Naval Health Research Center Study Indicates U.S. Troops Who Saw Combat More Likely to Experience Mental Health Issues

For the past 20 years – and longer before that – service members have returned from deployment talking about mental health concerns and illness they believed were linked to their time in the military, with many of their concerns backed by a variety of studies. Now, a study that has been following military personnel, both […]

A humvee filled with Marines conducting a mounted combat patrol cruises through the desert of Iraq during the setting sun near Al Asad, Iraq, in 2006. US Marine Corps Photo

For the past 20 years – and longer before that – service members have returned from deployment talking about mental health concerns and illness they believed were linked to their time in the military, with many of their concerns backed by a variety of studies.

Now, a study that has been following military personnel, both active-duty and veterans, for 20 years supports the theory that experiencing combat can lead to adverse physical and health effects.


Researchers at the Naval Health Research Center and Veterans Affairs Puget Sound have studied service members across all branches for the past 20 years, using surveys to determine what outcomes time in the military might have on mental and physical health as part of the Millennium Cohort study.

Researchers working on the study have written articles about their work since it began, but they recently published a literature review in the journal Annals of Epidemiology, which means the authors analyzed the previously published material to identify larger trends over the years.

Although the study, which is sponsored by the Department of Defense, included 260,000 service members from all branches, it did not break down results by branch. The Army had the most participants, making up 40 percent of those enrolled, said Rudy Rull, a senior epidemiologist at the Naval Health Research Center.

Of those enrolled, two-thirds were active-duty and the remaining one-third were reserve or National Guard. Of the people surveyed, 60 percent are now no longer in the military, Rull said.

The Millennium Cohort study is a prospective study, which has continued to enroll participants over the years. It was originally slated to stop in 2022, but the study was extended until 2068, with the most recent participants enrolled between September 2020 and August 2021.

The most recent enrollment period resulted in 50,000 more participants, including some from the Space Force.

Cpl. Corey D. Stewart, from Greenville, Ohio, a vehicle commander with 4th squad, Security Company, Combat Logistics Battalion 2, provides security during a seven-day long convoy from Al Asad, to Sahl Sinjar and Al Taqaddum, Iraq, Feb. 8, 2009. US Marine Corps Photo

One of the reasons for the study is that members of the military community noticed health issues among service members and veterans, but it was difficult to look at causes through retrospective studies, said Dr. Margaret Ryan, ​​regional medical director of the Defense Health Agency Immunizations Office. Ryan was one of the people involved in the early days of the study.

The cohort study was born out of the need for prospective studies, Ryan said, adding that it was based on other ones, like the Nurses’ Health Study I and II, which look at women’s health.

“And the Millennium Cohort was conceived in that vein, by a senior epidemiologist who said, ‘if we had this cohort study, we could answer so many questions so much better than we ever had before,’” Ryan said.

One of the most consistent findings in the literature review was that combat increases the likelihood of experiencing adverse mental health, with post-traumatic stress disorder the most common outcome. Service members who experienced combat while deployed were three times more likely to report PTSD symptoms or a diagnosis compared to those that did not deploy. Depression was also higher among those who saw combat while deployed.

The risk of post-deployment PTSD was also higher for those who had been physically or sexually assaulted before deploying, had poor baseline mental health, were severely injured during combat or deployed multiple times, according to the study.

At the same time, there were many who did not experience PTSD. Of the sampled service members who deployed and did not experience combat, 90 percent had low levels of PTSD symptoms. Of those who did see combat while deployed, 80 percent had low levels of PTSD.

The study also looked at suicide research done with the Millennium Cohort and found that there was no direct relationship between deploying and risk of suicide. Instead, it found that risk factors were being male, having bipolar or depression disorders, or having problems with alcohol use. Another study the literature review examined found that experiencing high combat severity or particular incidents, like killing a civilian, were associated with suicide attempts, although mental disorders were a confounding factor.

Overall, with physical health, deployment was not necessarily harmful, although specific exposures from deploying were associated with poorer health. The literature review looked at respiratory health, autoimmune conditions and cardiometabolic health, but did not look at cancers.

In particular, the study looked at the effects of burn pits on autoimmune and respiratory illnesses, according to the study. A recent episode of “The Problem with Jon Stewart” examined burn pits, which some veterans have associated with an array of health issues, including cancers. President Joe Biden also referred to the concerns around burn pits during his State of the Union address on Tuesday, saying they may have affected his son, Beau, who died of brain cancer in 2015.

U.S. Army Soldiers conduct convoy operations in northeastern Syria Sept. 27, 2020. US Army Photo

The study is based on surveys, which means it comes with the unavoidable survey bias, which is when people answer surveys based on what they think surveyors want to see. In order to combat this bias, the researchers also used Department of Defense medical records to compare surveys, Rull said.

The partnership with the Department of Veterans Affairs, has also allowed the researchers to access those files, Rull said.

When it comes to mismatches between medical records and surveys, typically the researchers found that a person would not deny that they had a condition, said one of the founding investigators, Dr. Edward Boyko, a staff physician at the VA Puget Sound.

However, people would say they had conditions not listed on the records, Boyko said. That did not mean they did not have the condition, just that the records may not reflect it.

Other limitations include recall and selection, Ryan said.

And, because it is a longitudinal study, there are challenges with keeping participants engaged, Rull noted.

As the researchers will continue the study for more than 40 years, they said there are more findings to come.

One finding will likely be the effect of COVID-19 on service members, as the last panel enrolled before the pandemic began.

It is interesting that the first and last enrollment periods happened during life-changing events, Ryan said.

“​​We had this … world event that happened on Sept. 11, [2001] right as we were in the middle of enrolling that first panel in the cohort,” she said.” And … it changed things a bit, just as the pandemic has. And yet the study team itself is resilient and figures it out.”