But parents don't restrict their own energy intake, according to the report, which joins a growing body of work on the effects of menu labeling.

For the study, researchers randomly assigned 99 parents of 3- to 6-year-olds to one of two groups. Both were shown McDonald's-like menus and asked to choose foods for themselves and their kids. However, one group's menu included the calorie content next to the price for each item.

On average, parents whose menus did not list calories chose a meal of about 670 calories, while those who did have that information chose a meal of 570 calories.

One hundred calories may seem like a small amount, but over time it could make a significant difference, said Dr. Pooja Tandon, a pediatrician at the University of Washington, who led the study. For example, she said, an extra 100 calories per day in adults can lead to 10 extra pounds in a year.

From 1977 to 1996, American kids tripled the amount of restaurant food they ate, according to the new report.

Although menu labeling is still the exception rather than the rule, a few states--including California and Oregon--and cities--including New York--have already passed policies requiring chain restaurant to provide calorie information. The intention is to help combat the nation's obesity problem by raising consumer awareness of just how many calories lurk in their burgers, sandwiches, fries and desserts.

Such policies have had mixed results. In October, an independent study of New York's law concluded that menu labeling had done nothing to change consumer habits in the city's low-income neighborhoods. Shortly thereafter, the city's health department released preliminary data from a larger study suggesting that New Yorkers had, in fact, started buying fewer calories at 9 of 13 fast-food and coffee chains included in the research.

And a study published in December found that restaurant menus that include calorie information do seem to encourage diners to exercise some restraint.

The new study is the first to investigate how labeling affects what parents order for their children, and Tandon thinks the findings send an optimistic message.

This could be one piece in changing the childhood obesity epidemic, she told Reuters Health.

The 100-calorie difference in what parents selected persisted even after Tandon and her colleagues took into account other factors such as body mass index, a standard measure of the relationship between weight and height.

It's exciting to see that parents will use this information, said Lisa Harnack of the University of Minnesota, who was not part of the new investigation.

Harnack, an expert in obesity prevention, said the impact of menu labeling is less clear-cut in adults. In the new study, parents did not change their own energy intake significantly based on calorie information.

When people eat out they think it's a special occasion, she said. They put nutrition on the back burner.

Earlier studies have found that people tend to value taste over price, and price over nutrition, when they eat at restaurants. One possibility, she said half-jokingly, is that parents' preferences may change when they're not eating the food themselves. And children who learn healthy eating habits at a young age might carry those habits into adulthood.

It's difficult to say if the results will hold true in real-life settings, Tandon said: Things might change when your belly is growling and you are in a hurry.

Still, I'd like parents to think about the small changes they can make, Tandon said, adding that just choosing milk or fruit juice instead of a fountain drink can make a big difference in the long run.