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.

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