WASHINGTON (AP) -- A new study offers an additional reason for men to exercise: It could add years to their sex lives.

Researchers hope men will learn from this, and exercise to protect their potency even if they haven't exercised to protect their hearts.

Men over 50 who kept physically active had a 30 percent lower risk of impotence than men who were inactive, the study found.

As men aged, the odds rose that they would have erectile dysfunction. But physical activity seemed to slow the process.

"One could postulate that it would at least add years to your ability to have and maintain an erection," said researcher Eric B. Rimm, an associate professor at the Harvard School of Public Health. "Those who were physically active had a lower risk."

Rimm and his colleagues looked at data from questionnaires by 31,742 men ages 53 to 90 in the Health Professionals Follow-up Study of dentists, optometrists, podiatrists, pharmacists and veterinarians. The researchers excluded men with prostate cancer because impotence can be a side effect of surgery for the cancer.

The researchers were checking which lifestyle and health factors affected the risk of erectile dysfunction, which the study defined as the ability to have and maintain an erection adequate for intercourse. Their findings were published in the August issue of the journal Annals of Internal Medicine.

Men who were physically active reported better erections, and the most physically active men did best. Men who were able to run at least 3 hours a week had a 30 percent lower risk of impotence than men who did little or no exercise. These men seemed to have the sexual ability of men two to five years younger than they were, Rimm said.

But Rimm said there was still a 15 to 20 percent reduction in the risk of erectile dysfunction among men who were able to do a brisk walk of 30 minutes most days of the week. This much activity is the minimum level that federal officials recommend for good health.

Being a vigorous exerciser and adding other healthy lifestyle factors such as not smoking, staying lean and drinking only moderately had the effect of adding 10 years to a man's sexual status, Rimm said.

Exercise seems to benefit the small arteries that control erections, much as exercise benefits other arteries, such as those that feed the heart, Rimm said. Thus, what happens to the penis may be an early warning of what could happen to the heart, such as a heart attack, he said.

Rimm and other doctors hope men will worry about their penises even if they haven't been worried about their hearts.

"We are hoping that would be a motivating factor," said Dr. Ira Sharlip, an assistant clinical professor at the University of California, San Francisco and a spokesman for the American Urological Association.

"Maybe the penis is like a barometer of what could happen to your heart and to your brain for stroke," said Dr. William Steers, professor and chairman of the urology department at the University of Virginia. Steers was not involved in the Harvard study.

"Go in IC (intensive care) and ask how many men were impotent before your heart attack, and more than 80 percent in any study had erectile dysfunction way before their heart attacks," said Dr. Irwin Goldstein, a professor of urology at Boston University School of Medicine.

But lifestyle changes such as exercise have to be made before the impotence develops, because there's no evidence that exercise afterward will undo the damage, the doctors said.

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