The signs were a proxy for an actual food label, but they made the point. They effectively led to fewer juice and soda purchases, and to purchases of smaller sizes (12-ounce cans instead of 20-ounce bottles). Bleich also saw learned behavior; even after the signs came down, the local patrons continued to buy less soda and juice.

"The problem with calories is that they're not very meaningful to people," Bleich told me. "The average American doesn't know much about calories, and they're not good at numeracy."

That concern is the impetus for a growing movement to make nutrition information as simple and practical as possible. Some have proposed a three-tiered stoplight system, where healthy foods are labeled with a green light (Go!), and junk bears a damning red. Yellow is ... everything else. Others have proposed an even simpler thumbs-up, thumbs-down dichotomy.

"Let's say you do know that you need to take in about 2,000 calories a day—which most American's don't know," Bleich said. "Let's say that a hamburger at McDonald's has 250 calories. To figure out the percent that 250 represents of 2,000 is tough, mentally. Most people can't do that, and they certainly can't do it quickly when they're trying to place an order."

The notion is only partly cynical, in that most people only glance at labels for a few seconds, so simplicity is apropos. And the people who consume the most junk calories are also those with the least education and health literacy. These are the populations with the highest rates of obesity and related chronic diseases. These labels need to reach and speak to, most importantly, those at-risk groups.

Bleich launched her initial 2011 foray into exercise-labeling investigation after driving around some of Baltimore's low-income neighborhoods. She saw boarded-up houses and drug trade on the corners. "I thought, do the people who live here care about the calories in the food they eat?" She posted the exercise-equivalent signs in one corner store, to good effect. "In fact, they did."

Today's new results explore a larger, more substantial version of that original study, including the subtleties of changes in people's decisions. The research looked exclusively at black middle-school and high-school students in urban Baltimore. On average, black adolescents drink twice as much soda and juice as the American Health Association guidelines recommend at maximum. The exercise equivalents were based on average metabolic rates for 15-year-olds weighing 110 pounds.

So the conversion from calories to exercise is far from universal. But everything in nutrition is based on averages, Bleich notes, including the generic dietary recommendation of 2,000 calories daily, which is a very rough estimate that depends on age, weight, activity levels, basal metabolic rate, and even environmental temperature. Some athletes need closer to 4,000. But, she says, providing a usable benchmark is much better than an absolute number like a calorie count, which has little relevance to many people. "That's the beauty of this system," she said. "Federal regulations already require that this nutrition information is conveyed. Why not give it in the most digestible form, in a way that's shown to have the largest impact on behavior?"