It's probably no big surprise to learn that the more tired your brain — after a long day at work, say — the more you eat. But a new study shows that there's a very simple way to fix this: A 15-minute workout.
Researchers at the University of Alabama at Birmingham lured 38 college students to the lab with a free pizza dinner and told them to devour as much as they wanted. Then, on separate occasions, all of the students fasted for two hours before completing 20 minutes of rigorous graduate-level reading comprehension and math problems. Following the exhausting mental work, half of the group got to relax and do nothing for 15 minutes, while the rest hopped on the treadmill for 15 minutes of high-intensity intervals. Then it was time for pizza.
Sure enough, the students who chilled out after the exam consumed 100 more calories, on average, than they did at the first pizza feeding. This showed, just like past studies have, that taxing the brain can make people eat more. “Even though you’re not physically moving, the brain is still doing work and utilizing the body’s limited energy stores, primarily glucose,” says lead researcher William Neumeier. “Glucose is tightly regulated, so the brain signals, 'Hey, we need to replenish all this energy we’ve used,' even though the number of calories burned is very minimal.”
Exercising after a tough mental task proved to have the opposite effect. The participants who worked out afterward ate 25 fewer calories than they had before, suggesting that busting a sweat after busting your brain may make you less hungry. At first, 25 fewer calories may not seem so significant. But once you factor in the calories the kids burned on the treadmills, the differential was really more like 200 calories, says Neumeier.
So what’s going on here? Most likely, it’s a mix of psychological and physiological factors. For one, exercise might distract the brain from thinking about food. “It puts a buffer between completing tough mental work and having immediate access to food,” Neumeier says. Also, previous research has shown that one hour or less of exercise can increase satiety. As Neumeier explains, intense physical activity appears to affect hormones such as leptin, which regulate hunger and fullness. If you go longer than an hour, you’ll have depleted your glucose stores and need to refuel. But if you do shorter bouts of exercise that don’t drain your tank, that seems to suppress appetite.
Based on this evidence, the idea that exercise makes you eat more is looking like mere myth. “According to our study, it appears exercise will reduce food consumption, at least in the short term, in addition to the fact you’ve burned calories,” Neumeier says. “I’d definitely suggest a quick workout after work. If nothing else, you’re expending energy, which is extremely beneficial.”