Researchers analyzed data from 1.1 million Swedish men born between 1950 and 1987 and underwent a medical exam for military service. The men were followed into adulthood.
"Being in poor physical shape at 18 years of age, measured as the test results on an exercise bike during their medical exam for compulsory military service, can be linked to a risk of suicidal behavior as an adult that is 1.8 times greater," study co-leader Margda Waern, of the University of Gothenburg in Sweden, said in a university news release.
The increased risk of suicide among men with poor fitness at age 18 was evident even 42 years later, according to the study, which was published online recently in the journal Psychological Medicine.
A previous study by the same team of researchers found that adults who were in good physical shape as teens were less likely to suffer severe depression.
"But even when we exclude individuals who suffer from severe depression in connection with suicide or attempted suicide, the link between poor physical shape and an increased risk of suicidal behavior remains," Waern said.
Depression is a strong predictor of suicidal behavior in later life, but the situation among younger people is complex, and many factors are involved, the researchers said.
"One theory is that the brain becomes more resistant to different types of stress if you are physically active," study co-leader Maria Aberg, also of the University of Gothenburg, said in the news release.
Physical exercise should be considered in suicide prevention efforts aimed at young people, the researchers said. This study, however, found merely an association between physical fitness and suicide, not a cause-and-effect link.
The U.S. Centers for Disease Control and Prevention has more about preventing suicide.