The city's ban on trans fats in fast food is making a difference in New Yorkers' diet, a Department of Health study published Tuesday shows.

The department surveyed customers and collected receipts for nearly 15,000 lunchtime purchases at fast-food chains around the city in 2007 and 2009, before and after the ban was in place.

The amount of trans fat in each lunch sold dropped an average of 2.4 grams after the ban, researchers report in Tuesday's edition of Annals of Internal Medicine. The biggest drop, 3.8 grams, occurred in hamburger chains, followed by Mexican food and fried chicken chains.

"While we just looked at fast-food meals in this study, every time a New Yorker walks into a restaurant they know that their meal won’t have high levels of trans fat. So it's really good news for everyone,” said Christine Curtis, study co-author and director of nutrition strategy at the city’s health department.

Trans fat is widely considered the worst kind of fat for the heart. The American Heart Association has long recommended that people limit trans fat to fewer than 2 grams a day.

The 2008 ban that meant fast-food chains could serve no more than 0.5 grams per serving. It was the first ban of its kind in the country, and prompted similar restrictions elsewhere in the country.