Inflammation – which is the body's natural process of fighting infection – may also be a key feature of depression, according to scientists.
UK researchers have found those with depression have higher levels of a protein in their bodies associated with inflammation, compared to those without.
This was even when taking into account factors such as ill health, unhealthy behaviours or socio-economic background, the experts said.
They added that their findings, published in the journal American Journal of Psychiatry, could help pave the way for new targeted treatments for mental health disorders.
Professor Carmine Pariante, from the National Institute for Health Research Maudsley Biomedical Research Centre, said: 'Our large-scale analysis of data removed socio-economic background, ill health, unhealthy habits as well as genetic predisposition to immune dysfunction as the only explanations for the relationship between depression and inflammation.
'By this process of elimination, we show that there may be a core biological process that …