Men who are physically active but have trouble losing weight are helping their hearts, which seems to pay off in a lower risk of premature death.

To reach that conclusion, researchers in the U.S. studied the fitness levels of 14,345 men using treadmill tests over six years. People can worry less about their weight as long as they continue to maintain or increase fitness levels, researchers say. Ben Stansall/AFP/Getty

After more than 11 years of follow-up, those who maintained or improved their fitness levels lived longer, regardless of whether the numbers on the scale stayed the same or even went up, the researchers reported in this week's issue of Circulation: Journal of the American Medical Association.

"Given the great difficulties of losing weight and maintaining a reduced weight over the long term, this study underscores the benefits of maintaining and improving fitness to reduce mortality risk independent of weight change," said the study's lead researcher, Duck-chul Lee, a physical activity epidemiologist in the department of exercise science at the University of South Carolina.

"You can worry less about your weight as long as you continue to maintain or increase your fitness levels," he added.

Participants were an average of 44 years old and they had at least two comprehensive medical exams.

Factors such as age, family history of heart disease, beginning fitness level, changes in lifestyle factors such as smoking and physical activity, and medical conditions such as high blood pressure or diabetes were taken into account in the analysis.

During the follow-up period, 914 deaths from all causes and 300 deaths from heart disease and stroke were recorded.

Each increase in their endurance level on the treadmill test resulted in a lower risk of death, the researchers found.

The researchers noted that the participants were largely well-educated white men from middle to high socioeconomic backgrounds.

Since most were slightly overweight, the results may not apply to severely obese or extremely unfit individuals, they cautioned.

"In conclusion, maintaining or improving fitness is associated with a lower risk of premature deaths from all-causes and cardiovascular disease in men," the study's authors wrote.

"Increased attention needs to be placed on strategies to maintain or improve fitness."

The study was funded by the U.S. National Institutes of Health and an unrestricted research grant from Coca-Cola.