You’re eyeing the menu at [insert favorite food chain here], debating between the decidedly not good-for-you onion rings and better-for-you salad, which boasts 120 fewer calories. You (of course) make the right choice, but according to a surprising new study, going with fewer calories may not mean you’ll end up getting less.Starting in 2008, U.S. cities and counties began passing ordinances, which obligated chain restaurants to list the calorie content of foods on their menus. But the menu labeling may be off—in some cases, more than 100 calories off—especially at sit-down chain restaurants where portion sizes may vary.

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Lorien E. Urban, Ph.D. and colleagues at Tufts University analyzed the energy content—kcal (or calories) per portion—of 269 take-out food items from fast food and sit-down restaurants.Of the measured food items, 40 percent were at least 10 calories higher than the listed amount, while about half were 10 calories or lower than stated. So it appears that you’re getting the amount of calories you think you’re eating.

Yet if you take a closer look, some of the findings were surprising. One particular item was 1,000 calories more than the listed amount! Sure, this is just one case, but nearly one-fifth of the foods were more than 100 calories greater than labeled. So if you’re watching your diet, one menu choice may lead you astray.Of the foods with highest calories discrepancies, the average difference in food was a whopping 289 calories. Soups and salads tended to be the most deceiving healthy choices.

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“We think it is very important that lower calorie foods not contain more calories than listed because such foods are purchased by people trying to control their weight,” said author Susan B. Roberts, Ph.D. in a press release. “They will find that harder to do if they are eating more than they think.”

But can the occasional wrong calorie-count really make a difference in your overall eating habits and body health?

Before researchers can answer this question, it’s important to first look at studies of calorie listings on menus—do they cause people to make healthier food choices?

“The data seems to be pretty clear that putting calorie information on the menu does not have any effect,” said David Levitsky, Ph.D. Professor of Nutritional Sciences at Cornell University.

Other studies conducted in the past two years find that this nutrition information didn’t push people to purchase foods with less calories. A 2010 study of New York City sales at Starbucks did find one effect—a net decrease of only six calories per transaction on average.

National legislation will go into effect in 2012, which will force food chains (with 20 or more locations) to put calories on the menu. “I would love to think that people do put [calorie labels] into consideration, but there’s little evidence that they do,” Dr. Levitsky said.So how might this new law decrease the rise in body weight the U.S. has seen over the past 25 years?

“It’s not going to affect obesity rates,” Dr. Levitsky predicts.

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