Running Can Trip Up Health, Expert Says

Running 10 miles a day, every day, in rain, snow and cold could make you a hero to the running club, but it may not impress a germ. One expert says that tough workouts may lower resistance to disease.

Intense aerobic workouts may increase the body’s production of hormones that depress the immune system, said David C. Nieman, an associate professor in health, leisure and exercise science at Appalachian State University in North Carolina.

On the other hand, moderate aerobic activities such as walking can increase the activity of cells that feast on bacteria and viruses, making you better able to resist disease, Nieman said.

While Nieman was at Loma Linda University in California, he and his colleagues looked at how many of the 2,300 runners who had completed the Los Angeles Marathon in 1987 came down with a cold or flu.

More than 13% were sick within a week, compared to 2% of those who trained but did not compete, Nieman said.

And it is not simply the stress of competition, the researcher said. In the two months before the race, about 40% got sick. Those who trained by running more than 60 miles a week had twice the risk of those who ran fewer than 20 miles a week, he said.

The cause could be hormonal, Nieman said. Ten experienced runners who worked out for three hours at their fastest marathon pace had higher levels of the hormone cortisol--close to 60% above their pretest levels, he said.

Cortisol is valuable for people who do long, hard exercise, because it fights inflammation. But it also depresses the activity of natural killer cells, which destroy virus-infected cells, Nieman said. His work with the 10 experienced runners, although preliminary, indicated that natural killer-cell activity fell by more than 30% for almost six hours.

He advises serious athletes to do no more than two workouts a week at their hardest level, to give the immune system time to recover.

However, exercisers who do not work out so hard do not risk weakening their immune systems as they try to improve their fitness, Nieman said.

“The 20- to 30-minute type of thing, we don’t feel that’s long enough to worry about,” he said. “I don’t think the public should be at all concerned about short jogs.”

Instead, regular, moderate exercise may improve one’s immune system, he said. He bases this on a study of 50 women, 12 of whom were started on a program of brisk 45-minute walks, five days a week. The others were in an inactive comparison group.

The walkers increased their natural killer-cell activity, Nieman said.

In addition, walking increased the activity of neutrophils, the dominant form of white blood cells, which can kill bacteria and viruses before they enter a cell, he said.

Apparently as a result, he said, the 12 walkers had half the cold or flu days of their non-walking colleagues during the 15 weeks of the study.

The increase in natural killer cells and neutrophils does not last long but may be enough to do some valuable anti-infection housecleaning, Nieman said.

If you get an upper respiratory infection such as a cold, you probably can still exercise, even though you may not feel like doing as much, said Leonard H. Calabrese, head of clinical immunology at the Cleveland Clinic Foundation.

But, if you have a lower respiratory infection such as “a real chest cold or flu,” Calabrese advises a break in exercise.