Researchers at Indiana University-Purdue University Indianapolis opine that depression leads to elevated inflammatory proteins in the human body.
Led by Dr. Jesse Stewart, researchers found that depressive symptoms are associated with increases over time in interleukin-6, an inflammatory protein that predicts cardiovascular events.
On the other hand, levels of interleukin-6 were not linked to subsequent increases in depressive symptoms.
The new study is the first to examine both directions of the depression-inflammation connection and to measure the physical symptoms of depression, such as fatigue and sleep disturbance, in addition to the cognitive-emotional symptoms, such as pessimism and sadness.
While many previous studies have linked depression to increased inflammatory protein levels measured at the same time, but they couldn't speak to which is the cause and which is the effect.
"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," said Dr. Stewart.
The participants in the study consisted of 263 healthy men and women aged 50-70 years at the start of the study.
They 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 related to depression.
Stewart said that the strength of the association of depression with future heart disease is similar to that of traditional risk factors like smoking, high blood pressure and elevated cholesterol.
"Promotion of inflammation may be one pathway through which depression may 'get under the skin' to negatively influence cardiovascular health. The link to cardiovascular disease demonstrates that there may be physical as well as mental health reasons to treat depression," said Stewart.
The study has been published in the latest issue of the journal Brain, Behavior and Immunity.