If further research bears this out, people might someday be tested for this fat, just as they are for cholesterol now, to see if they're in danger of having a heart attack. The study found that levels of the fat strongly correlated with the risk of heart disease, especially in people under age 60.

"It is an important study," said Judith Berliner, a professor of medicine at the University of California, Los Angeles, who had no role in the research but wrote an editorial accompanying it in Thursday's New England Journal of Medicine.

Doctors say the findings give people another reason to limit fat in their diets.

No one really knows what causes the formation of blockages, which can squeeze blood vessels shut and deprive the heart of nourishment.

"Bad Cholesterol'

"Most of the studies in the past have revolved around cholesterol," but other factors also must be involved because cholesterol levels are normal in many heart attack victims, Berliner said.

Scientists have long suspected that one such factor might be oxidized phospholipids, a type of fat that's a major component of LDL or "bad cholesterol."

Research in animals has found that this fat, floating in the bloodstream, contributes in many ways to blockage formation. The new research, led by Dr. Sotirios Tsimikas at the University of California, San Diego, is the first to show the same is true in people.

Tsimikas studied 504 people being tested for clogged arteries.

Oxidized Phospholipids

Among those 60 or younger, people with the highest levels of oxidized phospholipids were three times more likely to have blockages than those with the lowest levels.

Those who had high phospholipids and high cholesterol were at even greater risk. Getting a measurement of the level of this fat must be done separately from tests for total cholesterol and LDL.

More research of phospholipid levels in all types of people -- not just the largely white group already suspected of heart problems in this study -- is needed, said Dr. Sidney Smith, director of the center for cardiovascular diseases at the University of North Carolina at Chapel Hill and past president of the American Heart Association.

The research also raises possibilities for new approaches to treating and preventing heart disease, Berliner said.

The study was funded by the federal government and the La Jolla Specialized Center of Research in Molecular Medicine and Atherosclerosis and two foundations. Several authors have consulted for drug companies with heart disease products.