Nothing works up a good appetite like an intense sweat session. But does a hard workout give you license to eat whatever you want?

No-but while you know deep down that indulging in a 600-calorie protein shake can undo all of your hard work, it's still easy to feel like you've earned a treat. And that's exactly what a new study out of the U.S. and Taiwan found: People who are on a diet often see working out as a justification to indulge. And this sense of "exercise entitlement" may be one reason you can't seem to lose weight.

In the study, researchers asked college students whether they were currently on a diet or rarely watched what they ate. Each person was then given a bag of chips and told they could eat them before a workout, after it, or while they watched a movie. Surprisingly, the group that inhaled the most chips were dieters who planned to exercise after eating-they gobbled up nearly twice as many calories as the non-dieters and three times as many as the dieting non-exercisers!

At first glance, this doesn't seem to make sense: The dieters in the study said they were tracking all of their calories. The problem? We're not as good at guessing how many cals we burn through exercise as we may think we are, says Susan Albers, Ph.D., a psychologist at the Cleveland Clinic and author of But I Deserve this Chocolate. You might forget that running three miles at a 10-minute mile pace only burns about 300 calories-the amount in one cream-filled doughnut. "Research indicates that people underestimate the amount they eat by about 40 percent if they are overweight and 20 percent if they are at a normal weight," she explains.

But the scientists add that chowing down post-workout has a lot to do with attitude too: When we view eating right and exercising as hard work, we want to to reward ourselves with something decadent. Changing your mindset could be the solution, though! In the study, dieters who were told they were exercising for fun ate significantly fewer chips than those who were told that the workout was tough. Albers says we have to stop using food as a reward and exercise as a punishment-period. (Not all packaged foods are the enemy! These 10 Packaged Foods Are Surprisingly Healthy .) "Rewarding yourself with food is a dangerous habit," she adds.

So the next time you're pushing yourself through bootcamp class, instead of calculating how many scoops of ice cream you've "earned," focus on enjoying the moment, taking satisfaction in a job well done, and treating your body with the love and respect it deserves.