"This study just reinforces the idea that trans fat is bad -- worse than saturated fat -- and we need to make a concerted effort to reduce trans fats, including individuals, food manufacturers and policy-makers," said Dr. Frank Hu, senior author of the study and an associate professor of nutrition and epidemiology at the Harvard School of Public Health in Boston.

Results of Hu's study are expected to be published in the April 10 issue of the American Heart Association's journal, Circulation.

Trans fats, also called hydrogenated fats, are man-made compounds made from processed liquid oils. These harmful fats will raise bad cholesterol and could lower a person's good cholesterol levels. Because these fats are so thick and stiff, they can also clog up arteries and blood vessels, which can lead to heart attack or stroke.

Trans fats are commonly found in processed foods such as potato chips, cookies, doughnuts, cakes and many fast foods. Previous research had already implicated dietary trans fat as a major player in the development of heart disease. However, past research had been done using self-reported dietary information. The current study includes an objective measure of trans fat intake -- trans fat levels in red blood cells. Hu explained that because red blood cells live for six months or more, trans fat levels in those cells are a good indicator of average trans fat intake. The Harvard team examined blood samples collected from almost 33,000 women participating in the ongoing Nurse's Health Study. During the six-year study period, 166 women developed heart disease. The researchers then pulled information on 327 healthy women to serve as controls.

The women were grouped into four different quartiles based on the levels of trans fats in their blood.

The researchers found that women in the fourth quartile -- those with the highest trans fat levels -- had three times the risk of heart disease when compared to women with the lowest levels -- those in the lowest quartile. Women in the second and third quartile had a 60 percent greater risk of heart disease.

Hu and his colleagues also estimated the average daily trans fat intake from the trans fat blood levels. Women in the lowest quartile were estimated to have an average daily trans fat intake of 2.5 grams, while women in the highest quartile were estimated to take in 3.6 grams per day intake of trans fats. Hu stressed that these averages should be considered rough estimates.

The U.S. Food and Drug Administration (FDA) estimates that the average American diet contains about 5.8 grams of trans fat daily. The American Heart Association advises that trans fat should make up no more than one percent of your daily caloric intake.

Although no men were included in this study, Hu said he believes the findings are also applicable to men.

"Trans fats are a dangerous and unnecessary component of our diet. When you look at data like this, it's scary. Just a small change gives you a great increase in risk," said Dr. Nieca Goldberg, medical director of the Women's Health Program at New York University Medical Center and author of The Women's Healthy Heart Program: Lifesaving Strategies for Preventing and Healing Heart Disease in Women.

"People really need to be aware of trans fat and take it out of their diets," she said, adding that the FDA's recent labeling change -- introduced in January 2006 -- makes it easier for people to know what's in packaged foods. It's still difficult to know what's in restaurant or fast foods, however.

But Goldberg believes that a law recently passed in New York City to ban restaurants from serving foods containing trans fat after 2008 may "have a domino effect" across the country. However, until that law takes effect, she suggests avoiding fried foods and ordering fruit for dessert when eating out.

When buying packaged goods, look for foods that have no trans fat. However, under current rules, products with 0.5 grams or less trans fat can label their products as having zero grams of trans fat. That means if you have four foods with 0.5 grams of trans fat each, you've unwittingly eaten two grams of trans fat. Given that this study found that averaging just one extra gram daily can significantly increase your heart disease risk, Hu said the labeling is probably "something we should consider."

In the meantime, if a product is labeled zero grams of trans fat, but the ingredient list includes "partially hydrogenated vegetable oil," the food does contains some trans fat, Hu noted.

More information

To learn more about trans fats, visit the American Heart Association.