Spring is finally here, and if the weather looks good, many of us will head outside this weekend to stretch our legs. Some will content themselves with leisurely strolls in the park while those looking for more of a challenge may prefer to chase that legendary, though misunderstood, “runner’s high”—the heightened sense of well-being and euphoria that endurance running can produce.

Scientists used to chalk these feelings up to an increased concentration of endorphins, but we now know that the runner’s high comes from the so-called endocannabinoids, or eCBs (named for their molecular resemblance to the active ingredient of cannabis). These are powerful painkillers, released during long-distance running, that also stimulate the secretion of the neurotransmitter dopamine from various neuron populations in the brain. Dopamine is often popularly seen as the brain’s pleasure-inducing reward chemical, and while there is more to it than that, many recreational drugs—including cannabis, of course, but also nicotine, heroin and cocaine—do indeed stimulate the release of dopamine, inhibit its absorption or mimic its actions.

If running feels so good, why don’t we do more of it? Many societies suffer from a generally inadequate level of physical activity. The World Health Organization says that obesity has nearly doubled since 1980; even back in 2008, about 35% of the global adult population was overweight and thus at increased risk of cardiovascular diseases, diabetes and some cancers. Our energy-rich diet is partly to blame, particularly in the West (a relic of our hunter-gatherer past—we’re programmed to crave calories). But that wouldn’t be such a problem if we had more collective will to run off those calories.

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One clue to resolving this paradox is that only certain kinds of exercise induce the eCB high. Consider the context in which this particular neural-reward system evolved. Our ancestors needed it to persuade them to chase prey for hours on end, which would have put their systems under considerable stress. The function of the eCBs was to suppress the sensation of these unpleasant side effects in favor of the long-term gain. Simply going for a walk doesn’t stress the body nearly enough to switch on the eCB system.

This Catch-22 doesn’t just apply to humans. In a paper published in 2012 in the Journal of Experimental Biology, David Raichlen of the University of Arizona and his colleagues showed that dogs—the ultimate running junkies—are in the same bind. The concentration of eCBs in the bloodstream of his canine test subjects went up markedly after a 30-minute bout of endurance running, but not after walking.

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