Boston - The conventional wisdom that has held unsaturated fats are harmless to the heart is wrong, according to a study in Thursday's New England Journal of Medicine.
The results of a 14-year study of more than 80,000 female nurses found that one type of unstaturated fat - trans-unsaturated fat - actually increases the risk of heart disease.
U.S. consumers have been getting complicated signals in the last month about dietary fat.
One study, reported at last week's meeting of the American Heart Association in Orlando, Florida, concluded that extremely low-fat diets may hinder concentration. Another study, published in last week's Journal of the American Medical Association, found that aggressively eliminating fat may cut too much of the "good" cholesterol used by the body to protect the heart.
The New England Journal study supports the well-established notion that the saturated fat found in red meat and dairy products is harmful.
Using data from the Nurses' Health Study of over 80,000 women, Dr. Frank B. Hu of the Harvard School of Public Health and his colleagues found that replacing 5 percent of a woman's energy intake from saturated fat with energy from two types of unsaturated fats -- mono-unsaturated fat and polyunsaturated fat -- reduced the heart disease risk by 42 percent.
But the work failed to support the conventional wisdom that unsaturated fats are universally good. Trans-unsaturated fat turned out to be harmful.
The researchers discovered that cutting out 2 percent of a person's intake of trans-unsaturated fat and replacing it with mono-unsaturated fat or polyunsaturated fat reduced the risk of heart disease by a hefty 53 percent.
Trans-unsaturated fats are hardening agents found in margarines, shortening, packaged cookies, crackers and many fast foods. They allow otherwise healthy fat to remain firm at room temperature through a process known as hydrogenation.
Unfortunately, U.S. consumers do not have a quick way to check the amount of trans-unsaturated fat in the food they're eating.
Although U.S. food labels list the amount of saturated fat, which is already known to be harmful, they do not make distinctions between trans-unsaturated fat and other fats.
The only suggestion that a product contains trans unsaturated fats appears on the list of ingredients. Trans-fats are listed as "partially hydrogenated" oil.
"The matter of food labels is important, since many of the 'hidden fats' in processed foods such as pastries and savory snacks are trans fatty acids," said Dr. Tim Byers of the University of Colorado School of Medicine, in an editorial in the Journal.
The U.S. Food and Drug Administration is considering a citizens petition to list the amount of trans-unsaturated fats on food labels, Byers said.
Many manufacturers of tub margarines have already cut the amount of trans- fats in their products, he said, so it may not be difficult to get the producers of other foods to make similar adjustments.
About 5 to 10 percent of fat in the U.S. diet consists of trans-unsaturated fat.
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