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 thatcan be chalked up to a rise insomething 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 allto do with the so-called "runner's high." Instead, scientists think the effectcould be attributed to other chemicals in the body that produce similar pain-relieving and happy feelings.The 'endorphins make you happy'ideaThe idea that increased levels ofendorphins are responsiblefor your happy feeling after a strenuous workout arosein the 1980s, whenscientists found that endorphin levels in the blood spiked after prolonged exercise. Some researchers also assumedthese 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 largemolecules. So large that they can'tmove from the bloodinto the brain. The blood-brain barrier is key to keeping the brain safe; it keeps certain pathogensand molecules from passing from the blood into the brain. But because endorphins can't get through, itmeans it's unlikely that theyare the solechemicalresponsible for yourpost-run high.Turning to endocannabinoidsEndorphins aren't the only chemicals whose levelsincrease when you exercise. A chemical called anandamide does too, aSeptember 2015 study in miceand a small 2004 studyin peoplesuggested.Anandamide is a type of endocannabinoid, a chemical that's part of a system that's in charge ofmoderatingthe 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 onmice as they ran on running wheels.The researchers found that, in addition toappearing more calm lesssensitive to pain after running, the micehad 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 ofone 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 animalsremained more relaxed and pain tolerant. But when they blocked the effects of the endocannabinoids, the symptoms of the mice'srunner's highs disappeared.Their findings suggestthat the mice's elevated endorphinlevels, then,had littleto 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 wayfor a mouse!).Otherfactors at playStill other studies suggest that neither endorphins nor endocannibinoids are the cause of the runner's high. One, for example,(also done inSeptember 2015),found that mice withlow levels of a hormone called leptin tendedto 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, its 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 demonstratedin mice, it doesn't mean they'll necessarily be found inhumans too. Andthere might be a combination of factors at play, so it might be some time before we have definitive evidenceof whatexactly iscausing a runner's high.An earlier version of this post was written by Tanya Lewis.NEXT UP:8 killer arm workouts you can do virtually anywhereSEE ALSO:13 exercises to keep your legs strong at any ageJoin the conversation about this storyNOW WATCH: Scientists say this may be the best exercise for your brain health
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