We all chase the elusive runner’s high. Some people describe it as “feeling strong” while other describe it as a sense of mental clarity. Suffice to say, it feels good.

But most highs are addictive; the more of the stimulus you get, the more of it you need to feel that same level of pleasure. A runner’s high is different. According to recent research published in Medicine & Science in Sport & Exercise (MSSE), the fitter you get, the better the high.

Fitness is table stakes for the runner’s high

In the scientific literature, there’s little consensus about what a runner’s high actually is. The problem is that it can’t be reliably stimulated in a research environment, and runners’ descriptions of the pleasurable sensations they feel vary wildly. Together, these two issues make the phenomenon difficult to study in a rigorous, consistent way. However, one thing that has been shown consistently is that no one experiences a runner’s high, whatever it may be, early in a run.

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“It’s usually at least 30 minutes into a run, which automatically tells you that you have to be able to run without being exhausted for at least 30 minutes,” said Panteleimon Ekkekakis, Ph.D., a professor of exercise psychology at Iowa State University.

“You have to be a pretty fit runner to even have the biological substrate to feel whatever runner’s high is.”

Fitness and beta-endorphins

A likely candidate for the “biological substrate” Ekkekakis is referring to is beta-endorphin: a type of naturally occurring opiate that boosts mood and reduces pain similarly to its humanmade counterpart, morphine. The original study connecting beta-endorphins to “runner’s high” was conducted in 2008. Researchers used positron emission tomography (PET) to scan athletes’ brains before and after they ran for two hours. They found that not only were levels of beta-endorphins higher in the athletes’ brains after they ran, but also that the athletes’ feelings of euphoria correlated with higher levels of beta-endorphins.

The recent study has extended the original research by comparing subjects’ self-reported levels of physical activity and fitness with levels of beta-endorphins in their brains. The researchers found that the fitter the subject, the higher the levels of beta-endorphins. Furthermore, on the PET scans of subjects’ brains, the researchers saw that the beta-endorphins were released specifically in the brain networks that deal with pain and emotional regulation. As the authors of the study wrote, “more trained individuals showed greater opioid release acutely following exercise in brain regions especially relevant for reward and cognitive processing.”

The chicken or the egg

So greater fitness leads to a better runner’s high, right? Well, maybe, maybe not.

There are two caveats to consider. The first is that the sample size of this study was small—64 men—and the number of subjects who had their brains scanned was even smaller. Therefore, as any statistician will tell you, it’s too early to make any sweeping conclusions.

The second caveat is that the authors found a correlation between fitness and beta-endorphins, and correlation isn’t the same as causation. In other words, there’s a “chicken or egg” problem: Does your brain produce more endorphins because you become more fit? Or do you become more fit because you happen to have a brain that produces more endorphins?

The promise of brain plasticity

To answer this question, the next step researchers could take would be to conduct a larger training study where beginner runners have their baseline levels of beta-endorphins measured, they follow a training program for a period of six months to increase their fitness, and then they have their beta-endorphins remeasured. If those levels increase, causation can be inferred.

While there is no evidence yet, Ekkekakis suspects that if such a study is conducted, this is the result researchers will find.

“If I were a betting man and you asked me to put my money on whether the genetic predisposition comes first or you can get an effect from training, I’d say you can probably get an effect from training because we know that the brain is plastic,” he said. “The brain adapts. So if you consistently stimulate any system, including systems in the brain, over time, you’re going to get an adaptation.”

Keep the faith, new runners

Now what if you haven’t worked up to 30 minutes of running yet? Are you doomed to never experience a runner’s high?

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Fortunately, the brain has developed a reward mechanism for this situation, too. Instead of offering up “natural morphine,” the way it does when you put in a prolonged, strenuous effort, your brain administers “natural pot,” otherwise known as endocannabinoids. These are a type of neurotransmitter that is released when you do something enjoyable. It’s not same rush you’ll feel after a HIIT class (that’s likely beta-endorphins); it’s the pleasant fulfillment you feel when you go for a walk outside or a light jog with a friend.

“These two neuromodulatory systems—the opioid and endocannabinoid systems—have somewhat similar effects,” said Tiina Saanijoki, a researcher at the University of Turku and Åbo Akademi University in Finland and the lead author on the MSSE study.

“Both are involved, for instance, in pain processing, addiction, and mood regulation. They also interact with each other and likely they both have a role in the various mood effects of exercise.”

So whether you’re just getting started or in the thick of regular training, your brain is doing its best to keep you running.

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