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Many diners at fast food restaurants don't know what they're eating and consume far more calories than they think, a new study suggests.

The 2010 to 2011 study published in the journal BMJ revealed that teenagers underestimated the calories in fast-food meals by 34 percent, parents of school-age children by 23 percent and adults by 20 percent.

Lead investigator Jason Block of Harvard Medical School and Harvard Pilgrim Health Care Institute surveyed about 3,385 adults, teens and parents of school-age children who dined at 89 fast food restaurants including McDonald's, Burger King, KFC, Subway, Dunkin' Donuts and Wendy's. 

Survey participants were asked to guess the number of calories in their meals. Block and his team then collected the participants' receipts and counted how many calories the meals actually contained.

Researchers found that two in three participants in the study underestimated the calorie content of their meals and one in four underestimated the number of calories in their fast food meals by at least 500 calories.

Teenage diners ordered meals that contained an average of 756 calories, but underestimated their fast food meals by an average of 259 calories.  Adult diners ordered meals that contained an average of 836 calories, but underestimated their meals by an average of 175 calories. School age children aged three to 15 years old got meals that had an average of 733 calories, but their parents' underestimated their meals by 175 calories.

Surprisingly, people who ate at Subway underestimated the number of calories in their orders by a larger amount than those who ate at McDonald's, Burger King, KFC, Wendy's and Dunkin' Donuts. The study revealed that adults and teens eating at Subway underestimated by 20 percent and 25 percent more than those eating at McDonald's.

Many American states and cities have passed laws requiring chain restaurants to print calorie content on menus, and the U.S. Patient Protection and Affordable Care Act of 2010 included a provision that will soon require restaurant chains with more than 20 U.S. sites to print calorie content on menus.

Block told the USA Today that the latest findings show that "diners don't really know what they are eating in terms of calorie content".

"They need this information to help guide their choices," Block said, according to USA Today. "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."

Study authors recommend that in addition to providing calorie content on menus, U.S. policy makers could also improve menu labeling by supporting social marketing campaigns to help consumers make more informed decisions.