By GRETCHEN REYNOLDS
| April 16, 2012, 2:35 pm
Some people respond to exercise by eating more. Others eat less. For many years, scientists thought that changes in hormones, spurred by exercise, dictated whether someone’s appetite would increase or drop after working out. But now new neuroscience is pointing to another likely cause. Exercise may change your desire to eat, two recent studies show, by altering how certain parts of your brain respond to the sight of food.
In one study, scientists brought 30 young, active men and women to a lab at California Polytechnic State University in San Luis Obispo for two experimental sessions, where they draped their heads in functional M.R.I. coils. The researchers wanted to track activity in portions of the brain known as the food-reward system, which includes the poetically named insula, putamen and rolandic operculum. These brain regions have been shown to control whether we like and want food. In general, the more cells firing there, the more we want to eat.
But it hasn’t been clear how exercise alters the food-reward network.
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.
“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.
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.