People are wired to run, according to a new study that looked at the roots of the pleasurable sensation known as runner's high.
Experienced during moderate to intense aerobic exercise, runner's high occurs when natural chemicals called endocannabinoids activate the part of the brain associated with "feeling good," said study co-author Greg Gerdeman.
"Endocannabinoids are molecules that are often referred to as the body's own marijuana-like substances, because they activate similar cellular receptors," said Gerdeman, a biologist at Eckerd College in St. Petersburg, Florida.
To test if running—and its feel-good effect—is hardwired in more active animals, Gerdeman and colleagues set up an experiment with two species of natural athletes, humans and dogs, and one species that doesn't run, the ferret. (See National Geographic Adventure's running guide.)
The team, led by the University of Arizona's David Raichlen, recruited ten humans to run and walk on a treadmill. Eight dogs and eight ferrets were specially trained to do the same.
The scientists collected blood samples before and after the subjects had exercised for 30 minutes.
Analysis of the blood samples revealed that, after exercise, a type of endocannabinoid called anandamide increased in all of the dogs and the people.
However, the ferrets saw no change in their endocannabinoid levels after exercise.
The team also gave the human runners a mood-evaluating questionnaire and found that all the subjects reported feeling happier after exercise. What's more, the greater the surge of anandamide, the bigger the boost in mood, Gerdeman noted. (See "Running Barefoot Reduces Stress—On Feet.")
The results support the idea that athletic species have a built-in motivation to run in the form of runner's high while non-athletic species, such as ferrets, do not.
Runner's High Influenced Evolution?
The findings may hint at what spurred people to evolve for long-distance running—a practice that's not only exhausting but also "makes you prone to injury in a predator-dominated world," Gerdeman noted.
If early humans experienced a runner's high, he said, "that's the neurological reward. It would have promoted ... repeated behaviors."
"But the real evolutionary payoff would have been an improved chance of survival and reproduction because of those behaviors."
For instance, the added endurance would have allowed hunter-gatherers to catch prey such as gazelle, which can run fast but not far. This strategy likely made early peoples more successful hunters, the study authors say.
Dan Lieberman, a human evolutionary biologist at Harvard University, noted that runner's high also could have made early hunters more alert.
"When you get a runner's high, everything becomes more intense," Lieberman said. "Blue becomes bluer, and you have a heightened sense of awareness."
In 2004, Lieberman and colleague Dennis Bramble published a study making the case that humans evolved to run long distances about two million years ago.
The team listed several physical adaptations—such as springy tendons and short forearms—to support their theory. (Explore the human body.)
The new study "extends that from physical features to neurobiological features as well," Lieberman said.
"You'd expect if running long distances is important for our ancestors to hunt and gather, there should be a feedback mechanism to help people do that-and runner's high is a kind of [positive] feedback."
People Evolved to Be Athletes
Though most people don't need to chase down their dinner today, running still has benefits, both scientists noted.
"If our physiology evolved for us to be effective exercisers, that may be why our cardiovascular health, our metabolic health, and our mental health" depends on it, study co-author Gerdeman said.
Harvard's Lieberman added, "The larger issue is that [many] people are out of touch with their bodies-people just don't understand what it's like to be regularly very physically active."
For instance, the average hunter-gatherer walked about 5 to 9 miles (9 to 15 kilometers) a day, he said.
Like it or not, he said, "our bodies evolved to be athletes."
The runner's high study was published in April in the Journal of Experimental Biology.