MANNHEIM, Germany, Oct. 9 (UPI) -- The high reported by runners, generally ascribed to endorphins, was found by researchers in Germany to be caused by endocannabinoids -- suggesting it is similar to the high people experience after consuming marijuana.
Endorphins, natural chemicals produced by the body, have pain relief qualities similar to morphine. During intense exercise, the stretching and tearing of muscles causes the body to increase endorphin production.
To test this, researchers at the University of Heidelberg ran three experiments with mice to find which of the chemicals is responsible for runner's high.
"If you place a running wheel in a mouse's cage, it will run voluntarily, covering great distances of about 10 km/day over twelve hours each day," Dr. Johannes Fuss, lead author of the study, told The Daily Beast. "They're really motivated to run; there are strong biological processes motivating these mice to run in running wheels, therefore they're a good model to study why humans are motivated to do exercise."
Researchers initially worked with two groups of mice, allowing one group to run on wheels in their cages and not the other. Anxiety and sensitivity to pain were measured in each of the groups, showing that running alleviates both feelings for mice. In the mice that were permitted to run, higher levels of endocannabinoids were detected.
The running mice were then given drugs to block either endorphins or endocannabinoids while running. When endorphins were blocked their anxiety and sensitivity to pain was reduced as usual. The mice whose endocannabinoids were blocked, however, no longer enjoyed these benefits of running.
In the third experiment, researchers bred mice genetically altered not to have cannabinoid receptors in their brains. These mice, when put in cages with wheels, would run as normal at first but lose interest after a few days, leading the researchers to determine that endocannabinoids are responsible for the high from running.
"A runner's high is a subjective sense of well-being some humans experience after prolonged exercise," researchers wrote in the study, published in the Proceedings of the National Academy of Sciences. "For decades, it was hypothesized that exercise-induced endorphin release is solely responsible for a runner's high, but recent evidence has suggested that endocannabinoids also may play a role."