Why do people feel good after they exercise? Thirty years ago, scientists thought they'd come up with the answer: endorphins, hormones that the body produces during exercise that have opiate, or morphinelike, properties. This explanation for what's known as "runner's high" -- the euphoric feeling some people get from exercise -- soon became the stuff of sports legend. But just how these natural chemicals affect us -- and what other body systems might be at play -- is an ongoing debate. Here, we sort through the latest theories.
The big breakthrough
Endorphins were discovered when researchers looked into a well-known phenomenon -- our ability to get intoxicated from morphine and heroin. They reasoned that we evolved this capacity not to get high from these synthetic drugs but to benefit from endogenous compounds -- that is, substances our bodies produce naturally. In the mid-1970s, scientists identified several such opiatelike chemicals that act as messengers throughout the body. Combining the words endogenous and morphine, they coined a new word, endorphin.
Some 25,000 studies later, researchers have yet to figure out whether the increases in endorphin levels in the bloodstream produced by exercise have any impact on endorphin levels in the brain, which would be necessary to improve mood. Because of their particular chemical makeup, synthetic opiates like morphine and heroin can enter the bloodstream and pass through the blood-brain barrier, the semipermeable tissue that protects the brain from potential toxins. By contrast, endorphins in the bloodstream cannot penetrate this barrier, which throws their role in any true runner's high into question.
Defining runner's high
The notion of runner's high itself has its critics, in part because what any one person means by "high" is impossible to quantify. Although some people feel euphoric after their daily workout, others find the experience gratifying but hardly exhilarating. People who feel especially good may simply be releasing more endorphins; on the other hand, their state of bliss could also be related to improved body image, a sense of accomplishment and a host of other factors ranging from an enhanced immune system to oxygen deprivation.
Most likely, credit for the feel-good effect -- reportedly prompted not just by running, but by cycling, tennis and nearly any other intense cardio exercise -- has to do with a number of body chemicals and systems that constantly interact. For example, feeling a rush points to the involvement of the hormone adrenaline, says Murali Doraiswamy, M.D., chief of the biological psychiatry division at Duke University School of Medicine in Durham, N.C. "Adrenaline evolved as part of our fight-or-flight response to stress, and it's what gives us that geared-up feeling."
The satisfaction associated with exercise probably also involves dopamine. This chemical messenger turns on the brain's reward system and makes people want to repeat enjoyable experiences, everything from munching chocolate or having sex to running, drinking or using illicit drugs.
Exercise and addiction
Regardless of exactly which neurotransmitters are involved in runner's high, perhaps the more important question is whether the need for a buzz can cause dependency. Traditionally the measure of addiction to any substance has been whether withdrawal from it makes you physically sick, but some researchers now think that the true test is how it affects you mentally and emotionally.
"You might not actually be physically ill if you don't run," says George Koob, Ph.D., a professor in the Department of Neuropharmacology at The Scripps Research Institute in La Jolla, Calif. "But if you feel miserable when you're not running even though you're damaging your body and wrecking your work and personal life [by doing so], you've crossed the line into addiction."
If your level of exercise is wreaking havoc on your life, clearly it's time to cut back and perhaps to seek counseling. But it seems unlikely that most of us need to worry about such a fate, if only because for most people exercise simply doesn't produce enough opiatelike substances to be dangerous or, apparently, to make us feel stoned. For this large majority, the high of exercise doesn't come from doing it, but from the feelings generated by health and fitness benefits that occur afterward. "If exercise was truly addicting, no one would put it off," says Duke's Doraiswamy. "Gym wait lists would be longer than those at the hottest restaurants, and doctors would go out of business from lack of patients. Everyone would be rail-thin."