You’ve probably heard of it. Perhaps you’ve experienced it. Runner’s high is the “high” produced by strenuous exercise, such as long distance running. Symptoms include analgesia (pain relief) and a subjective feeling of well-being or euphoria. There has long been a scientific debate about whether there is such a thing as a runner’s high, and if so, what mechanisms are behind it. Today, the consensus seems to be that there is, and we think we know roughly how it works.
The term endorphin, short for endogenous morphine, is often invoked to explain runner’s high. Endogenous means originating within the body, as opposed to introducing exogenous morphine, e.g., by injection. Endorphins are not literally morphine, but rather morphine-like in that they share with morphine an affinity for binding to the so-called opioid receptors in the brain. These are the same receptors that heroin (which metabolizes to morphine), oxycodone and codeine bind to, and activating these receptors in turn leads to the release of dopamine, one of the neurotransmitters principally responsible for pleasure in the brain.
Endorphins are connected to runner’s high through circumstantial evidence and evidence from animal studies. One particularly interesting study forced mice to swim every two hours. When the mice were let off their exercise routine, they exhibited symptoms similar to morphine withdrawal. The mice, apparently, had become physically addicted to opiates produced in their own body during exercise, and when they stopped the exercise regimen, they acted as if they’d been cut off from a serious morphine dependency.
In humans, indirect measures such as endorphin levels in the blood serum pointed towards endorphins being behind runner’s high. On the other hand, this need not indicate that there are heightened levels of endorphins in the central nervous system, which is where they need to be to impact mood and pain sensations. More important, perhaps, is the fact that runner’s high can be negated with naloxone, an anti-opioid agent often administered to people who overdose on opiates. This is what you’d expect if the effects were really due to endogenous opioids.
In 2008, German researchers tried measuring endorphins directly in humans after exercise. In their pilot study, they recruited ten experienced distance runners. The subjects were screened for drug abuse, and only runners who stated that they had experienced runner’s high were included. The volunteers were then subjected to PET scans before and after a two-hour long distance run. They were also asked to rate their mood before and after. The researchers found that, as expected, the runners reported being significantly happier right after a long run than right before or on rest days. Accompanying their subjective reports, the objective brain scans found significant activation of opioid receptors thirty minutes after a long run. This was the first direct, in vivo evidence of an endorphin-generated runner’s high.
The last word on runner’s high hasn’t been said. After all, not everyone who exercises experiences the “high.” In addition, there are other factors that obviously could account for feeling up after a hard workout: the satisfaction of completing a hard challenge, bettering one’s self-image through developing a fitter body, and so on.
If endogenous opioids are really responsible for the feelings of well-being working out gives us, that could explain the well-documented inverse relationship between depression and regular exercise. Some studies have found that regular exercise is as effective, or even more effective than antidepressants or cognitive-behavioral therapy for treating depression. In addition, you’re significantly less likely to become depressed in the first place if you work out. Finally, it could explain stories of injured athletes who continue to train despite doctors’ advice to the contrary and injure themselves further. Perhaps, like mice, humans really can get physically addicted to the good feeling of a hard workout.