People who stay even moderately fit as they age may live longer than those who are out-of-shape, a new study suggests.

The study, of nearly 4,400 healthy U.S. adults, found that the roughly 20 percent with the lowest physical fitness levels were twice as likely to die over the next nine years as the 20 percent with the next-lowest fitness levels.

That was with factors like obesity, high blood pressure and diabetes taken into account -- underscoring the importance of physical fitness itself, researchers report in the journal Medicine and Science in Sports and Exercise.

Our findings suggest that sedentary lifestyle, rather than differences in cardiovascular risk factors or age, may explain (the) two-fold higher mortality rates in the least-fit versus slightly more fit healthy individuals, lead researcher Dr. Sandra Mandic, of the University of Otago in Dunedin, New Zealand, noted in an email to Reuters Health.

She pointed out that nearly two-thirds of the least-fit study participants were not getting the minimum recommended amount of exercise -- at least 30 minutes of moderate activity, like brisk walking, on five or more days a week.

These results emphasize the importance of improving and maintaining high fitness levels by engaging in regular physical activity, Mandic said, particularly in poorly fit individuals.

The study included 4,384 middle-aged and older adults whose fitness levels were assessed during exercise treadmill tests sometime between 1986 and 2006; they were then followed for an average of about nine years.

When Mandic's team separated the participants into five groups based on fitness levels, they found that one-quarter of the least-fit men and women had died during the study period, versus 13 percent of those who were slightly more in shape.

Among adults in the most-fit group, only 6 percent died during the follow-up period.

Overall, the five groups showed little difference in their reported exercise habits over their adult lives. Where they did differ was their activity levels in recent years.

Since it is recent physical activity that offers protection, Mandic said, it is important to maintain regular physical activity throughout life.

And since fitness is linked to longevity regardless of weight and health conditions like high blood pressure and high cholesterol, exercise is important for all, according to Mandic.

That, she said, includes people who are thin and in generally good health.

SOURCE: Medicine and Science in Sports and Exercise, August 2009.