Does Exercise Distort Your Perception of Hunger?

The answer? It’s complicated.

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Should you trust your hunger meter after a workout? Two studies published this year reported that contrary to popular belief, exercise doesn't make people hungrier. In fact, results showed that brisk exercise can decrease interest in food.

But other experiments have turned up evidence to the contrary. Some researchers have found that not only does exercise cause a perceived need to eat more, but that our pleasure response to food is heightened following a workout—and that even thinking about exercise can whet our appetites.

So does exercise make us hungry or doesn't it? And if it does, should it? The relationship between physical activity and calorie compensation seems to have proved complex, not just for the average person, but for scientists, as well.

A Perfect Cure for the Midday Munchies?

The most recent study to challenge the notion that people can "work up an appetite" came from Brigham Young University, in Provo, Utah. In the study, 35 women were shown pictures of food, once after a brisk 45-minute morning walk and again a week later, at the same time of day but following no exercise. During the slideshow, an electroencephalogram (EEG) that measured activity in the food reward regions of participants' brains showed decreased desire for food, aka food motivation, after the workout. (Interestingly, though food reward effects were lower on exercise day, participants reported eating approximately the same amount after viewing the food pictures on both days.)

The BYU study wasn't comprehensive, since it tested just one case each of exercise and no exercise, and the study subjects were all female. But in a similar 2012 study, scientists from California Polytechnic State University found the same results. Following 60 minutes of aerobic exercise in the morning, the brain waves of test participants, 17 men and 13 women, showed significantly reduced drive to eat.

"Hungry by Association" Phenomenon

If exercise lowers hunger, how do we explain why some people get that "empty stomach" feeling … from even just thinking about working up a sweat? A 2011 study published in Appetite identified this effect. At a mall, 94 volunteers were divided into three groups. People in group No. 1 were prompted to read a scenario about an exhausting workout. Those in group two were asked to think about a fun workout. Group three wasn't asked to consider an exercise scenario. All three groups were then offered candy and Chex Mix as a "thank you" gift for participating the activity. People in the first two groups took at least 50 percent more of each snack than those in the no-exercise-scenario group.

Obviously, hunger levels shouldn't have been affected by hypothetical exercise. What was going on? Perhaps the compensatory response is the brain's unconscious attempt to replenish energy stores used up during exercise. Or it could simply be an "I've earned a splurge" mentality. Either way, exercise-induced calorie sprees could be a problem for those exercising to manage or lose weight, according to study coauthor Brian Wansink, a Cornell University professor and author of Mindless Eating. Especially if just reading a fitness article could make you reach for snacks.

Pleasure Eaters

It's not just hypothetical exercise that can tip hunger scales. Studies have found that many people really do make a beeline for the fridge after a workout. And evidence suggests that this refueling may be psychologically, not physiologically, driven.

Psychologists at the University of Leeds, in England, observed that compensatory eating post-exercise is common among "hedonic eaters"—people who eat for pleasure rather than to maintain energy balance, according to the Journal of Clinical Endocrinology and Metabolism. In the study, "compensators" showed signs of hedonic hunger. Not only did they eat more than "non-compensators" after a high-intensity workout, but they also rated the food more palatable and had more interest in high-fat, sweet foods.

Maybe You Really Have Earned It

All in all, it seems that there are two types of exercisers: those who experience a spike in food appeal shortly after a workout, and those who don't.

For the first group, exercise-induced hunger may be psychological, and it may be connected to hedonistic eating tendencies. Then again, it may not be. Scientists say there are many variables at play in the relationship between exercise and hunger, including metabolism, gender, regularity of exercise, intensity and duration of exercise, and whether you ate before your workout. (And, by the way, you should eat before your workout.)

Besides, you may have license to be hungry after a rigorous workout. High-intensity interval training (HIIT) burns significantly more calories than does low-intensity exercise. And intense bouts of aerobic activity are believed to boost metabolism for up to several hours after a workout—referred to as the "afterburn" effect.

If you find yourself doubling-up on calories after the gym, just check in with yourself to make sure you're really hungry and not indulging for another reason. Having a healthy snack beforehand can also help curb unrestrained eating after a workout.

For the second group, you may still want to keep an eye on calorie compensation—your hunger could strike later. Some research suggests that the suppression of hunger following exercise can be explained by the temporary redistribution of blood flow during exercise, away from the intestines and to the muscles. Sometimes called exercise-induced anorexia, the effect, they say, lasts only a short time after a workout.

What's your compensation M.O.? Do you belong to the famished post-exercise club or do you find it easy to pass on food after a workout?

Hungry for more? Write to eatandrun@usnews.com with your questions, concerns, and feedback.

Chelsea Bush is a Utah-based journalist on a mission to tap the secrets of psychology to end laziness, cheeseburger addictions, and other annoying habits that keep us flabby. Join the cause here in the comments and at @chelseawriting.