Anyone who has run more than a few miles with some regularity has experienced what is usually called a "runner's high," an overwhelming feeling of euphoria and well-being that makes the running experience something far more rewarding than just moving forward toward an end point.
As a dedicated endurance trail runner, I can attest to this feeling and the craving for more. Although this is not the only reason why people run, we come back, again and again, hoping for these almost magical moments, that come and go as we move along the road or the trail.
Until recently, people associated such feelings with the release of endorphins, a group of hormones that activate the body's opiate receptors, acting as pleasant painkillers. However, a study published last fall in the Proceedings of the National Academy of Sciences proposes different body-secreted chemicals as the trigger, endocannabinoids, cousins of the familiar cannabinoids in marijuana.
Apparently, runners do get high from our body's own drug.
Endorphins are large molecules, too big to flow freely within brain receptors. They work well in muscles to alleviate pain, but seem to have little to do with feelings of euphoria and well-being that must be triggered within the brain.
Endocannabinoids, on the other hand, are smaller molecules, and have been conjectured to be the culprits of the much-coveted runner's high. To test this hypothesis, scientists from the Central Institute of Mental Health at the University of Heidelberg medical school in Mannheim, Germany, put a group of healthy lab mice to run on a wheel, an activity they seem to enjoy. Indeed, scientists have noted elevated levels of both endorphins and endocannabinoids in mice and in people after an enjoyable but strenuous run.
First, the scientists put the mice in cages with bright and dark spots. When anxious, mice tend to go to the dark spots, in search of shelter. A more relaxed mouse would instead be more comfortable in a bright spot. As expected, the mice tended to spend more time in bright spots after a good run. This behavior didn't change when scientists blocked the receptors for endorphins: The mice still experienced their version of a runner's high. However, when the scientists blocked the receptors for endocannabinoids leaving open those for endorphins, the mice acted as anxious as ever, showing none of the calming effects from their run. They recoiled to the dark spots in their cages.
The study also showed that the benefits from the endocannabinoids only kick in after some considerable mileage: The mice showing the less anxious behavior averaged more than three miles everyday on their wheels. It's not clear how these numbers translate to humans, but we can assume that we need to work hard to experience a legitimate runner's high. I imagine the effects are different for different people, and it will take some experimenting before you find your groove.
From an evolutionary perspective, the results make a lot of sense. We evolved to hunt fast moving animals and to escape from equally-fast predators. Such prolonged runs are tiring and painful. If we get a reward from working hard, our chances of running longer and faster are increased. And, with them, our chances of survival. As Christopher McDougall pointed out in Born to Run, his inspiring book about long-distance running, running is in our DNA, in how our bodies evolved to have an Achilles tendon and sweat glands and a series of other adaptations that make us all into potential marathoners.
Not that we need to run 26.2 miles to experience a runner's high. Fortunately, lions and antelopes don't run that far. As most physiological benefits of running seem to be at a peak between 90 minutes and 2 hours, 30 minutes — a 10-mile run at a 10-minute pace is plenty. It may sound like a lot to the non-runner, but you build-up your distance slowly and carefully to avoid injury. I should know, as I've been training for my first trail ultra-marathon this coming Saturday. I picked a shorter distance, 33 miles, to see how it goes. And, more importantly, I picked a beautiful place, Tuscany, so that — if things don't go well — I can at least enjoy the scenery and, endocannabinoids lacking, I can try some of the amazing local wines after the race.
Soon-to-be ultra-marathoner Marcelo Gleiser is a theoretical physicist and cosmologist — and professor of natural philosophy, physics and astronomy at Dartmouth College. He is the co-founder of 13.7, a prolific author of papers and essays, and active promoter of science to the general public. His latest book is The Island of Knowledge: The Limits of Science and the Search for Meaning. You can keep up with Marcelo on Facebook and Twitter: @mgleiser.