Also, mean menu calories only marginally decreased and only new menu items showed the 12 percent drop.
At restaurants that had a specific food focus, calorie declines were bigger among those that were not central to that focus. Indeed, core items at burger and chicken restaurants actually became more caloric.
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The findings could have broad implications for the obesity epidemic since a big chunk of people eat at fast-food restaurants each day. About a third of children, 41 percent of adolescents and 36 percent of adults dine there on an average day, the report noted.
"The main take-home point of the study is it doesn't take a lot of additional calories to increase weight over time," Bleich said.
"If you can get each one of those people to take in 60 fewer calories, that could potentially have a significant impact on obesity," she added.