Lead researcher, Jason Block, an HMS assistant professor of population medicine at Harvard Pilgrim Health Care Institute, said: "We found that people, especially teens, are consuming more calories than they think they're getting when they eat fast food."
The study was the first large scale analysis of its kind to evaluate the difference between what adults and teens estimated the calorie content of their fast food meals were versus their real calorie content.
34 percent of teens underestimated the number of calories in their meals followed by 23 percent adults who were parents of school-age children and 20 percent of other adults.
In 2011 and 2012, the researchers surveyed close to 3,400 adults, teens, and children at 89 different fast-food outlets in the New England area.
Adults, on average, ordered meals with 836 calories and underestimated the calorie count of their meals by around 175 calories.
Teens tended to order meals containing around 756 calories and underestimated the calorie count by 259 - the largest gap.
An overwhelming twenty five percent of the participants thought their meals had 500 fewer calories than they actually had.
They also saw differences by food chain. Of all the different fast food chains observed, people who ate at Subway were the most likely to underestimate the number of calories in their meals by the largest amount.
Block noted that: "participants also ordered side dishes with more calories at Subway. Dieticians also falsely considered equivalent calorie meals to be lower calorie at Subway than McDonald's.3 Our study extends these findings by showing that this "health halo" is unique to Subway across the six chains and is present across age groups in a diverse sample."
"These findings tell us that many people who eat at fast-food restaurants may not be making informed choices because they don't know how many calories they're consuming. Having the information is an important first step for anyone wanting to make changes."
The authors emphasized that during the time of this study, the fast food chains didn't post calorie information on their menus.
The research was conducted by the Harvard Medical School/Harvard Pilgrim Health Care Institute in conjunction with leaders of the Massachusetts and Connecticut Departments of Public Health.
Fast food poses serious health threatNational figures for 2007-2010 show that on average, adults in the US get more than 11% of their daily calories from eating fast food. This is of particular concern considering that some restaurant chains "are scientifically engineering these extreme meals with the express purpose of promoting obesity, diabetes, and heart disease", according to Michael F. Jacobson, executive director of CSPI.
Fast food chains continue serving meals associated with serious health risks. Their meals may sound and look extremely appealing, but they are actually incredibly fattening and can eventually harm human health if eaten regularly over the long-term. A recent issue of Nutrition Action Healthletter listed dishes with the unhealthiest ingredients in the U.S.- some boasting more than 3,000 calories.
Obesity rates have not risen for the first time in thirty years, according to a report by the Robert Wood Johnson Foundation and Trust for America's Health, published in August 2013.