We recently reported on a study that says serious runners marking their miles tend to think about how running kind of sucks. But how do you square that negativity with the well-known, widely reported euphoria known as runner's high? 

A new study in the journal Cell Metabolism suggests that high is the result of a decrease in the hormone leptin, which is associated with feeling sated. As your levels of leptin dwindle, they send a hunger signal to your brain, which releases dopamine to increase your stamina and keep you running in the pursuit of food. 

In the study, mice were genetically altered to interfere with their brains’ leptin signaling pathways, disrupting the release of dopamine in response to leptin. These mice ran twice as many miles on their wheels as normal mice. The decrease in leptin signaling seemed to increase the dopamine-related reward associated with running, inducing positive feelings about the task at hand—an advantage if your body thinks you're hungry and needs nutrients.

"Based on these findings, we think that a fall in leptin levels increases motivation for physical activity as a means to enhance exploration and the pursuit of food," study co-author Stephanie Fulton, a researcher at the University of Montreal, says in a press statement. “Our study also suggests that people with lower fat-adjusted leptin levels, such as high-performance marathon runners, could potentially be more susceptible to the rewarding effects of running and thus possibly more inclined to exercise,” she explains. 

Leptin probably isn’t the only hormone involved in runner’s high, but it does seem to play a vital role. Previous research has also connected lower leptin levels with the desire to move, finding associations with low leptin levels and exercise addiction as well as hyperactivity in anorexic patients

[h/t: Washington Post]