That rush of euphoria after a long run doesn't come from endorphins - long believed to be the source - but from the same chemical compounds found in marijuana, suggests new research on mice, reported by the The New York Times.
Since the 1980s, runners and other athletes have commonly uttered the phrase, "my endorphins are kicking in" as they start to sense the feel-good, floaty feeling that can come with prolonged exercise. That notion originated with studies in that decade that found endorphin levels in the blood spiked after exercise. The thinking was that the chemicals produced that euphoric sensation in the brain.
One problem, however, is endorphins are too big to fit through the brain-blood barrier. So what was behind the sensation? In recent years, scientists have zeroed-in on another possibility - the same compound found in marijuana.
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It turns out our bodies create endocannabinoids - basically a naturally-made version of the cannabinoid molecules found in pot.These molecules are small enough to cross into the brain and attach to receptors to create that runner's high sensation.
To be sure that endocannabinoids are at play and not endorphins, researchers with the Central Institute of Mental Health at the University of Heidelberg medical school in Mannheim, Germany, gathered a group of lab mice, tested their anxiety levels and then let them run. As expected, after running on their wheels for the equivalent of an impressive three miles, the mice appeared more relaxed, less sensitive to pain and hung out longer in well-lit parts of their cages - something anxious mice are less prone to do.
Next the team gave the mice two sets of drugs. One set blocked the effects of endorphins, while the other blocked endocannabinoids. When endorphins were blocked, the mice still showed all the signs of runner's high. When endocannabinoids were blocked, however, the mice showed none of the signs of an anxiety-free, post exercise buzz.
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The team published their research this week in Proceedings of the National Academy of Sciences.
The findings aren't definitive since they were carried out in mice, not people. But they do at least point to a much more probable explanation for just how our bodies reward us for pushing through pain - and running long.