Another factor why running can make one happy has been discovered. A certain hormone called leptin, nicknamed the "satiety" hormone, has been unveiled as another ingredient to a runner's happiness.
Leptin is from the Greek word Leptos, which means thin. It is called "fat hormone" as this is generated by fat cells. An increased leptin level sends a signal to the brain indicating you are "full" thus resulting in increasing the metabolic rate.
However, when leptin level decreases, it signifies hunger. Metabolism, hunger and energy expenditure are primarily regulated by leptin. Furthermore, this hormone not only enriches mood but also enhances brain function, mental sharpness and boosts better memory.
In an experiment conducted by Stephanie Fulton and colleagues, they revealed that the "runner's high" is not only caused by endorphins but is also affected by a dopamine, neurotransmitter for motivation.
"We discovered that the rewarding effects of endurance activity are modulated by leptin, a key hormone in metabolism. Leptin inhibits physical activity through dopamine neurons in the brain," said Fulton, the corresponding author of the published article in the journal Cell Membrane and researcher from the University of Montreal Hospital Research Centre (CRCHUM).
The study was conducted through comparing a normal and a genetically engineered deficient STAT3, a leptin-sensitive protein that sends signals to release dopamine, mice by voluntary wheel running in cages. Results revealed that normal mice ran at about six kilometers per day, while genetically engineered mice ran almost two times faster -- 11 kilometers per day.
The first author of the study said that "mice that do not have the STAT3 molecule in the dopaminergic neurons run substantially more. Conversely, normal mice are less active because leptin then activates STAT3 in the dopamine neurons, signalling that energy reserves in the body are sufficient and that there is no need to get active and go looking for food."
Even though the study was performed on mice. This can actually support previous researchers in humans on an association of leptin levels and exercise addiction and quick marathon times. A conclusion that marathon runners are driven by the insufficient amount of leptin level in the body can thus be drawn. This means, their desire to run is urged because of inadequate leptin. "Our 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," Fulton said.
Although, Fulton admittedly consider the fact that there are other factors affecting runner's high, the entire team "think(s) that a fall in leptin levels increases motivation for physical activity as a means to enhance exploration and the pursuit of food."
