The study, published in Brain, Behavior and Immunity, found depressive symptoms associated with increases over time in interleukin-6, an inflammatory protein that predicts cardiovascular events. The study determined levels of interleukin-6 were not related to later increases in depressive symptoms.
"There is two-way communication between the brain and the immune system, so we had to determine whether activation of the body's immune system sent a signal to the brain to affect mood and behavior or whether the depression activated the immune system," study leader Dr. Jesse Stewart of Indiana University - Purdue University Indianapolis said in a statement. "The link to cardiovascular disease demonstrates that there may be physical as well as mental health reasons to treat depression."
The study involved 263 healthy men and women ages 50-70, who were tested at baseline and again six years later to determine their levels of depressive symptoms and interleukin-6.
Levels of C-reactive protein -- another inflammatory protein -- were also measured but were not found to be related to depression.