Runner's high has been described as many things: euphoria, a burst of energy, or feelings of zen and clear-headedness, to name a few. It's also been a biological mystery for years, and an elusive goal for some runners who have never experienced it themselves. Up to this point, most research on runner's high has suggested that the phenomenon occurs when the body releases pain-blocking endorphins and addictive, drug-like endocannabinoid chemicals in response to physical stress of exercise. But a study published this month in the journal Cell Metabolism suggests another explanation. Leptin, a hormone secreted by fatty adipose tissue that regulates feelings of hunger and satiety, also appears to play a role in the rewarding effects of running.
"Our study shows that this phenomenon is not just a matter of endorphins," says lead author Maria Fernanda Fernandes, a post-doctoral fellow at the University of Guelph in Ontario. When she and her colleagues tested two groups of mice—one of which had been genetically modified to have impaired leptin levels—they found that the low-leptin critters logged nearly twice as many miles on a running wheel as their genetically normal peers.
This may serve an evolutionary purpose, she says, since running was once a means of finding and pursuing food sources. "We speculate that for humans, low leptin levels increase motivation to exercise and make it easier to get a runner’s high," she explains. "Ultimately, leptin is sending the brain a clear message: When food is scarce, it’s fun to run to chase some down."
Previous research has also shown a correlation between leptin and marathon times: the lower people's leptin levels are, the faster they run.
So does lower leptin equal faster and more enjoyable runs? It's very likely—which may be why short and easy pre-breakfast runs, when leptin is naturally low after you've fasted all night, can be so invigorating, says Fernandes. But don't start skipping meals just yet, at least not before long or hard runs.
"Our findings do not mean that people should purposefully run too far or too often on an empty stomach, as this can be very harmful for the health and act as an unpleasant experience far from euphoric," she says. "It's important to fuel the body properly before running exercise so that you can run and feel your best."
There may be one way you can safely and permanently alter your body's leptin production, however: Run more.
"Chronic running exercise is well known to reduce leptin levels," says Fernandes. So the benefits may be cyclical: Running gives you lower leptin levels, which may make you more susceptible to runner's high and more inclined to exercise.
The bottom line: Leptin seems to be at least partially responsible for the rewarding effects of running, but it may not be the only metabolic chemical at play. More studies, preferably in humans, are needed—and of course there are plenty of other factors (including psychological ones) that will affect how much a person enjoys exercise.
But if you've never felt runner's high yourself, keep chasing it. The more often and more consistently you run, this study suggests, the better your body may be conditioned to achieve it.