TUESDAY, July 26 (HealthDay News) -- Only about one-sixth of fast food restaurant customers in New York City use calorie information provided on menus and menu boards, but they do tend to buy food with fewer calories, a new study shows.

Under a law introduced by the city in 2008, chain restaurants with 15 or more locations nationally must provide consumers with calorie information.

For this study, researchers analyzed survey data collected from 7,309 adult customers in 2007 and 8,489 customers in 2009. The surveys were conducted during lunchtime hours at 168 randomly selected locations of the top 11 fast food chains in the city.

Between 2007 and 2009, there was no overall decline in the amount of calories in foods bought by customers. But there were significant reductions at three major fast food chains. Average calories per purchase fell by 5.3 percent at McDonalds, by 6.4 percent at KFC, and by 14.4 percent at Au Bon Pain. Together, these three chains accounted for 42 percent of all customers in the study.

But the researchers also found that average calories per purchase increased by 17.8 percent at Subway, where large portions were heavily promoted.

Overall, 15 percent of the consumers in the study said they used the calorie information and, on average, these people purchased 106 fewer kilocalories than those who did not see or use the calorie information at the fast food outlets.

The study was published in the July 27 online edition of the BMJ.

Nutrition labeling is a step forward in fighting growing rates of obesity in many western nations, but changes in the food supply must follow, Dr. Susan Jebb, of the MRC Human Nutrition Research Center in Cambridge, U.K., noted in an accompanying editorial.

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