There are plenty of reasons to avoid foods such as french fries and potato chips: excess calories and a high fat content among them. But last April, scientists in Sweden added another drawback to the list. They announced that carbohydrate-rich foods that had been heated could contain elevated levels of acrylamide, a chemical known to cause cancer in rats at high doses. How the substance came to contaminate the foods was unclear, however. Now two papers published in this week's issue of the journal Nature hint at its source.

During the cooking process, amino acids and sugars combine in what is known as the Maillard reaction. The new work shows that this reaction can sometimes produce acrylamide as well. Donald S. Mottram of the University of Reading, England, and his colleagues combined the amino acid asparagine--which accounts for 40 percent of the amino acid content of potatoes used to make chips--with glucose. They found that temperatures above 100 degrees Celsius were sufficient to produce the substance and that temperatures above 185 degrees C prompted significant acrylamide formation. A second team, led by Richard T. Stadler of the Nestle Research Center in Lausanne, Switzerland, reached a similar conclusion after testing the reactivity of 20 amino acids at elevated temperatures. Plant-based foods such as cereals, wheat and rye flour are also rich in asparagine, the scientists note.

The effects of acrylamide on humans remain unclear, but it causes cancer in rats and fruit flies when ingested in concentrations 1,000 times those found in the average diet. The World Health Organization has listed the substance as "probably carcinogenic to humans" and is organizing efforts to coordinate further research into its consequences. An understanding of how heat leads to acrylamide formation could inform methods of limiting its production, either through modifications to the Maillard reaction or by utilizing ingredients with lower asparagine content. For now, scientists say that people should be more concerned with eating a healthy, balanced diet, which includes plenty of fruits and vegetables, than they should be with what might be lurking in their order of french fries.