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Inflammatory markers, such as elevated levels of C-reactive protein, are well-documented in people diagnosed with depression. What has not been fully understood though, is why. It has been posited that this association may be symptom-specific, meaning that higher levels of inflammation are likely to affect only certain depression symptoms, specifically the depression symptoms that characterize sickness behavior such as fatigue, lack of appetite and motivation, and withdrawal. From an evolutionary standpoint, scientists hypothesize that these symptoms could have had a beneficial origin, namely that these symptoms can serve an important role in the preservation of energy resources needed for fighting infection and promoting healing.

In a study recently published in The Journal of the American Medical Association (JAMA), researchers decided to test the hypothesis that this inflammation response could be a pathogen host defense.

To analyze the association between C-reactive protein and depression symptoms, researchers looked at the data from three cross-sectional surveys from the National Health and Nutrition Examination Survey (NHANES), specifically 2005-2006, 2007-2008, and 2009-2010. 15,071 participants with a mean age of 47.5 years were included in the analysis. C-reactive protein was measured using standard procedures and depressive symptoms were assessed using the Depression Screener Questionnaire.

They found that C-reactive protein was associated with all specific depression symptoms when not adjusting for other depression symptoms. When adjusting for other depression symptoms, C-reactive protein was independently associated with sleep problems, tiredness or lack of energy, and changes in appetite. And the independent association between C-reactive protein and anhedonia almost reached statistical significance.

As researchers summarized,

“This pattern of results is consistent with the evolutionary view linking inflammation and depression with pathogen host defense because tiredness, lack of energy and reduced appetite are primary characteristics of sickness behavior. Further research is needed to determine whether changes in inflammation predict changes in specific symptoms and to identify metabolic pathways that mediate such changes.”

We know that mental and physical health are often closely tied. This study more precisely demonstrates the emotional/psychological correlation our bodies can produce in response to physical health and circumstances. The more we understands about the association between inflammation and depression, the better we are able to serve patients suffering form all degrees of depression.

Dr. Michael Reed is a Nashville based Psychiatrist. Please visit his main website for information about his career.