WASHINGTON (AP) - Sometimes, exercise can weaken an athlete's ability to fight off a cold. Other times, researchers say, exercise won't make a difference - or might strengthen the ability.

The difference seems to depend on how much exercise the athlete does.

"Recent evidence suggests that athletes experience mild suppression of immune function during periods of intense training," said researcher Laurel T. Mackinnon of the University of Queensland in Brisbane, Australia. "In contrast, moderate exercise training appears to have either no effect on, or enhance resistance to, upper respiratory infection."

Mackinnon's report was in a special supplement to the American College of Sports Medicine's journal Medicine and Science in Sports and Exercise. The July supplement reviewed the latest research about the effects of working out on an athlete's response to upper respiratory tract infections. These infections largely are colds, but can include other conditions, such as laryngitis.

It appears that harsh, demanding workouts slightly impair the production of white blood cells that fight infection, Mackinnon's article said. The combination of declines in several types of infection-fighters may set up the athlete for illness, it said.

It may be that the body reduces its inflammation-fighting immune cells a bit to give the exercise injuries a chance to heal by themselves, Mackinnon said.

Researchers say athletes seem more prone to symptoms of these infections after long periods of intense training, or after an exhausting competition such as a marathon. Researcher David Nieman of Appalachian State University in Boone, N.C., found that 50 percent to 70 percent of athletes report symptoms in the second week after a marathon.

Having symptoms, however, is not the same as having a full-blown upper respiratory disease, which is far less common, Nieman said. And the immune suppression itself is not as extensive as a more serious clinical immune deficiency, Mackinnon said.

Some researchers believe that the period of reduced immune protection amounts to an open window for infection. But the research to prove the window exists has not yet been done, Nieman said in an article in the supplement.

The test would be whether the athletes with the greatest suppression in their immune system would have the greatest likelihood of infection, Nieman said.

Other factors, such as stress, lack of sleep and bad nutrition may also raise risk, Nieman said. The way to aid recovery is simple: Reduce stress, get more sleep, cut back on the exhausting training - and, before an important event, avoid large crowds and sick people, he said.

For ordinary exercisers, the effect of a workout or a race seems different. If it has any effect, exercise may raise disease resistance a bit, Mackinnon said. In some of Nieman's studies, sedentary women who took up walking 45 minutes a day had 40 percent to 50 percent fewer days of symptoms than did similar women who remained sedentary.

But other researchers say the effect of moderate exercise - what the sports medicine group considers a healthy level - may have to build over time to become meaningful.

David G. Rowbottom and Katherine J. Green of Queensland University of Technology in Brisbane, Australia, say individual bouts of moderate exercise don't seem to produce enough of an effect to change the immune system.

Copyright 2000 The Associated Press. All rights reserved.