Depression and other types of mental illness, alcohol problems and being male -- strong risk factors for suicide among civilians -- were all linked to self-inflicted deaths among current and former members of the military.
But the researchers found deployment and combat did not raise the risk.
"The findings from this study are not consistent with the assumption that specific deployment-related characteristics, such as length of deployment, number of deployments, or combat experiences, are directly associated" with suicides, the authors wrote.
The results echo smaller studies focusing on a specific branch of the military, but this is the first to look at a sampling from the entire military population, said lead author Cynthia LeardMann, a researcher with the Naval Health Research Center in San Diego.
More than 145,000 people from all branches took part, including active-duty service members, reservists and retirees, and they were followed from 2001 to 2008, a period in which the U.S. waged wars in Iraq and Afghanistan. The findings were published Tuesday in the Journal of the American Medical Association.
A recent increase in the military suicide rate has raised concerns about a possible link between suicide and deployment, including long or repeated tours of duty in Iraq and Afghanistan. But the new results should lay those concerns to rest, said Dr. Nancy Crum-Cianflone, another researcher with the Navy center.
She is leading a larger study on the health effects of serving in the military. The newly released findings are based on a subset of participants in that study.
The 2001-08 study looked at a small portion of the thousands of suicides among active-duty service members and veterans during that time.
There were 78 suicides among the study participants, or an average of almost 12 per 100,000 people followed for one year. The rate was about two times higher among men and people with depression, and a little higher than that among those with alcohol problems. But it was four times higher among those with bipolar disorder.
Pentagon data show there were 349 suicides last year alone among active-duty troops, the most since 2001.
Crum-Cianflone said the military suicide rate climbed sharply between 2005 and 2009, to about 20 per 100,000 people followed for one year. At the same time, there was an increase in the number of people with mental illness in the military. The reason for that is unclear, the study authors said.
The suicide rate in the general population also increased in recent years, to almost 18 per 100,000 in 2010, according to a JAMA editorial.
David Rudd, scientific director for the nonprofit National Center for Veterans Studies, said the study provides only a snapshot and doesn't answer whether combat exposure increases the lifetime risk of suicide.
Rudd said evidence suggests most service members who attempt suicide had pre-existing psychiatric problems and may have been suicidal before entering the military. That, he said, suggests a need for better screening and treatment.
In the study, depression was present in about 23 percent of those who committed suicide and almost 11 percent of those who didn't take their lives. Six percent of the suicides involved bipolar disorder, compared with less than 1 percent of the non-suicides. Alcohol-related problems afflicted 30 percent of the suicides and 14 percent of the non-suicides.
Post-traumatic stress syndrome was uncommon and by itself was not found to be a suicide risk factor.
But Dr. Charles Hoge, a study co-author and retired Army psychiatrist, said: "Service members with PTSD often experience co-existing depression or alcohol problems, which would increase their risk" of suicide.
Rachel Yehuda, director of traumatic stress studies at Mount Sinai School of Medicine in New York, said the study "calls into question the previously assumed relationship between length of combat exposure and suicide" but doesn't address other ways combat affects mental health.
Hoge said service members are routinely and extensively screened for mental illness before enlisting and afterward and those who are seriously ill are rejected. But he noted that some mental illnesses typically emerge first in young adulthood.
He said the military has made great efforts to offer treatment to those affected.
"There's been a huge increase over the last several years in the number of mental health professionals working at military facilities," Hoge said. These include combat stress teams in the field and counselors back home.
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