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You may avert your eyes from them, or purposefully cover them up when you’re scanning the menu at your favorite restaurant, but increasingly, they’re hard to ignore — the unsettling number of calories in the meal you’re about to consume. And as if the information isn’t sobering enough, here’s something to make you dislike it even more — those numbers, it seems, may be low-balling what you’re actually eating.

That’s what the latest study of menus at dozens of popular eateries found. Susan Roberts, director of the Energy Metabolism Laboratory of the USDA Human Nutrition Research Center at Tufts University, reports that while most listed calorie counts are accurate, those for the lowest calorie options are on average 100 calories below than what the foods really contain. And the listed counts often average the amount of calories over several different plates of the same meal, so there’s no guarantee of exactly how many calories you’re consuming with any one serving.

“We found huge discrepancies,” Roberts says. “Our statistician gave me this analogy: let’s say you sell 10-lb. bags of sugar, and you sell 15-lb. bags to your friends and 5-lb. bags to your enemies, and you call them all 10 pounds. The average would be accurate but it wouldn’t be fair to individual buyers. We found that’s what may be happening in restaurants. Overall, the main calories on the plate were not very different from what was listed on websites. But low calorie foods that are appropriate for weight control have more calories than listed, and high calorie foods have less calories than listed.”

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That means that those who chose options such as salads or other healthier foods, may be taking in about 100 more calories than they bargained for, which can lead to up to 15 pounds of weight gain over a year.

The misleading information, says Roberts, is “disgusting. Anybody trying to lose weight or avoid gaining weight — and that’s about 50% of the American public — are ordering the lowest calorie foods when they eat out, so this is an important group of foods for American health. And the information about their calories is inaccurate.”

Roberts and her team published their findings in the Journal of the American Medical Association, drawing their data from the calorie counts of 269 food items at 42 fast food and sit-down restaurant chains, including McDonald’s, Olive Garden, Outback and Boston Market. The scientists ordered some of the most popular items off the menus in three cities — Boston, Indianapolis and Little Rock, which represented a geographic sampling of the country — then froze the meals and shipped them to a central lab where they were analyzed for their calorie content. Overall, the items contained about 10 calories more than what the chains’ websites claimed they carried, which was within the statistically acceptable range. But for 19% of the foods, all of which fell into the low-calorie category, the lab technicians recorded about 100 extra calories than what the restaurants were claiming the foods contained.

The most egregious inaccuracies occurred with sit-down restaurant foods, says Roberts, and not fast food chains, which often shoulder the blame for America’s calorie glut. “Everybody labels fast food as the bad guys, but they are the good guys in this case; their numbers were much better than those of sit-down restaurants.” That’s because there’s more consistency in the portion sizes and cooking procedures at fast food chains than in restaurant kitchens, where chefs aren’t always using identical cuts of meat or pre-portioned ingredients.

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But given that most Americans get about a third of their daily calories from eating out, what can the weight-conscious do if they can’t rely on the information on menus? Simply not eat out as much. That’s the advice Roberts gives to her weight-loss group. “I tell my group not to eat out as much because of the huge portions and because you end up eating more calories than you meant to,” she says. If you do find yourself in a restaurant, Roberts recommends choosing the salad entrée, getting the dressing on the side and holding the cheese.

As for the restaurants themselves, diet experts hope this latest study triggers more honest reporting of calorie counts. “Restaurants need to step up to the plate and acknowledge the role they play in the obesity epidemic, and in helping Americans to get thin,” says Roberts. “They need to offer more portions of lower calorie foods, and to honestly put on people’s plates what they list on their websites. That would be a big help for all Americans.”

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny . You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.