Put the butter back on the table.

Researchers at Wageningen University in the Netherlands have discovered that the type of fat in margarine and vegetable oil reduces the ability of blood vessels to dilate or expand and decreases the level of good cholesterol more than the saturated fats found in butter and meats.

The researchers wanted to determine whether different diets play a role in the blood vessels’ ability to dilate or expand. Such reduced function of the blood vessels is a common factor among people with coronary heart disease.

The scientists randomly assigned 29 healthy men and women to a diet high in trans fatty acids or a diet high in saturated fats. The ability of blood vessels to expand and contract of those fed the diet high in trans fatty acids was reduced by one-third. In addition, blood levels of the “good cholesterols,” high-density lipoproteins, were reduced by one-fifth.

“This might be part of the explanation of why a high intake of trans fats increases the risk of coronary heart disease,” said Nicole de Roos, an expert in human nutrition and epidemiology at Wageningen University and lead author of the study. 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. Nutrition labels that list “hydrogenated” or “partially hydrogenated” oils refer to these trans fatty acids.

The research was reported in the July issue of the Journal of Atherosclerosis, Thrombosis and Vascular Biology, which is published by the American Heart Association.