The study involved surveying and checking receipts for more than 7,300 customers in the 12 months prior to the implementation of the calorie-posting law and for 8,500 customers nine months after the law went into effect.
On average, customers bought foods with 44 fewer calories at McDonald's, 80 fewer at Au Bon Pain and 59 fewer at KFC. However, the other leading chains' customers showed little change in their behavior.
Some of the change likely resulted from chains offering less caloric options now -- such as Starbucks having made low-fat milk its default in coffee beverages and Applebee's introducing menu options with 550 or fewer calories, reported Reuters.
"We think, overall, these initial findings are positive," Dr. Lynn Silver, director of New York City's Office of Science and Policy and co-author of the report, told Reuters.