WASHINGTON — Sorry, couch potatoes, the verdict is in: People who exercise regularly really do live longer.

In fact, people who get a good workout almost daily can add nearly four years to their life spans, according to the first study to quantify the impact of physical activity this way.

The researchers looked at records of more than 5,000 middle aged and elderly Americans and found that those who had moderate to high levels of activity lived 1.3 to 3.7 years longer than those who got little exercise, largely because they put off developing heart disease — the nation's leading killer. Men and women benefited about equally.

"This shows that physical activity really does make a difference — not only for how long you live but for how long you live a healthy life," said Oscar Franco of the Erasus M.C. University Medical Center in Rotterdam, the Netherlands, who led the study published yesterday in the Archives of Internal Medicine. "Being more physically active can give you more time."

Previous studies have found that being physically active has a host of health benefits. It reduces the risk of being overweight and of developing many illnesses, improves overall quality of life and lowers the mortality rate. But the new study is the first to directly calculate the effect on how long people live.

"This should encourage people to be more active — to take a more active role in their own health and not just sit and wait for a pill to prevent this or that or save your life," Franco said.

Franco and his colleagues analyzed data from the Framingham Heart Study, a well-known research project that has followed 5,209 residents of one Massachusetts town for more than 40 years, collecting information about lifestyles and health.

The researchers calculated the effects of low, moderate or high levels of physical activity on life span, accounting for the possible effects of factors such as age, sex, education, whether they smoked or had serious health problems.

People who engaged in moderate activity — the equivalent of walking for 30 minutes a day, five days a week — lived 1.3 to 1.5 years longer than those who were less active. Those who took on more intense exercise — the equivalent of running half an hour a day, five days a week — extended their lives by 3.5 to 3.7 years, the researchers found.

The findings show that even for people who are middle-aged, exercising more can add years to their lives, Franco said.

"This shows it's never too late to start following a healthy lifestyle. It's never too late to start exercising," Franco said. "For example, instead of taking your car to your office, why don't you take your bike or walk? Physical activity is very important for a healthy lifestyle."

Other experts said the study was consistent with the growing body of evidence that exercising on a regular basis is one of the most important things people can do for their health.

"At the end of the day, this is more evidence that the sedentary lifestyle is the most devastating to health, longevity and chronic disease development," said James Hill of the University of Colorado in Denver, adding that he hoped it might motivate people to exercise. "Putting it in terms of life expectancy is something that's relevant to people."

While adding between one and four years may not sound like a lot to some people, Franco, Hill and others said exercising regularly also enables people to live healthier lives, free from a host of chronic illnesses that can make it hard for people to enjoy life.

In addition, recent studies have also found that exercise has payoffs for the mind, too. It has been shown to improve overall well-being, reduce stress and depression and cut the risk of Alzheimer's and other forms of dementia, several experts said.

"The benefits of physical activity extend well beyond the effects on longevity," said JoAnn E. Manson of Harvard's Brigham and Women's Hospital in Boston.

The trouble is, many people seem to ignore the evidence, government recommendations and public health campaigns to be physically active.

Most Americans still fail to exercise regularly, and the number who exercise in their leisure time has been dropping, according to a recent report from the Centers for Disease Control and Prevention.

Franco and others noted that this and other studies show that people do not have to be exercise fanatics to reap benefits. Adding just a little activity to the daily routine can have major benefits.

"What we're talking about is small changes," Hill said. "We're telling people to get out and walk more. Fifteen, 20 or 30 minutes of walking each day is probably enough."