People who know how many calories are in their food choose to eat healthier, according to a study.
While millions enjoy meals in restaurants and cafes, the food cooked by the chef can add surprisingly.
Researchers have discovered that people actively choose healthier options when they can see how many calories are in their meals.
In one study, people reduced their calorie intake by three percent when they were given a menu with the nutritional information in it.
But people still treat themselves, despite a small reduction of calories in their appetizers and main courses, the choices of desserts and drinks were not modified.
Even the cooks themselves were surprised at how many calories there were in some of their dishes.
The investigation comes as the UK government is considering passing a law requiring all restaurants, cafes and take-aways to display calories on their menus, while many states in the United States already have similar rules.
People who eat in restaurants that show the number of calories in a meal on the menu order starters and main courses with, on average, 45 fewer calories than people who receive normal menus.
A study by Cornell University in Ithaca, New York, monitored a total of 5,550 diners at two restaurants in the United States.
Groups of people received random menus with or without the amount of calories displayed next to each meal.
Their options were noted and they completed questionnaires about themselves so that the experts could be sure that their results were not influenced by other factors.
The research found that people who knew how many calories they would consume in their food ate an average of 45 fewer calories, three percent of the total.
THE UK COULD PUT FORCES TO ALL POWER SUPPLIES TO SHOW CALORIES
The British government could push for a law requiring all restaurants, cafes and take-aways to provide customers with calories on their menus.
A public consultation has been initiated on the plans, which have met opposition from another area of government and a health charity.
The Treasury is said to oppose the measure, warning it would be "onerous" for 26,000 small businesses and could force them to raise prices and cut jobs.
And the health charity Beat warned that it would exacerbate eating disorders and cause great anguish to patients with conditions such as anorexia or bulimia.
But public health minister Steve Brine argued: "Families want to know what they're eating when they're on the move.
"This is not about forcing someone to eat certain things, or companies to behave in a certain way, but I firmly believe that we have the right to know the nutritional content of the food we give to our children."
Although 45 calories are not many, it is the equivalent of about half a banana or a whole apple, they could be added for people who often eat out.
The options of appetizers and main dishes improved slightly, but the drinks and desserts were not changed, suggesting that people could still eat what they wanted.
The study's author, Professor John Cawley, said: "Even if you are an educated person who eats a lot and knows nutrition, there can still be amazing things in this calorie count."
Scientists found that people find caloric information useful and suggest that requiring restaurants to provide it by law could be an efficient way to improve health.
"It is clear that people value this information," added Professor Cawley.
"It's a cheap policy to put in place, and the fact that there is a reduction in the calories ordered makes it attractive."
Even the cooks who cooked the dishes were surprised by the amount of calories in some foods, such as a tomato soup and a grilled cheese sandwich.
"They would have said it was one of the least calorie items on the menu," added co-author Alex Susskind.
All the meals in the study had a calorie count between 200 and 1,840 calories, and people's orders contained an average of approximately 1,500 calories.
The recommended amount of calories that an average adult should eat in a day is 2,000 for women or 2,500 for men.
The team's findings were published in the National Office of Economic Research.