The runner's high: every athlete has heard of it, most believe in it and many say they've experienced it.

But scientists have reserved judgment because no rigorous test confirmed its existence.

Until now.

Researchers in Germany, using advances in neuroscience, report in the current issue of the journal Cerebral Cortex that the folk belief is true: running does elicit a flood of endorphins in the brain. The endorphins are associated with mood changes and the more endorphins a runner's body pumps out, the greater the effect.

Leading endorphin researchers not associated with the study say they accept its findings.

"Impressive," says Solomon Snyder, a neuroscience professor at Johns Hopkins and a discoverer of endorphins in the 1970s.

"I like it," says Huda Akil, a professor of neurosciences at the University of Michigan. "This is the first time someone took this head on. It wasn't that the idea was not the right idea. It was that the evidence was not there."

For athletes, the study offers a vindication that runner's high is not just a New Age-y excuse for their claims of feeling good after a hard workout.

Some people reported they felt so good when they exercised that it was as if they had taken mood-altering drugs. Others who say they'd experienced a runner's high found it to be infrequent – they might feel relaxed or at peace after exercising, but only occasionally euphoric.

Often, those who say they experienced an intense euphoria report that it came after an endurance event. Some, such as marathon runner Marian Westley, had such volatile emotions that the sight of a puppy could make her weep. A short, intense effort, like a five-kilometre race, induced the high in still others who pushed themselves near to the point of collapse.

And yet, there are those like Annie Hiniker, who says that when she finishes a 5-K race, the last thing she feels is euphoric. "I feel like I want to throw up," she says.

For more than 30 years, the runner's high hypothesis has been unproven since spinal taps were the only way to measure endorphins in the brain. The theory proposed there were real biochemical effects of exercise on the brain as the release of endorphins – the brain's naturally occurring opiates – could change an athlete's mood. Running was not the only way to get the feeling; it could also occur with most intense or endurance exercise.

But now medical technology has caught up with exercise lore.

For athletes and nonathletes alike, the results are opening a new chapter in exercise science. It is now possible to define and measure the runner's high, and should be possible to figure out what brings it on.

The results also offer hope for those who do not enjoy exercise, but do it anyway, as motivation to elicit a feeling that makes working out positively addictive.

The lead researcher for the study, Dr. Henning Boecker of the University of Bonn, says he got the idea of testing the endorphin hypothesis when he realized that methods he and others were using to study pain were directly applicable.

The idea was to use 3-D imaging PET scans, combined with recently available chemicals that reveal endorphins in the brain, to compare runners' brains before and after a long run. If the scans showed that endorphins were being produced and were attaching themselves to areas of the brain involved with mood, that would be direct evidence for the endorphin hypothesis. And if the runners, who were not told what the study was looking for, also reported mood changes with intensities matching the amount of endorphins produced, that would be another clincher for the argument.

Boecker and his colleagues recruited 10 distance runners and told them they were doing a broader study of opioid receptors in the brain and not specifically the release of endorphins, and the runner's high. The athletes had PET scans before and after two-hour runs. They also took a standard psychological test that indicated their moods before and after running.

The data showed that, indeed, endorphins were produced during running and were attaching themselves to areas of the brain associated with emotions – in particular the limbic and prefrontal areas.

The limbic and prefrontal areas, Boecker says, are activated when people are involved in romantic love affairs or, he says, "when you hear music that gives you a chill of euphoria, like Rachmaninoff's Piano Concerto No. 3." The greater the euphoria the runners reported, the more endorphins in their brain.

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"Some people have these really extreme experiences with very long or intensive training," says Boecker, a casual runner and cyclist who says he feels completely relaxed and his head is clearer after a run.

That calm and clarity also occurred in the study subjects: "You could really see the difference after two hours of running. You could see it in their faces," he says.

In a follow-up study, Boecker is investigating whether running affects pain perception. "There are studies that showed enhanced pain tolerance in runners," he says. "You have to give higher pain stimuli before they say, `Okay, this hurts.'''