  • Eat less of the following foods:
    • Margarine (particularly stick margarine)—Use olive oil, canola oil, liquid, or trans-fat-free tub margarines instead.
    • Cookies, cakes, and pastries
    • Crackers, cereals, and snack foods containing partially hydrogenated vegetable oil
    • French fries and other fried foods
    • Doughnuts
  • If you enjoy margarine on your morning toast, use tub margarine that contains no trans fat and is low in saturated fat and use a smaller amount.
  • Check the list of ingredients. If partially hydrogenated oils appear on the list, the food contains trans fats, even though it may be a very small amount.
  • Foods high in trans fats tend to be snack foods, so fill up on whole grains , vegetables , lean meats , and fish. Choose fruits or low-fat dairy products for snacks.
  • Limit saturated fats. Saturated fats and trans fats often go hand in hand, so limiting your intake of saturated fats should automatically lower your trans fat intake.


American Dietetic Association

American Heart Association


Canada's Food Guide

Dietitians of Canada


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Board of Health votes to phase out artificial trans fat from New York City's restaurants. New York City Department of Health and Mental Hygiene website. Published December 2006. Available at: Accessed July 17, 2008.

Center for Food Safety and Applied Nutrition. Questions and answers about trans fat nutrition labeling. US Food and Drug Administration website. Available at: . Accessed September 10, 2006.

FDA acts to provide better information to consumers on trans fatty acids. US Food and Drug Administration website. Available at: . Accessed September 10, 2006.

Frequently asked questions about McDonald's USA food & nutrition. McDonald's website. Available at: Accessed July 17, 2008.

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Nutrition. Kentucky Fried Chicken website. Available at: Accessed July 17, 2008.

Trans Fat Now Listed With Saturated Fat and Cholesterol on the Nutrition Facts Label. US Food and Drug Administration website. Available at: Updated January 2006. Accessed July 17, 2008.

Last reviewed June 2008 by Dianne Scheinberg, MS, RD, LDN

Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.

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