Depression Alters Immune Systems By Decreasing Physical Activity

"We also found that depression was associated with greater tobacco and caffeine consumption, less physical activity, and poorer sleep quality," said Gregory E. Miller, PhD, lead author of the study.

The researchers believe they have found a brain-behavior connection that links the altered immune response of mildly to severely depressed women outpatients to their typically low level of physical activity. From 42 to 63 percent of the differences in specific immune functions between depressed and non-depressed study participants was related to physical activity, the researchers found. They used the participants' production of lymphocytes under stimulation by mitogens to measure the impact of various depression-related factors on their immune function.

The study, appearing in the November/December issue of Psychosomatic Medicine, presents the first published data to identify a behavior that might be responsible for the immune system alterations that occur in depressed women, the researchers pointed out.

These new findings have potentially wide future impact because the observed immune differences between depressed and non-depressed women could help to explain the higher rates of sickness and death observed repeatedly among depressed individuals, the scientists said.

The Pittsburgh scientists worked with 32 non-hospitalized clinically depressed women and 32 healthy non-depressed women matched as controls. Miller and colleagues Sheldon Cohen, PhD and Tracy B. Herbert, PhD, investigated a broad spectrum of both endocrine and health practice pathways through which depression might influence immune function.

The possibility that depression might influence immune function through the neuroendocrine system was tested by measuring participants' levels of hormones such as norepinephrine, cortisol, estradiol, epinephrine, and progesterone. Researchers found, however, that hormone levels did not account for differences in immune response between the groups of depressed and non-depressed women.

The health practices assessed by the researchers were those often associated with depression: alcohol, tobacco and caffeine use, nutrition, and sleep quality and efficiency -- as well as physical activity. While a variety of the health practices were associated with immune system processes, physical activity was the only one to explain why depressed women had immune alterations compared with the control group.

"An important next step of this research is to determine whether interventions aimed at increasing physical activity can buffer people from the immunologic changes associated with depression," said Miller.

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Support for the study was provided by the National Institute of Mental Health and the National Institutes of Health.