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 suggest, 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 in which their heads were draped in functional MRI 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.
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 MRI tables. Each volunteer then swapped activities for the second session.
Immediately afterward, participants watched a series of photos flash onto computer screens. Some depicted fruits and vegetables or nourishing grains, while others showcased glistening cheeseburgers, ice cream sundaes and cookies.
In the volunteers who had been sitting for an hour, the food-reward system lit up, especially when they viewed 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.
“Responsiveness to food cues was significantly reduced after exercise,” said Todd Hagobian, a professor of kinesiology at California Polytechnic who oversaw the study, published in The Journal of Applied Physiology. “That reduction was spread across many different regions of the brain,” he continued, “including those that affect liking and wanting food, and the motivation to seek out food.”
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.
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 nonresponders, also had displayed the highest brain responses to food cues following exercise when the study began. After three months, they retained that lead. Their food-reward networks lit up riotously after exercise at the sight of food, and in fact showed more enthusiasm than at the start of the study.
The responders’ brains, in contrast, responded with a relative “meh” to food pictures after exercise.
What all of this suggests, Hagobian of Cal Poly said, is that “exercise has a definite impact on food reward regions.” But that effect may depend on who you are and what kind of exercise you do, he said. His fit young subjects, he noted, completed strenuous endurance sessions. “It’s likely that in order to achieve weight loss and weight maintenance, you need to do a fair amount of exercise, and do it often,” he said.
For exercise to noticeably dampen your desire for food, in other words, you may need to sweat for an hour. It may also help if you’re already lean and in shape.