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Depression is associated with systemic inflammation and autoimmune conditions, including rheumatoid arthritis (RA). Researchers in Japan studied the relationship between C-reactive protein (CRP), depression, and pain perception in RA patients. CRP is synthesized in the liver in response to the inflammatory cytokines IL-6 and IL-1, and is commonly used as a measure of inflammation. Depression was quantified by the Beck Depression Inventory II. Both inflammation and depression were independently associated with higher levels of pain and more need for treatment.
The authors explained that inflammation may cause depression, or depression may cause inflammation, or both. As one hypothesis, depression may promote poor health habits that lead to infection. In the other direction, inflammation is widely known to produce depressive symptoms in humans and animals, as when a person with the flu shows reduced energy, appetite, and interest in normal activities. Reference 1 cites numerous studies from the medical literature showing that inflammation and depression have a bidirectional association: either one can activate the other. This type of system is analogous to the “vicious cycle” of poison ivy rashes, where itching causes scratching and scratching can damage the skin to produce more itching.
Another study from San Francisco showed that depression is associated with differences in how RA patients and their physicians evaluate their health status. In their investigation, 30 percent of patients rated their disease severity significantly higher than their physicians did. Depressed patients had the highest differences between doctor and patient scores.
Researchers at Tulane University in New Orleans found a strong association between depression and severity of rheumatoid arthritis. Patients with Class III RA disease were 5.92 times more likely to be depressed as patients with Class I RA.
Many authors note that pain and disability can be significant factors for depression. But there's more to it: inflammation appears to contribute directly to depressive symptoms.