NEW YORK — Being out of shape could be more harmful to health and longevity than most people expect, according to a new, long-term study of middle-aged men. The study finds that poor physical fitness may be second only to smoking as a risk factor for premature death.

It is not news that aerobic capacity can influence lifespan. Many past epidemiological studies have found that people with low physical fitness tend to be at high risk of premature death. Conversely, people with robust aerobic capacity are likely to have long lives.

But most of those studies followed people for about 10 to 20 years, a lengthy period of time for science but nowhere near most of our actual lifespans.

Some of those studies also enrolled people who already were elderly or infirm, making it difficult to extrapolate the findings to younger, healthier people.

So for the new study, which was published this week in the European Journal of Preventive Cardiology, researchers from the University of Gothenburg in Sweden and other institutions turned to an impressively large and long-term database of information about Swedish men.

In 1963, almost 1,000 healthy 50-year-old men in Gothenburg who had been born in 1913 agreed to be studied for the rest of their lives, to help scientists better understand lifetime risks for disease, especially heart disease.

The men completed baseline health testing in 1963, including measures of their blood pressure, weight and cholesterol, and whether they exercised and smoked. Four years later, when the volunteers were 54, some underwent more extensive testing, including an exercise stress test designed to precisely determine their maximum aerobic capacity, or VO2 max.

Using the results, the scientists developed a mathematical formula to estimate the aerobic capacity of the rest of the participants.

Aerobic capacity is an interesting measure for scientists to study, because it is affected by both genetics and lifestyle.

Some portion of our VO2 max is inherited from our parents. But much of our endurance capacity is determined by our lifestyle. Being sedentary lowers VO2 max, as does being overweight. Exercise raises it.

Among this group of middle-aged men, aerobic capacities ranged from slight to impressively high, and generally reflected the men’s self-reported exercise habits. Men who said that they seldom worked out tended to have a low VO2 max.

To determine what impact fitness might have on lifespan, the scientists grouped the men into three categories: those with low, medium or high aerobic capacity at age 54. Then they followed them for almost 50 years.

During that time, the surviving volunteers completed follow-up health testing about once each decade.

The scientists also tracked deaths among the men, based on a national registry.

Then they compared the risk of relatively early death to a variety of health parameters, particularly each man’s VO2 max, blood pressure, cholesterol profile and history of smoking. (They did not include body weight as a separate measure, because it was indirectly reflected by VO2 max.)

Not surprisingly, smoking had the greatest impact on lifespan. It substantially shortened lives.

But low aerobic capacity wasn’t far behind. The men with the lowest VO2 max had a 21 percent higher risk of dying prematurely than those with middling aerobic capacity, and about a 42 percent higher risk of early death than the men who were the most fit.

Poor fitness turned out to be unhealthier even than high blood pressure or poor cholesterol profiles, the researchers found.

Highly fit men with elevated blood pressure or relatively unhealthy cholesterol profiles tended to live longer than out-of-shape men with good blood pressure and cholesterol levels.

Of course, this study cannot prove that poor fitness shortens lifespans, or explain how VO2 max might affect lifespan.

However, the findings raise the possibility that by strengthening the body, better fitness may lower the risk of a variety of chronic diseases.

As this study also involved Swedish men, whether the findings are applicable to other people, particularly women, is uncertain.

But “there is no reason not to think” of any beneficial associations between fitness and longevity, said Dr Per Ladenvall, a researcher at the Sahlgrenska Academy at the University of Gothenburg, who led the study. Past studies involving women have found such links, he said.

Encouragingly, if you are concerned about the state of your particular aerobic capacity, you can increase it just by getting up and moving. “Even small amounts of physical activity,” Dr Ladenvall said, “may have positive effects on fitness.” NEW YORK TIMES