As several big chains roll out new healthy offerings for weight-conscious consumers, a recent study found that restaurants often provide substantially inaccurate calorie counts on menus. Some of the restaurants mentioned in the study, however, are questioning its accuracy.

In the study, published in the Journal of the American Dietetic Association’s January issue, Tufts University researchers found that on average, the calorie counts of different items at 29 quick-serve and sit-down restaurants were 18 percent higher than stated by the brands. Federal regulations generally allow for a 20 percent standard deviation.

The researchers found that Taco Bell’s Express Taco Salad with chicken had 607 calories, which is 281 calories, or 86 percent, more than stated by the company. But Taco Bell is disputing the findings, saying the researchers used an inaccurately small value for the portion size, which skewed the results.

“Their data is incorrect,” says Taco Bell spokesman Rob Poetsch. “I don’t know where they got their numbers.”

Susan Roberts, one of the study’s authors, said in an e-mail that the “numbers were accurate for the Web reports of the restaurant at the time of the study and for what we ordered.”

Not all of the items analyzed understated calorie counts, though. Researchers found that a large Domino’s Thin Crust Cheese Pizza had 141 calories, which is 71 calories, or 33 percent, lower than advertised. For consumers trying to lose weight, unknowingly eating fewer calories might sound like a good thing, but Roberts said that is not necessarily true.

“When a food is 30 percent low that is not great either,” she said. “It means you will get hungry soon and want to eat something else.”

A spokesman for Domino’s defends the company by saying it is virtually impossible to list calories exactly.

“We make a million pizzas a day, all by hand, all by individuals, in 5,000 stores across the United States,” Tim McIntyre says. “We’re not robots. You’re going to find an anomaly every single time.”

McIntyre also casts doubt on the study because it tested only one example of each food item.

“The sample size is one,” McIntyre says. “That is statistically insignificant.”

Roberts said the purpose of the study was to establish a phenomenon within the restaurant industry, not to single out any one company.

McDonald’s, whose McChicken Sandwich had 17 percent fewer calories than stated, according to the study, issued a statement saying that it adhered to federal regulations.

“Based on the data provided, the calorie counts for McDonald’s menu items fall into the 20 percent margin, set forth by the FDA,” the statement said.

Dunkin’ Donuts, the only other quick serve featured in the study, released a similar statement saying any “discrepancies” in calorie counts “are likely due to minor formulation variances during the baking process or improper in-store production.” The company also affirmed its adherence to federal guidelines.

While some measure of human error might be inevitable, health guru Joanne “Dr. Jo” Lichten, author of Dining Lean: How to Eat Healthy When You’re Not at Home, calls the calorie discrepancies “fairly avoidable.” She says that having a dietician on staff is one way restaurants could provide customers with more trustworthy nutritional data.

“Restaurants need to have accountability for the information that they put out there in the public,” Lichten says.

The Tufts research comes out just as some of the restaurants in the study are promoting new lower-calorie options. This month, Dunkin’ Donuts started offering additional breakfast sandwiches and wraps with egg whites, and its DDSMART menu, a “selection of better-for-you foods and beverages,” now includes a low-fat cranberry-orange muffin.

Taco Bell is promoting its Drive-Thru Diet of seven items with less than nine grams of fat. The promotion’s pitchwoman is Christine Dougherty, who lost 54 pounds in two years while regularly eating Fresco items off the menu.