[Running] The 'runner's high' isn't really an endorphin rush. Scientists say it's your body dosing itself with something else entirely

[Brandon18]

For decades, popular science credited endorphins for the euphoric feeling long distance runners describe after sustained exercise.

That explanation has a fatal flaw though, beta-endorphins are simply too large as molecules to cross the blood-brain barrier, meaning whatever is happening in the blood cannot directly be causing changes in the brain itself. Researchers have since shifted attention to endocannabinoids, compounds the body produces naturally that act on some of the same receptors as THC in cannabis. Unlike endorphins, these molecules are lipophilic and cross into the brain easily, and a landmark 2015 study on mice led by Johannes Fuss found that blocking cannabinoid receptors entirely eliminated the anxiety reduction and pain relief associated with running, while blocking opioid receptors had no effect at all on those same responses.

Anthropologist David Raichlen has proposed the runner's high might be an evolutionary reward mechanism, one that motivated early hunter-gatherers to keep pursuing prey across long distances. His comparative research measured endocannabinoid spikes after exercise in humans, dogs, natural long distance runners, and ferrets, sedentary animals, finding the effect was strongest in the two species actually built for endurance.

The Truth Behind Runners High And Other Mental Benefits Of Runningwww.hopkinsmedicine.org