People suffering from depression or psychological distress appear to have higher than normal blood levels of C-reactive protein (CRP), an indicator of inflammatory disease, according to new research from Denmark.
Previously, CRP has been linked with an increased risk for heart disease. "People with increased CRP have a two- to threefold risk of depression," said lead researcher Dr Borge Gronne Nordestgaard, from Copenhagen University Hospital.
Whether increased CRP is a cause of depression or only a sign of it isn't known, he noted. "We can't really say what is the chicken and what is the egg," Nordestgaard said. And whether lowering CRP will help relieve depression isn't clear, he added.
Nordestgaard noted that increased levels of CRP are associated with people "with an unhealthy lifestyle - obese people and people with chronic diseases." But their findings held even when they accounted for these factors.
How the study was done
For the study, published online in the Archives of General Psychiatry, Nordestgaard's team collected data on more than 73 000 adults who took part in population studies in Copenhagen. Specifically, they looked at self-reported antidepressant use, antidepressant prescriptions and hospitalisation for depression.
Among people taking antidepressants, the odds of also having high CRP levels were almost three times higher than for those not taking these drugs, the researchers noted.
In addition, elevated CRP levels were associated with more than doubled odds of hospitalisation for depression.
Millions suffer from depressed
More than 21 million Americans suffer from depression, a leading cause of disability, according to Mental Health America. Earlier studies have suggested that low-grade systemic inflammation may contribute to its development, the researchers noted.
Even though someone suffering from an inflammatory disease might not show symptoms, it is possible that it could lead to depression, Nordestgaard said.
Not everyone agrees with Nordestgaard's conclusion. One expert said that because the study results are based on cross-sectional analyses, it is impossible to determine whether CRP levels cause depression.
"In other words, simply finding a connection between inflammation and depression, however strong, says nothing about the underlying mechanism that connects them," said Simon Rego, director of psychology training at Montefiore Medical Center/Albert Einstein College of Medicine in New York City.
For example, the inflammation may lead to depression, depression may lead to the inflammation, or the association might be caused by a third and entirely different factor, he explained. "Clearly, more research is needed to establish the direction of the association between CRP and depression," Rego said. Another expert agreed.
"I am not certain what clinical utility this study will have," said Dr Bryan Bruno, acting chair of psychiatry at Lenox Hill Hospital in New York City. "It's clear they are not establishing a causative role between elevated CRP and depression."
"The study does, however, remind us that there is a biological basis for depression," Bruno said. "It reminds us just how much a brain disease depression is," he said.
For more information on depression, visit the US National Institute of Mental Health.
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