The Affordable Care Act will soon require all fast food chains to post calorie labels, in hopes of encouraging diners to make healthier choices.

Whether that will actually happen is unclear. New York City, a mecca of public health experiments, has used calorie labels since 2008. There, research on whether they work is mixed, with many studies showing that the new information had no effect on eaters' decision-making.

A new study out of North Carolina, flagged by Aaron Carroll, suggests that the labels could be more effective if they included additional information—namely, the exercise required to work off the calories at hand.

Researchers showed 804 employees of the University of North Carolina Medical Center one of the four menu formats, shown above. They were asked to "imagine you are at a fast food restaurant" ordering a meal. Then they selected all items they would purchase.

Those provided with no calorie labels at all ordered, on average, 1020 calories. A menu with calorie labels dropped that down to 927 calories. But the one that did best was the third option, which showed how far someone would need to walk to burn off their meal. Under that scenario, study participants ordered an average of 826 calories.

Those who saw the distance label mostly tended to shy away from burgers, it seems, but ordered the same amount of sides, drinks, salads and desserts.

"Our results suggest that people might be more responsive to physical activity information in the form of miles walked rather than minutes walked as demonstrated by the overall lower calorie selections made in response to a men with mile labels," the researchers wrote.

Granted, this is just one study and it did not happen in an actual fast food restaurant. These were just questionnaires filled out online, rather than actual lunch orders placed. One especially interesting study, published just after the New York menu labels took effect, found that many New Yorkers said the calorie labels lead them to make healthier choices.

But when researchers looked at what they actually ordered, it was no different from calories purchased prior to the law. So it's not clear what would happen if these labels were put into action.

New York City is, in a way, already testing this out. As part of a public health campaign, it has posted maps like this one that translate a 20-ounce soda into a three-mile walk to burn off the calories.