News stories on the heart risks of trans fats seem to sway the public's shopping habits, but only for a short time, a new study suggests.

Researchers found that Los Angeles shoppers tended to buy fewer products containing trans fats in the week following media coverage of the artery-clogging fats. But the effects waned soon thereafter.

The findings, reported in the American Journal of Preventive Medicine, point to a need for sustained public health campaigns to remind consumers why they should limit foods high in trans fat.

"While news coverage is a potentially valuable source of information, and one that can help the public to make informed decisions about their health, this study shows that news coverage alone is not enough to sustain changes in consumer behavior," co-researcher Dr. Dominick L. Frosch, of the University of California Los Angeles, said in a written statement.

Trans fats have become notorious because they not only raise "bad" LDL cholesterol — like the saturated fats in meat and butter do — but also lower levels of the "good" HDL cholesterol.

Trans fats are formed during food processing when hydrogen is added to vegetable oil to make it solidify; foods that list "partially hydrogenated vegetable oil" on the label contain trans fat.

Traditionally, that has included most commercially prepared baked and fried foods - such as cookies, crackers, chips, breads and french fries — but manufacturers and restaurants have been increasingly removing trans fats from their products.

Since 2006, the Food and Drug Administration has required all food product labels to list the amount of trans fat, if that amount exceeds 0.5 grams per serving.

But since that policy went into effect, "there has been no coordinated effort to educate the public about the dangers of trans fat," said Frosch.

One recent study of U.S. adults found that while most knew they should avoid trans fats, few could name the foods that typically contain them.

For the current study, Frosch and colleague Dr. Jeff Niederdeppe of Cornell University in Ithaca, New York used weekly sales data from a major grocery chain in Los Angeles, covering 129 weeks between 2005 and 2007.

They looked at the relationship between local news coverage of trans fats and sales of several trans-fat-rich products: buttered popcorn, hot dogs, vegetable shortening, stick margarine and packaged cookies and biscuits.

In general, the researchers found, news coverage seemed to spur a dip in trans-fat sales, with the influence being stronger after the FDA labeling rule went into effect.

However, the impact of each media blitz began to fade after only one week.

"In the absence of broader changes in food policy and public education," Niederdeppe said in the statement, "news coverage may be insufficient to produce lasting reductions in trans-fat purchases and consumption."