Running too Far may Release a Stress Hormone

David Nieman surveyed 2,311 runners training for the Los Angeles marathon, including in his study both those who raced and those who dropped out for nonmedical reasons. He found that those who raced were six times more likely to get colds afterwards, a sign of how stressful the long race can be.

To see which immune components might be involved, he then coaxed 50 marathoners into his lab - by offering $ 100 a head - and asked them to run for 90 minutes to three hours on a treadmill at 80 percent of maximum aerobic capacity.

By measuring blood levels of immune cells - neutrophils, monocytes, natural killer cells, T cells and B cells - before and after the run, he found immune cells "leave the blood and go to muscles," presumably to repair damaged muscle fibers.

This suggests, he says, that the risk of infection may increase temporarily after a long, hard workout because "the front lines don't have as many soldiers," though immune function does bounce back to normal nine to 24 hours later.

Nieman also measured runners' levels of cortisol, a hormone and powerful immunosuppressant that soars in response to both mental and physical stress. He found that after a 3-hour morning run, cortisol soars and stays high all day.

For that reason, Nieman recommends that exercisers stay out of "cortisol country." The way to do that, his studies show, is to work out at 70 percent of your maximum heart rate for no more than 90 minutes per session.

The bottom line, says exercise physiologist Edmund Burke at the University of Colorado, is that while exercise is necessary, if done to excess "it's a double-edged sword."

And while most of us need to hear primarily the first half of that message, some need to hear the last. Indeed, Bicycling magazine, citing two small but alarming studies in Poland and Australia, recently warned that while riding hundreds of miles a week leads to fitness, it may also lead to impaired immunity.

Nieman came to that same conclusion two years ago. After running 58 marathons, he quit. "My own research was convincing me it was not a healthy thing to do," he said.