Diners badly underestimate calories in fast-food meals

• Some diners underestimate the calories in fast-food meals by at least 500 calories
• Teens are eating about 259 more calories in fast-food meals than they realize
• Adults are off the mark on calories by 175

Many diners are gobbling far more calories in their fast-food meals than they realize, a new study shows.

Teens underestimated the calories in fast-food meals by 34%; parents of school-age children by 23%; adults by 20%, says lead researcher Jason Block of Harvard Medical School and Harvard Pilgrim Health Care Institute.

Block and colleagues surveyed about 3,400 adults, teens and parents of school-age children who visited 89 fast-food restaurants, including McDonald's, Burger King, KFC, Subway, Dunkin' Donuts and Wendy's. People were asked to estimate the calories in their meals, then the researchers collected their receipts and figured out how many calories the meals actually contained. The study was conducted in 2010 and 2011.

Among the findings, published Thursday in BMJ, a journal of the British Medical Association:

• One-fourth of participants underestimated the calories in their meals by at least 500 calories.

• Teens' fast-food orders contained an average of 756 calories, but they underestimated their orders by an average of 259 calories.

• Adults ordered meals containing an average of 836 calories, but they underestimated by 175 calories.

• School-age children got meals that had an average of 733 calories, but their parents' guestimates were 175 calories too low.

• Diners at Subway underestimated the calories in their orders by a larger amount than diners at McDonald's, Burger King, KFC, Wendy's and Dunkin' Donuts.

"These large underestimations show that diners don't really know what they are eating in terms of calorie content, and they need this information to help guide their choices," Block says.

"They could get it from the company websites or in some other form in the restaurants, such as wall posters, napkins or cups, but soon they'll be directly faced with it when they see it on the restaurant menu boards before they order their meal. Customers can already do this at McDonald's -- and in some cities," he says.

The study was funded in part by the Robert Wood Johnson Foundation.