April 17 (HealthDay News) -- Military personnel who have poor mental or physical health before they go into combat are more likely to experience post-traumatic stress disorder following their battlefield experience, a U.S. military study finds.

The study of 5,410 personnel found that 395 of them -- 7.3 percent -- had a diagnosis or symptoms of PTSD after fighting in Iraq or Afghanistan, according to the report in the April 17 online edition of BMJ.

More than half of the cases of PTSD occurred in those with the lowest health scores on a questionnaire they filled out between 2001 and 2003, the report said. The incidence of PTSD in the group was assessed by a second questionnaire completed between 2004 and 2006, after combat experience.

The data came from the Millenium Cohort Study, a long-running study of the health of American military personnel.

The result is not surprising, said Keith A. Young, vice chair for research at Texas A&M Health Science Center College of Medicine, who has done work on PTSD.

"It's pretty clear that people coming in with reduced mental capacity are more likely to develop PTSD after combat," Young said. "One of the major conditions that brings down the mental health score of individuals is depression. Our hypothesis is that depression and PTSD have a commonality of a brain anatomical variant."

And, he said, "it is relatively hard to disentangle poor mental health from poor physical health. People who have depression are likely to have poor physical health as well. They are less disposed to take proper care of themselves."

One of the notable findings of the study is that the military personnel had better scores of mental and physical health than the general U.S. population, said Bruce Dohrenwend, a professor of epidemiology in the department of psychiatry at the Columbia University School of Public Health.

"Actually, the scores of these veterans show that they are considerably healthier than we see in the normative scores [those of the population at large]," Dohrenwend said.

The finding that 58 percent of the symptoms or diagnoses of PTSD were seen in the lowest 15 percent of the study participants "has implications for screening," he said.

Those personnel at highest risk of PTSD could be targeted for prevention programs, early intervention after exposure to stress, or even protection from stressful exposures, the report said.

But the study results do not appear definitive enough to prompt such measures, Dohrenwend said.

"Answers on a questionnaire are far from optimal, and they do not have good data on exposure," he said. "So is this enough to base policy on? It probably isn't, but it certainly is enough to follow up with a more intensive investigation of the lowest 15 percent."

SOURCES: Keith A. Young, Ph.D, vice chair, research, Texas A&M Health Science Center College of Medicine, Waco; Bruce Dohrenwend, Ph.D, professor, epidemiology, department of psychiatry, Columbia School of Public Health, New York City; April 17, 2009, BMJ, online