To find out, 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. And as another provocative new study of brain activity after exercise found, some overweight, sedentary people respond to exercise by revving their food-reward systems, not dampening them. In that study, published last year in The Journal of Obesity, 34 heavy men and women began a supervised, five-day-a-week exercise program, designed so that each participant would burn about 500 calories per workout. They were allowed to eat at will throughout the experiment. Twelve weeks later, 20 of the group had lost considerable weight, about 11 pounds on average. But 14 had not, dropping only a pound or two, if any. Those 14, dubbed non responders, also had displayed the highest brain responses to food cues following exercise when the study began. After three months, they retained that undesirable lead. Their food-reward networks lit up riotously after exercise at the sight of food, and in fact showed more enthusiasm now than at the start of the study. The responders' brains, in contrast, responded with a relative ˜meh' to food pictures after exercise.
