WASHINGTON (AP) - If men ever needed a reason to exercise, this may be it. A study finds that exercisers are less likely to become impotent.

Compared with men who did not exercise, men who burned 200 calories or more a day in physical activity - a level that can be met with as little as two miles of brisk walking - had far less risk of erectile dysfunction.

"If you do at least that, you can reduce your risk by half," said researcher Carol A. Derby of New England Research Institutes of Watertown, Mass., a private health research firm. "Men who were sedentary had the highest risk."

Derby and her colleagues looked at data on 593 men in the long-running Massachusetts Male Aging Study. The men, who were 40 to 70 years of age at the study's start, completed a mail-in questionnaire about their sexual health. None reported erectile dysfunction at the start of the nearly nine-year study; at the end, 17 percent did.

The study, which experts said was the first long-term look at how to prevent impotence through a healthy lifestyle, found a benefit in taking up exercise. Even those who started exercise during the study period lowered their risk, said the report in the journal Urology.

"Even if you were sedentary at the beginning, the men who were active at the followup had a much lower risk ... comparable to the people who were active at both time points," Derby said.

The researchers also found that the more exercise the men did, the lower the chances were that the men would develop impotence. But there were too few vigorous exercisers in the study to give the scientists great confidence that they could tell exactly how much the risk falls as exercise rises, Derby said.

The findings make physiological sense, said Dr. Drogo K. Montague, a urologist and head of the Center for Sexual Function at the Cleveland Clinic Foundation. Regular aerobic exercise fights vascular disease, including fat clogs that narrow the arteries, said Montague, who is not connected to the study.

Such clogs can impede blood flow through arteries that engorge the penis, so exercise should help to keep those arteries healthy and clear, Montague said.

The study did not look at whether exercise could defeat impotence after the problem develops. But another urologist who is not connected with the study doubted that it could. Few studies have found that exercise can reduce the amount of fat in already-clogged blood vessels.

"There's no evidence that exercise is a treatment," said Dr. Ira Sharlip of the University of California, San Francisco, president-elect of the Society for the Study of Impotence. "Even though it has not been studied, I would very seriously doubt that a guy who develops erectile dysfunction and goes on an exercise program, his erections would come back."

But if men start exercise while they don't have erectile disfunction, they not only may raise the chances of retaining their sexual health, they get all the other healthful benefits of exercise, the doctors say. For instance, exercise reduces the risk of cardiovascular disease and diabetes, and both conditions raise the risk of impotence.

So the doctors see lowering the risk of impotence as a good all-around motivational tool.

"It's something men are concerned about, and it's something men are concerned about now," Derby said. "The risk of a heart attack later is not as much concern."

Copyright 2000 The Associated Press. All rights reserved.