Perhaps it's not surprising, but for members of the U.S. armed forces, combat duty in Iraq and Afghanistan increases the risk of depression, according to a new study.

Timothy S. Wells of the U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base in Ohio, studied more than 40,000 members of the U.S. military who had been free of symptoms of depression and had not taken medication for anxiety, stress, or depression before deployment. The subjects were in all branches of the military from some or all of 2000 to 2006.

Those who experienced combat had the highest rate of new diagnoses of depression - about 6 percent for men and about 16 percent for women. That compared with about 4 percent of men and about 8 percent of women who were not deployed, and about 2 percent of men and 5 percent of women who did not face combat.

Some of the differences in the group that was not deployed could be explained by the fact that only those who meet all of the U.S. military's health requirements are eligible for deployment, Wells told Reuters Health by email. In other words, It is likely that the nondeployed group had other risk factors, such as other mental health disorders and health conditions that placed them at increased risk of depression in comparison to those who deployed, but were not exposed to combat.

In their report in the American Journal of Public Health, the investigators note that male combat specialists had a lower risk for depression than men in health care or other supportive positions, suggesting that military hardiness may help lower risk.

Individuals who are expected to be exposed to combat may receive training that alters their risk for depression compared to non-combat exposed personnel, Wells said.

Male and female personnel with pre-existing PTSD were more likely to develop depressive symptoms, reflecting a well-known link between the two conditions.

Other risk factors for depression among men included younger age, smoking, alcohol dependence, and service in the Army or Marine Corps. Furthermore, women who were married, divorced, non-Hispanic white, on active duty, or served in the US Navy or Coast Guard faced an increased risk for depression.

SOURCE: American Journal of Public Health, online November 12, 2009.