Polyunsaturated vegetable oils from plants like soybeans and sunflowers are generally praised by nutritionists because they help people keep their cholesterol down and, it is believed, avoid heart disease.

However, according to a new study, when used to fry foods, the oils produce a toxic compound that has been associated with a variety of illnesses, including cardiovascular disease, Parkinson's and Alzheimer's diseases, and liver problems.

The findings, the researchers say, highlight the risk of reheating the oils or reusing them, since the amount of the compound, known as HNE, increases with each heating.

"It adds up," said the lead researcher, Dr. Saari Csallany, a professor of food chemistry and nutritional biochemistry at the University of Minnesota.

The study was presented at a meeting of the American Oil Chemists' Society. The co-author of the study was Christine Seppanen, a graduate student.

Other studies have shown that HNE is absorbed by food cooked in polyunsaturated oil, Csallany said.

The compound forms when the very component of unsaturated oils that is considered so healthful, linoleic acid, oxidizes. The study reported that three other toxic compounds related to HNE had also been found in heated soybean oil.