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There's quite possibly no better feeling in the world than the feeling of calm and happiness at the completion of a tough run.
So what's the reason for this runner's high? You've probably heard that can be chalked up to a rise in something called endorphins, the so-called "happy" chemicals that induce feelings of pain relief and pleasure.
But it's actually a bit more complicated than that.
Recent studies in mice suggest that endorphins may have nothing at all to do with the so-called "runner's high." Instead, scientists think the effect could be attributed to other chemicals in the body that produce similar pain-relieving and happy feelings.
The 'endorphins make you happy' idea
The idea that increased levels of endorphins are responsible for your happy feeling after a strenuous workout arose in the 1980s, when scientists found that endorphin levels in the blood spiked after prolonged exercise. Some researchers also assumed these chemicals produced the sense of euphoria we feel after a tough workout, and the idea caught on rapidly.
But there's a problem with this explanation: Endorphins are large molecules. So large that they can't move from the blood into the brain. The blood-brain barrier is key to keeping the brain safe; it keeps certain pathogens and molecules from passing from the blood into the brain. But because endorphins can't get through, it means it's unlikely that they are the sole chemical responsible for your post-run high.
Turning to endocannabinoids
Endorphins aren't the only chemicals whose levels increase when you exercise. A chemical called anandamide does too, a September 2015 study in mice and a small 2004 study in people suggested. Anandamide is a type of endocannabinoid, a chemical that's part of a system that's in charge of moderating the psychoactive, feel-good effects of marijuana. And unlike cumbersome endorphins, anandamide can smoothly make its way from the blood to the brain.
To tease out the effects of endorphins and endocannabinoids for their 2015 paper, researchers at the Central Institute of Mental Health at the University of Heidelberg medical school directly compared the effects of both of these groups of chemicals on mice as they ran on running wheels.
The researchers found that, in addition to appearing more calm less sensitive to pain after running, the mice had higher levels of both endorphins and endocannabinoids. They also spent more time in well-lit parts of their cage, something calm, less anxious mice tend to do. They were also slightly more pain-tolerant after their stints on the wheel.
As a way to ensure that they could measure the effects of each chemical individually, the researchers first gave the mice drugs to block the effects of one of the chemicals and then another type of drug to cancel out the effects of the other chemical. When they blocked the effects of the endorphins, nothing happened — the animals remained more relaxed and pain tolerant. But when they blocked the effects of the endocannabinoids, the symptoms of the mice's runner's highs disappeared.
Their findings suggest that the mice's elevated endorphin levels, then, had little to do with their post-workout buzz.
All this research has one obvious caveat: Mice aren't humans. And the study also revealed something disappointing: You probably need to run pretty far to experience a runner's high, since the mice ran an average of more than three human miles per day (a long way for a mouse!).
Other factors at play
Still other studies suggest that neither endorphins nor endocannibinoids are the cause of the runner's high. One, for example, (also done in September 2015), found that mice with low levels of a hormone called leptin tended to run farther than mice with normal levels of leptin.
Leptin, otherwise known as the "satiety hormone," inhibits a feeling of hunger in order to regulate our energy levels. The idea is that the less full (or more hungry) you feel, the more motivated you are to keep running. And that increased motivation might make it easier to get a runner's high. "Ultimately, leptin is sending the brain a clear message: When food is scarce, it’s fun to run to chase some down," lead author of the study Maria Fernanda Fernandes told Outside Magazine.
Again, just because these results have been demonstrated in mice, it doesn't mean they'll necessarily be found in humans too. And there might be a combination of factors at play, so it might be some time before we have definitive evidence of what exactly is causing a runner's high.
An earlier version of this post was written by Tanya Lewis.
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