U.S. researchers say partially hydrogenated vegetable oils have trans fats that can interfere with blood flow.

The study, scheduled to be published in the journal Atherosclerosis, finds trans fats reduce the amount of an enzyme -- prostacyclin -- that keeps blood flowing and helps prevents blood clots.

"This is the first time that trans fatty acids have been shown to interfere with yet another part of the blood-flow process," study leader Fred Kummerow, emeritus at University of Illinois at Urbana-Champaign, says in a statement. "This study adds another piece of evidence to a long list that points to trans fats as significant contributors to heart disease."

In 2007, Kummerow's team found trans fats inhibited the synthesis of arachidonic acid -- a precursor of prostacyclin -- from linoleic acid, even when there was plenty of linoleic acid available.

Kummerow points out trans fats displace, and cannot replace, the essential fatty acids linoleic acid, omega-6, and linolenic acid, omega-3, which the body needs for a number of reasons besides blood flow regulation.

Trans fats also increase low-density lipoproteins in the blood, increase inflammation of the arteries and make cells more likely to leak calcium -- all three factors believed to be involved in the formation of atherosclerosis.