Research from King’s College London has found new evidence of a link between low-grade systemic inflammation and depression. The large-scale case-control study identified higher levels of an inflammatory biomarker in depressed subjects compared to non-depressed subjects, however, not all experts are convinced inflammation plays a causal role in depression.
The relationship between inflammation and depression has been hotly debated over recent years. While the two conditions often appear closely connected, it’s still unclear whether inflammation plays a causal role in depression. Many diseases associated with inflammation are characterized by patient’s having an increased risk of depression but it is often reasonably argued that depression is a mere byproduct of illness.
Using data from the UK Biobank, an ongoing large survey tracking the health of thousands of people, the new research looked at the association between depression and an inflammatory biomarker called C-reactive protein (CRP). Just under 86,000 people were included in the study, with around a third having a clinical diagnosis of major depressive disorder.
The researchers report CRP levels were consistently higher in depressed subjects compared to non-depressed subjects. After adjusting for other clinical and behavioral factors the researchers still detected a link between this inflammatory biomarker and depression.
“Our large-scale analysis of data removed socioeconomic background, ill health, unhealthy habits as well as genetic predisposition to immune dysfunction as the only explanations for the relationship between depression and inflammation,” explains Carmine Pariante, joint senior author on the new study. “By this process of elimination, we show that there may be a core biological process that is behind the association between depression and increased inflammation. If we can identify this process and uncover more detail about its role in the development of depression, we can pave the way for trialling new treatments for this widespread mental health disorder.”
While the authors of this new research feel confident their findings point to some kind of yet-to-be-explained causal association between inflammation and depression, not everyone is convinced. David Curtis, from University College London, says these new findings are not novel and it was already known measures of inflammation can be associated with depression.
“This study confirms that levels of CRP, a marker of inflammation, tend to be somewhat higher in people who report being depressed at some time in their lives,” says Curtis. “I am doubtful that inflammation has a key role in causing depression and I’m not sure that the present study adds much to our understanding of depression.”
Previous research criticizing the depression/inflammation hypothesis has noted the relationship between the two factors may be indirect. Poor sleep or obesity, for example, are known to be linked to low-grade inflammation and depression, so it is difficult to home in on any causal mechanism.
Curtis is wary of the impact these kinds of research findings may have on people and cautions those suffering from depression against self-medicating with anti-inflammatory medications. If there is a causal relationship between inflammation and depression it is most likely precise drugs will need to be developed to target this mechanism, and plenty more research is needed before we get to that point.
“Certainly there is nothing to suggest that people should try to treat their depression with anti-inflammatory medications,” says Curtis. “As well as having no proven effect on depression, these medications have dangerous side effects whereas anti-depressants are safe and effective.”
The new research was published in The American Journal of Psychiatry.