It is no secret that regular exercise boasts a long list of health benefits and lots of research shows that being physically active can lead to a healthier and happier life. Physical activity can also affect psychological states like mood and self-esteem, contributing to good mental health Some mental health benefits can be felt immediately after bouts of moderate and intense aerobic exercise- like running or swimming, an effect popularly known as "The Runner's High" where people experience reduced feelings of anxiety and pain, mood elevation, calmness and even euphoria. People have even reported feeling as good as if they had taken mood-altering drugs. This has lead scientists to wonder what is going on in the brain and whether runner's high arises from similar changes in the brain biochemistry as taking certain drugs.
For decades, the endorphin theory of Runner's High was a popular one, dominating the public's understanding of the feel-good effects of exercise. Endorphins are produced within the body and brain and act on the opioid receptor system in the brain, binding to opioid receptors and activating a biological response. The main function of this system is to reduce pain signals and produce feelings of euphoria, and opioid agonist drugs such as Morphine produce similar effects by acting on this system.
This theory was loosely supported through research showing increased levels of endorphins in blood following exercise and it was thought that they were solely responsible for Runner's High. However, endorphin molecules are too big through the blood-brain barrier fully.
So, are endorphins actually involved in the neurobiological effects of Runner's High? In 2008, scientists in Germany decided to test this opioid-endorphin theory by injecting a group of runners with a tracer and scanning their brains before and after a 2-hour long run. This tracer allowed the researchers to see which brain areas were producing more endorphins. The runners also rated their mood before and after running and it was found that brain areas known to be involved in emotional regulation released opioids after endurance activity, and that this chemical release was closely related to increased happiness and euphoria ratings following running But recent research has suggested that the endocannabinoid system may also be involved.
This is the target of the main psychoactive ingredients of cannabis and its activation can also produce feelings of euphoria, reduced anxiety and wellbeing.
In 2015, German scientists carried out an elegant series of experiments using rats to demonstrate that the endocannabinoid system is a contributor to the effects of runners high. Rats were split into two groups: runners and nonrunners where The runners ran for 5 hours on an exercise wheel. Using the lightbox test, researchers found that the runners demonstrated less anxious behaviour. Rodents prefer darker areas, so if the rats spent more time exploring the lighter areas of the box, this may indicate reduced anxiety. The runner rats we're more likely to explore these bright and aversive areas than the rats that didn't run. Both groups of rats were then placed on a hotplate. The runners took a longer time to lick their paws or jump, suggesting a reduced thermal pain sensitivity.
They then did the same experiments again in rats injected with drugs that blocked the endocannabinoid receptors. They found the rats that received the endocannabinoid blockers still displayed high levels of anxiety, despite running for hours, suggesting a key role of endocannabinoid receptors in the anxiety-reducing effects of running.
Finally, mice that had been genetically modified to lack endocannabinoid receptors in the brain didn't experience the positive effects of running on anxiety at all. It seems that both the opioid-endorphin and endocannabinoid system in the brain contribute to the positive experiences of Runner's High, and that in a biochemical way, exercise can in fact, make you feel "high". But further research is needed in humans to show which exact effects are produced by which receptor system, if not both.