It has become fairly common to see calorie counts listed next to restaurant menu items. This helpful check for on-the-go eaters can prompt practical choices when calories are a concern. However, a new study of foods found in national chain restaurants suggests that some items contain more calories than listed.

The Journal of the American Medical Association recently published a study from the Jean Mayer USDA Human Nutrition Research Center on Aging (USDA HNRCA) at Tufts University that compared laboratory measurements of calories in 269 food items with restaurants’ stated calorie totals. Researchers randomly collected their samples from national fast-food restaurants and sit-down chain restaurants in Boston, Indianapolis and Little Rock, Arkansas.

The report revealed that while calories given on restaurant menus and websites were accurate on average, 19% of individual samples differed from laboratory measurements by 100 calories or more.

“On average, the food items measured 10 calories higher than the restaurants’ stated calories. That’s essentially accurate,” said senior author Susan B. Roberts, PhD, director of the Energy Metabolism Laboratory at the USDA HNRCA. “However, 19% of food items contained at least 100 calories more than listed, which suggests calories for individual foods can be unreliable. One item contained 1,000 calories more than listed.”

Lab analysis also indicated that lower-calorie food items purchased in sit-down restaurants tended to have more calories than listed. For example, based on their data, the researchers were able to predict that a sit-down restaurant item said to contain approximately 300 calories—and therefore be potentially suitable for weight loss or prevention of weight gain—could have about 90 calories more than stated.

Additionally, calorie listings for items often viewed as healthier than others—such as salads and soups—tended to be more unreliable in both sit-down and fast-food restaurants.

“Restaurants can play a powerful role in anti-obesity efforts if stated calories are accurate,” Roberts said, “because food purchased in restaurants accounts for one-third of the average American’s daily food intake. Examples of ways restaurants could help would be to increase the number of foods that calorie listings are provided for, make sure that lower calorie foods do not contain more calories than stated, and offer a wider selection of lower calorie items.”