The one thing runner’s chase harder than a personal record (and OK, brunch) is the elusive runner’s high: a euphoric feeling experienced during prolonged, hard exercise.

For a long time, runners and researchers alike believed this was caused by the brain's release of endorphins. In fact, a brain imaging study conducted in 2008 was the first to scientifically prove there's an increased release of endorphins in athlete’s jogging for two hours. But in the early 2000s, Dr. Scott Weiss, board certified athletic trainer and licensed physical therapist in New York City, told Medical Daily this thinking started to shift.

Crossing The Blood-Brain Barrier

Endorphins are large molecules — and large molecules have a hard time passing through the blood-brain barrier. The barrier separates circulating blood from the brain’s extracellular fluid as a way to protect the brain from foreign, potentially harmful substances. So when endorphins are released, they’re found in the neck and below, rather than the brain and spinal cord, Weiss said. This makes sense when you consider endorphins are released from the pituitary gland located at the base of the brain.

Endocannabinoids (ECs), on the other hand, are small molecules. Weiss said they’re a lipid (fat) “almost like the cannabinoids found in marijuana.” And actually, they’re part of a whole system. Scholastic defined the EC system as “a unique communications system in the brain and body that affects many important functions, including how a person feels, moves, and reacts.”

“Combine ECs with other enzymes that get released [during exercise] and that breaks the barrier, that’s really what gives someone the runner’s high,” Weiss said.

Describing this high can range from, “I feel like I’m running on a cloud,” to “I don’t feel my body,” “I’m in the zone,” and “I feel like I’m in The Matrix, like Neo.” Everyone has a different definition of the same thing, Weiss said. There are so many ways to describe it, which makes it hard to pin-point exactly how it feels; "you’re producing your own euphoria," he said.

Though the high isn’t limited to runners, it’s not something someone doing, say, sets of squats and pull-ups are likely to experience; the key is duration. Runners tend to spend more time training for long distance races, including marathons, thus the greater association between the high and running.

High Time

Researchers from the University of Iowa found runner’s high has the potential to promote heart health. After comparing the activity levels of rats, they found “exercised rates sustained significantly less heart damage from a heart attack than non-exercised rats, suggesting opiods are responsible for some of the cardiac benefits of exercise.”

But the more immediate effects have to do with performance. Weiss said runner’s high is proven to give those who experience it a little more V02max and strength. VO2 is a measure of the maximum volume of oxygen an athlete can use. And Active cited “research has shown that VO2 accounts for roughly 70 percent of the variation in race performances among individual runners…if you are able to run a 5K one minute faster than I can, it is likely that your VO2max is higher than mine by an amount that is sufficient to account for 42 seconds of that minute.”

Of course, more VO2 has its downsides. For starters, it can be painful to breathe fast and long for as fast and long as you're running. Second, with prolonged training, these receptors become less sensitive, Weiss said.

“As you train, you don’t high so much,” he added. “Your body basically goes, ‘You haven’t pushed yourself,’ ‘You haven’t changed anything up,’ so over time, runner’s high dissipates in intensity.”

Runners and otherwise endurance exercisers can’t hack a runner’s high; it happens when it happens. Or at least not yet. Weiss hopes more research is dedicated to understanding this phenomena in the future, so that maybe one day it’s something every runner can experience.