Putting calorie information on menus encourages healthy eating - but only in a limited way, a review of the scheme in the US shows.
Researchers quizzed customers before and after a law was passed in New York in 2008 forcing restaurants to display the nutritional information.
The study by the NY Health Department showed one in six used the information - with most reducing their intake.
It comes as UK restaurants are introducing a similar scheme.
A total of 32 firms signed up to displaying calorie information in UK outlets, including fast food chains McDonald's and KFC, as part of the government's "responsibility deal" set out earlier this year.
The initiative saw a host of voluntary agreements established in the fields of alcohol, physical activity and health at work as well as food.
And the New York experience has suggested calorie information could have a benefit.
Researchers surveyed more than 7,000 people in 2007 and another 8,500 in 2009 at 168 locations covering 11 of the top food chains in the city, the British Medical Journal's website reported.
Some 15% reported using the labels and these customers purchased 106 fewer calories than customers who did not use or see them.
However, overall there was no significant change in average calorie consumption before and after as some people were consuming more calories in 2009.
This was partly explained by changing practices at some of the restaurants. For example, energy consumption increased by nearly a fifth at Subway where large portions were heavily promoted.
Researchers said it was important if the scheme was going to be more of a success that education campaigns be set up to improve awareness.
Dr Susan Jebb, a nutrition expert from the Medical Research Council, who has advised the government on its strategy, said she would expect calorie information to have more of an impact in the UK.
"We are much more used to looking at front-of-pack labelling in supermarkets so I would expect more people to use calorie information than the one in six in New York.
"But this study also illustrates the importance of how the restaurant or food chain acts. It is going to require a combination of factors to make a big difference."
A spokeswoman for the Department of Health said: "This is a great example of how calorie labelling can influence the choices people make and lead to a healthier diet."
Beatrice Brooke, of the British Heart Foundation, added: "One in six meals in the UK is eaten away from home so it's essential we know what's in the food we're buying in restaurants and cafes.
"The New York research shows us just how valuable calorie labelling in fast food restaurants can be."