NEW YORK - Stroll the aisles of any grocery store and you're sure to spot labels declaring "zero grams trans fat" on the front of snack foods, cookies and crackers. But does zero really mean there's NO artery-clogging fat inside?

Maybe, maybe not.

Federal regulations allow food labels to say there's zero grams of trans fat as long as there's less than half a gram per serving. And many packages contain more than what's considered one serving.

"The problem is that often people eat a lot more than one serving," said Dr. Dariush Mozaffarian of Harvard School of Public Health. "In fact, many people eat two to three servings at a time."

Those small amounts of trans fat can add up, said Michael Jacobson of the consumer advocacy Center for Science in the Public Interest. To find out if there might be some trans fat, he said shoppers can check the list of ingredients to see if partially hydrogenated oil — the primary source of trans fat — is included.

"When it says zero grams, that means something different from no trans fat," said Jacobson. His group has urged the government to bar food producers from using any partially hydrogenated oils at all.

The Food and Drug Administration began forcing food companies to list the amount of trans fat on nutrition labels of packaged foods in January 2006. That led many companies to switch to alternative fats.

Trans fat occurs naturally in some dairy and meat products, but the main source is partially hydrogenated oils, formed when hydrogen is added to liquid vegetable oils to harden them.

Consumer groups and health officials have campaigned to get rid of trans fat because it contributes to heart disease by raising levels of LDL or bad cholesterol while lowering HDL or good cholesterol. Fast-food restaurants are switching to trans fat-free oils and New York City and Philadelphia are forcing restaurants to phase out their use of trans fat.

The American Heart Association recommends that people limit trans fats to less than 2 grams per day.

Julie Moss of the FDA's Office of Nutrition, Labeling and Dietary Supplements, said the half-gram threshold for labeling was adopted because it is difficult to measure trans fat at low levels and the same half-gram limit is used for listing saturated fat. She said the FDA would soon be doing consumer research on trans fat labeling, including whether a footnote such as "Keep your intake of trans fat as low as possible" should be added to food labels.