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Trans Fats: The Science and the Risks

This man-made fat was developed to protect us against butter. Turns out, it acts like butter inside our bodies.

By Denise Mann
WebMD Feature

Reviewed By Cynthia Haines


What exactly are trans fats? How are they made? How bad are they, really? And just how solid is the science that the FDA consulted when they voted to list trans fats on nutrition labels? To get to the bottom of these and other questions about trans fats, WebMD spoke to leading nutritionists.

What Exactly Are Trans Fats?

Trans fatty acids or trans fats are formed when manufacturers turn liquid oils into solid fats. Think shortening and hard margarine. Manufacturers create trans fats via a process called hydrogenation. Hydro-what? In a nutshell, hydrogenation is a process by which vegetable oils are converted to solid fats simply by adding hydrogen atoms.

Why hydrogenate? Hydrogenation increases the shelf life and flavor stability of foods. Indeed, trans fats can be found in a laundry list of foods including vegetable shortening, margarine, crackers (even healthy sounding ones like Nabisco Wheat Thins), cereals, candies, baked goods, cookies, granola bars, chips, snack foods, salad dressings, fats, fried foods, and many other processed foods.

Trans fatty acids are found naturally in small quantities in some foods including beef, pork, lamb, butter, and milk, but most trans fatty acids in the diet come from hydrogenated foods. So there is good news: When the new nutrition labels go into effect Jan. 1, 2006, it will be easier to screen these fats out of your diet. Until then, look at the package's list of ingredients. Products that contain partially hydrogenated oils or vegetable shortening may contain trans fats.

Where Did Trans Fats Come From?

Trans fats were developed during the backlash against saturated fat -- the artery-clogging animal fats found in butter, cream, and meats. Then food manufacturers realized that trans fats lasted longer than butter without going rancid. The result: Today trans fats are found in 40% of the products on your supermarket shelves.

"We used to use animal fats, and people said, 'saturated fats are bad,' so we switched to trans fats," says Ruth Kava, PhD, RD, director of nutrition at the New York City-based American Council on Science and Health. "This kind of gives us an unfortunate focus on ingredients rather than the whole diet when the problem isn't this fat or that fat, it's too many calories."

"Anything was good if it decreased saturated fat consumption in the 1950s through the 1980s," agrees Alice H. Lichtenstein, Dsc, professor of nutrition at Tufts University in Boston. "But then studies began to question trans fats," too. Finally, in the 1990s, the evidence became clear: When vegetable oil is turned into a solid, like butter, it acts like butter inside the body.

Next, learn about the risks.

What Do Trans Fats Do Inside the Body?

Like saturated or animal fats, trans fats contribute to clogged arteries. Clogged arteries are a sign of heart disease; they increase your risk of both heart attack and stroke. Here's how it works: Trans fats raise low-density lipoprotein (LDL) or "bad" cholesterol levels. This contributes to the buildup of fatty plaque in arteries.

"The science that shows that trans fats increase LDL cholesterol levels is outstanding and very strong. All evidence is pointing in the same direction," Lichtenstein tells WebMD.

In the Nurse's Health Study, women who consumed the greatest amount of trans fats in their diet had a 50% higher risk of heart attack compared to women who consumed the least.

Some researchers suspect that trans fats also increase blood levels of two other artery-clogging compounds -- a fat-protein particle called lipoprotein(a) and blood fats called triglycerides.

Equally worrisome, population studies indicate that trans fats may raise the risk of diabetes. Researchers at the Harvard School of Public Health in Boston suggest that replacing trans fats in the diet with polyunsaturated fats (such as vegetable oils, salmon, etc.) can reduce diabetes risk by as much as 40%.

How much trans fat is safe? No one really knows. Kava says the prestigious Institute of Medicine reported that there isn't enough research yet to recommend a safe amount of trans fats. "We know that like saturated fats, trans fats can raise bad cholesterol, but there is conflicting data about what it does to good cholesterol," she says. "I wish the data were stronger."

The FDA, while requiring manufacturers to put the amount of trans fats on nutrition labels, will not require a percent daily value (DV) for trans fat because there is not enough information at this time to establish a such a value, she says. Food labels do offer such information about saturated fats.

How Do Trans Fats Compare to Saturated Fats?

"Trans fats raise (bad) LDL cholesterol levels slightly less than do saturated fats," says Lichtenstein. "But saturated fats also raise levels of high density lipoprotein (HDL) or "good" cholesterol, and trans fatty acids don't." Trans fats may actually lower HDL. Thus, some researchers say trans fats are worse. Lichtenstein, however, figures the two fats probably cause equal harm in our diets because we eat far more saturated fat than trans fats.

The FDA estimates that Americans aged 20 and older eat 5.8 grams of trans fats per day -- that's about 2.6% of our daily calories. By comparison, we eat four to five times more saturated fat per day. About 40% of our trans fat intake comes from cakes, cookies, crackers, pies, and bread, while 17% comes from margarine.

Who Should Be Concerned About Trans Fats?

Of course, everyone should try to limit their consumption of trans fats and saturated fats. However, "individuals who are told by their physicians that they have elevated LDL cholesterol should be most concerned," Lichtenstein says. "They should minimize their intake of both trans fats and saturated fats."

Kava adds: "The most important thing is looking at the number of calories and then serving size. Then check out saturated fat and trans fat on the label. It might help some people make smarter decisions."

Are All Fats Bad?

Not at all. Polyunsaturated and monounsaturated fats -- found mainly in canola, olive, and peanut oils -- can result in less LDL and more HDL production in the body, says Boston-based community nutritionist Dana Greene, MS.

"That's a good thing," Greene says. But we should still limit our daily fat intake to 30% or less of our daily calories, she stresses. Her advice? "Choose heart-healthy fats including nuts, avocado, peanut butter, and trans-fat-free margarines such as Promise and Smart Beat."

Originally published July 17, 2003.

Medically updated October 2004.


SOURCES: Ruth Kava, PhD, RD, director of nutrition, American Council on Science and Health, New York. Alice H. Lichtenstein, Dsc, professor of nutrition, Friedman School of Nutrition Science and Policy, Tufts University, Boston. Dana Greene, MS, nutritionist, Boston.

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Last Editorial Review: 3/23/2005 6:56:47 PM


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