Scientists at the University of Montreal have discovered that the hormone leptin — nicknamed the "satiety hormone" — may be at play.
The primary purpose of leptin is in regulating energy stores. It signals to the body when it has enough fuel and energy, and in previous studies researchers have found that levels of leptin appear to fluctuate in people suffering from obesity, starvation, sleep deprivation and emotional stress. When you're in motion, your leptin levels may fall, and the researchers said this could "send a hunger signal to the brain's pleasure center to generate the rewarding effects of running."
Writing in the journal Cell Metabolism on Tuesday, Stephanie Fulton and her co-authors drew physiological parallels between people seeking out the rewarding effects of running and the reasons they eat when they are hungry.
In an experiment with mice, researchers compared normal mice to those that were genetically engineered to lack a leptin-sensitive protein called STAT3 that relays the leptin signal to release the reward chemical dopamine. The normal mice ran a decent amount each day, logging an average of six kilometers a day on a running wheel. But the genetically engineered mice ran an extraordinary amount, nearly twice as much as the normal mice — 11 kilometers — each day.
The new study supports previous research in humans that showed that low leptin levels are associated with exercise addiction and fast marathon times.
Fulton wrote that while leptin is probably not the only thing controlling the high of running, the study "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."