WASHINGTON The type of fat in margarine and vegetable oil reduces the function of blood vessels and drives down levels of good cholesterol more than saturated fats such as those found in butter and meat, Reuters reported on Thursday.

Researchers from the Netherlands sought to determine whether different diets play a role in the blood vessels' ability to dilate or expand. This function is impaired in patients with cardiovascular disease.

They gave diets high in trans fatty acids such as those in margarine to one group of subjects and diets high in saturated fats such as those found in animal products and coconut and palm oil to other subjects.

They found that the ability of the blood vessels to dilate was nearly a third lower in people who ate the diet high in trans fatty acids compared those on the saturated fat diet.

In addition, blood levels of high-density lipoproteins (HDL) -- the so-called good cholesterol -- were about one-fifth lower in the trans fatty acids group compared to the saturated fat group.

"This might be part of the explanation of why a high intake of trans fats increases the risk of coronary heart disease," Nicole de Roos, an expert in human nutrition and epidemiology at Wageningen University in the Netherlands and lead author of the study, said in a statement.

The scientists randomly assigned 29 healthy men and women to a diet high in trans fat or a high saturated fat diet in which the trans fats were replaced by saturated fats. After four weeks on one diet, the subjects were crossed over to the other diet.

Trans fats are created when hydrogen atoms are forced into liquid oils, such as corn or soybean oil, to make them solid at room temperature. The terms "hydrogenated" or "partially hydrogenated" oils on nutrition labels refer to this process.

French fries, doughnuts, cookies and crackers are examples of foods that are high in trans fatty acids.

The American Heart Association said it recommends that consumers use naturally occurring, unhydrogenated oils such as canola or olive oil and look for processed foods made with unhydrogenated oil rather than hydrogenated or saturated fat.