Calorie labels can be a little bit useless.

Yes, they do contain a number that indicates the amount of energy contained in a given food. But the units of measurement are unfamiliar and difficult to parse. What does 250 calories in one soda actually mean?

Researchers at Johns Hopkins University wanted to see what would happen if they made calorie labels more blunt — specifically, if the labels told consumers how much exercise it would take to burn off the energy in a soda.

They went into Baltimore and put up signs in six corner stores' fridges, right where the drinks were. They said it would take 50 minutes of running or 5 miles of walking to burn off the 250 calories in a 16-ounce soda. And they waited, over six weeks, to see if shoppers made different decisions.

Here's what happened among the teenagers who shopped at these convenience stores: they bought fewer sodas. Results published this week in the American Journal of Public Health show that consumption of sports drinks such as Gatorade went down as well.

At the same time that teens were buying fewer sodas, they were also more likely to purchase water — or leave the store with no drink at all.

In exit interviews, about one-third of those surveyed said that they noticed the signs. And among those, 40 percent say it changed their decision about what they would buy at the store. Those who did still buy sodas tended to buy smaller bottles than they did before the signs went up.

This suggests a relatively easy way to make calorie labels, which are becoming more ubiquitous, more powerful. The Affordable Care Act mandates that all chain restaurants with 10 or more locations post calorie information about their standard food items. But most research shows that these listings don't change consumers' behavior.

One study looked at New York City's calorie labeling law, which went into effect in 2008. It compared New Yorkers' consumption of fast food after those labels went up to that of residents of nearby Newark.

If the calorie labels made a difference, you would expect to see declines in New Yorkers' calorie consumption while Newark residents' consumption remained constant. But that didn't happen: while the researchers did find that about half of New Yorkers noticed the labels, there was no statistically significant change in how they ate.

What makes the new research encouraging is that it suggests a way to make the numbers on calorie labels more meaningful. "These results might also be relevant to other local or state initiatives in various settings (convenience stores, vending machines in schools or workplaces) that require point-of-purchase information," the authors write.

And, on a more personal note, this new study will make me think twice about grabbing the 50-minute run bags of Cheez-Its in Vox's office.