TUESDAY, July 19 (HealthDay News) -- In some states, it's already a requirement that restaurants with numerous locations must post the calorie counts of their foods. And, soon, federal law will require more restaurants across the country to provide such information to its patrons.

But, just how accurate are the calorie counts provided by restaurants?

That depends on where you eat. A new study that assessed foods from 42 restaurants -- including fast food places -- suggests that overall, the calorie counts are fairly accurate. But, the study also found that there was plenty of room left for improvement, especially in restaurants where you sit down to order your meal and for the foods classified as low calorie, such as salads.

"We did a random sampling of restaurant foods in three locations in the U.S., and included high- and low-calorie foods found at fast food and sit-down restaurants. The main finding of the study was that there was huge variation in how good the numbers for individual foods were, and in particular, foods listed as low-calorie," explained study author Susan Roberts, a scientist at the USDA Human Nutrition Research Center on Aging at Tufts University in Boston.

"We also found that sit-down restaurants were much worse in terms of being likely to give bad numbers than fast food places," she said.

In the restaurants overall, 19 percent of the foods -- almost one in five -- contained at least 100 more calories than what the menu stated. Of these foods, the 10 percent of the foods with the highest excess calories averaged more than 250 extra calories per food. One food item even had 1,000 calories more than what was listed on the menu, according to Roberts.

"And bear in mind, nobody eats just one food in a restaurant because of all the free sides, and perhaps drinks [and] desserts," Roberts continued.

While 100 extra calories in a day might not sound like much, Roberts said that over a year, 100 extra calories daily can lead to a 10- to 15-pound weight gain.

And, given Americans' propensity for restaurant meals, inaccurate calorie counts could really add up. Almost half of all Americans eat out at least three times a week, while one in six Americans eat out more than seven times weekly, according to background information in the study.

In addition, calories from restaurants make up about 35 percent of Americans persons daily energy intake, the study noted.

Results of the study are published in the July 20 issue of the Journal of the American Medical Association.

The 42 restaurants chosen for the study were in Massachusetts, Arkansas and Indiana, and the 269 sampled foods were purchased between January and June of 2010.

Overall, the researchers found a difference of about 10 calories per portion. However, there was significantly variability among foods, with 26 common foods containing an average of 273 calories more than what was listed on the menu.

"The variability in individual samples was unacceptable in my opinion," said Roberts. "The fact that low-calorie foods come out with more calories than listed is also a major problem. People buying low-calorie foods [may be] trying to watch their weight, and nobody can watch their weight effectively if they are unknowingly being given more calories than they ask for."

"The good news here is that there are efforts being made and things are moving in the right direction," said Linda Van Horn, a professor of preventive medicine at Northwestern University Feinberg School of Medicine in Chicago. Van Horn also wrote an accompanying editorial in the same issue of the journal.

"I think this provides a wake-up call to the restaurant industry, that there's still a long way to go before there is standardization," said Van Horn.

For consumers trying to control their caloric intake, she said, it's important to be aware that there may be variability in calorie counts, especially in sit-down restaurants. "In sit-down restaurants, there's much more creativity that can go into the preparation of foods, which may increase the caloric counts of those foods, as opposed to fast food restaurants where the food and portion sizes are supplied," said Van Horn.

"Right now, restaurants are a tough place to eat if you are watching your weight," said Roberts. She said that if you are eating out and concerned about the caloric content of your food, go ahead and order the salad, but ask for it without cheese, and ask for the salad dressing on the side. Salad dressings can add hundreds of calories to a salad if portions aren't controlled, she said.

More information

Get advice on eating healthy at restaurants from the American Heart Association.