Coruscation

The researchers had the volunteers either vigorously ride computerized stationary bicycles or sit quietly for an hour before settling onto the M.R.I. tables. Each volunteer then swapped activities for their second session.

Immediately afterward, they watched a series of photos flash onto computer screens. Some depicted low-fat fruits and vegetables or nourishing grains, while others showcased glistening cheeseburgers, ice cream sundaes and cookies. A few photos that weren't of food were interspersed into the array.

In the volunteers who'd been sitting for an hour, the food-reward system lit up, especially when they sighted high-fat, sugary items.

But if they had worked out for an hour first, those same people displayed much less interest in food, according to their brain scans. Their insula and other portions of the food-reward system remained relatively quiet, even in the face of sundaes.

"Responsiveness to food cues was significantly reduced after exercise," says Todd A. Hagobian, a professor of kinesiology at California Polytechnic who oversaw the study, published last month in The Journal of Applied Physiology. "That reduction was spread across many different regions of the brain," he continues, "including those that affect liking and wanting food, and the motivation to seek out food." Though he didn't follow the volunteers after they'd left the lab to see whether they might have headed to an all-you-can-eat buffet on days they exercised, on questionnaires they indicated feeling much less interested in seeking out food after exercise than after rest.

Those results may not be typical, though. The Cal-Poly subjects uniformly were in their 20s, normal weight and fit enough to ride a bike strenuously for an hour. Many of us are not.