Any runner knows the feeling of elation and calm they feel after an intense workout. You’ve probably heard that the feeling can be chalked up to something called endorphins, the so-called 'happy' chemicals. That may not be quite true.

A new study in mice suggests that endorphins may have nothing at all to do with the so-called runner’s high. Instead, these warm fuzzy feelings may be the result of chemical compounds made by the body called endocannabinoids. The endocannabinoid system is also responsible for moderating the psychoactive, feel-good effects of marijuana, The New York Times reports.

Endorphins are the body’s natural painkillers. Back in the 1980s, scientists found that endorphin levels in the blood spiked after prolonged exercise. From there, people made the logical leap to assume that these chemicals also produced the sense of euphoria in the brain. But there’s a problem with this theory. Endorphins are big molecules, and are too large to fit through the brain’s gatekeeper - the blood-brain barrier. So there’s no way they could be responsible for the post-run high.

Researchers have spent the last decade looking for other explanations, which led them to cannabinoids. Found in marijuana, these chemicals compounds are small enough to cross the blood-brain barrier and bind to receptors in the brain. And recent research has found higher levels of endocannabinoids (the ones the body produces naturally) in the blood of people and animals after exercise.

In the new study, researchers at the Central Institute of Mental Health at the University of Heidelberg medical school in Mannheim, Germany, directly compared the effects of endorphins and endocannabinoids. They put mice on wheels, which the animals like to run on for fun - maybe because they feel something akin to a runner's high.

The researchers found that, in addition to appearing more calm and less sensitive to pain after running, the mice had higher levels of both endorphins and endocannabinoids. In addition, they spent more time in well-lit parts of their cage, something only calm, less anxious mice do. They were also slightly more pain-tolerant.

To tease out the effects of endorphins and endocannabinoids, the researchers first gave the mice drugs to block the effects of one and then the other. When they blocked the effects of the endocannabinoids, the symptoms of the mice’s runner’s highs disappeared. When they blocked the endorphins instead, nothing happened - the animals remained more relaxed and pain tolerant.

Their findings suggest that the mice’s elevated endorphin levels, then, had little to do with their post-workout buzz.

Of course, there’s no guarantee that what happens in mice also happens in humans. And the study also revealed something depressing: You probably need to run pretty far to experience a runner’s high, since the mice ran an average of more than three miles (4.8 km) per day.

Still, the findings are interesting, and may explain how we evolved. Our ancestors had to go on long hunts to survive, so it makes sense that their bodies would produce a feel-good boost to make them feel less pain. So if you like the feel of a good long run, you may have your endocannabinoids to thank.

This article was originally published by Business Insider.

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