Poor physical fitness may be a runner-up second only to smoking in putting people at risk for premature death. That’s the finding of a new study of middle-aged men that focuses on aerobic capacity. The study’s results indicate a sedentary lifestyle does more damage to our health than most of us realize.

We inherit part of our maximum aerobic capacity from our parents, but our lifestyle determines a lot of our aerobic endurance capacity. A sedentary lifestyle and being overweight reduces aerobic capacity, while exercise raises it.

We already know that aerobic capacity influences longevity. Past research has concluded that people who are out of shape tend to be at high risk of premature death. Yet people who are physically fit and who have good robust aerobic capacity tend to live a long life.

This new study not only concludes that physical fitness prolongs life, but also puts physical fitness in context with other health factors influencing longevity. The research found a low aerobic capacity to be even unhealthier than high blood pressure or poor cholesterol.

Take the impact of elevated blood pressure and unhealthy cholesterol on very fit subjects and subjects in bad physical shape. Very fit men tended to live longer than out-of-shape men, even when the fit men had high blood pressure or bad cholesterol levels and the out-of-shape men had low blood pressure and low cholesterol.

The research found that the greatest detriment to longevity was smoking, which significantly shortened lives. But not far behind was low aerobic capacity. Subjects with the lowest aerobic capacity were at risk of dying prematurely 21 percent more than subjects with a mid-range aerobic capacity. They were at risk of suffering early death 42 percent more than the most fit men in the study.

Scientists surmise that better fitness reduces the risk of chronic diseases by building the body’s strength. The study didn’t prove that poor fitness shortened lifespans or explained how maximum aerobic capacity affects lifespans.

The new study by researchers from the University of Gothenburg in Sweden and other institutions was published this week in the European Journal of Preventive Cardiology. In 1963, they began studying 1,000 healthy 50-year-old men in Gothenburg who had been born in 1913. These men agreed to participate to help scientists look at lifetime risks for disease, especially heart disease. Researchers followed these men for almost 50 years. It is not certain whether the study results apply to women.

But one researcher who led the study said there is no doubt that all of us could draw useful conclusions between fitness and longevity. He noted that earlier studies involving women have found beneficial associations between the two. Even a little physical activity may have positive effects on fitness, the researcher says.