Trans Fats: the Bad Fat in Town
Trans fatty acids (trans fats) have gained quite a reputation. Do they deserve their bad rap? The answer is yes. 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 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 can contain trans fatty acids.
The Harm of Trans Fats
Trans fats not only raise LDL ("bad") cholesterol levels (as saturated fats also do), they also lower levels of HDL ("good") 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.
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.”
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:
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 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 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.
Lowering How Much Trans Fat You Eat
Use the following strategies to cut down on your trans fat intake:
American Dietetic Association
American Heart Association
Dietitians of Canada
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: http://www.nyc.gov/html/doh/html/pr2006/pr114-06.shtml. Accessed August 6, 2012.
Dietary recommendations for cardiovascular disease prevention. EBSCO DynaMed website. Available at: http://www.ebscohost.com/dynamed/. Updated July 30, 2012. Accessed August 6, 2012.
Lichtenstein AH, Ausman LM, Jalbert SM, et al. Effects of different forms of dietary hydrogenated fats on serum lipoprotein cholesterol levels. N Engl J Med. 1999;340:1933-1940.
Lichtenstein AH, Appel L, Brands M, et al. Diet and lifestyle recommendations revision 2006. A scientific statement from the American Heart Association Nutrition Committee. Circulation. 2006;114:82-96.
Nutrition. Kentucky Fried Chicken website. Available at: http://www.kfc.com/nutrition/. Accessed August 6, 2012.
Trans fats. American Heart Association website. Available at: http://www.heart.o.... Updated October 29, 2010. Accessed August 6, 2012.
Trans fat now listed with saturated fat and cholesterol. US Food and Drug Administration website. Available at: http://www.fda.gov.... Updated October 31, 2011. Accessed August 6, 2012.
Last reviewed August 2012 by Brian Randall, MD