It turns out that the runner's high isn't just thanks to endorphins. The hormone leptin, which is more commonly associated with appetite, might also come into play, according to a new study in mice.
The study from the University of Montreal Hospital Research Center found that exercise-induced bliss is ignited by low levels of leptin, the body's hunger hormone. When leptin dwindles, the theory goes, the body is told it has to go hunting and gathering, and the brain's reward and motivation center lights up.
"Our findings now show that this hormone also plays a vital role in motivation to run, which may be related to searching for food," said study co-author Stephanie Fulton, a professor in the University of Montreal's Department of Nutrition, in a statement.
Scientists suspect that our capacity for endurance running evolved to boost our chances of finding food (the longer you can run, the more likely you are to catch up to that delicious-looking rabbit). When leptin levels are high, your brain is told that it has enough food. When leptin levels are low, the brain is sent a motivated, caveman-like signal to keep going -- and is rewarded for this stick-to-itiveness with feelings of elation, the runner's high.
For the study, the researchers observed two sets of caged mice on running wheels. One group was genetically modified to have lower levels of leptin, and the other group was untouched. The modified group voluntarily ran twice as long compared to their unmodified counterparts.
That matches existing data on endurance runners -- long distance runners with brag-worthy marathon times, in particular -- which shows that the most elite among them have low levels of leptin. This new research suggests that athletic performance can, in part, be attributed to the evolutionary drive to run on empty.
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