A team of scientists said on Friday they had found "robust evidence" that a condition called Chronic Fatigue Syndrome (CFS) is a biological as opposed to a psychological disorder, but some experts questioned the findings.
The team from Columbia University in the United States identified in their research distinct immune changes in patients with CFS -- markers they said pointed to distinct disease stages and would lead to better diagnosis and treatment.
CFS, sometimes called myalgic encephalomyelitis or ME, is a controversial and debilitating condition characterized by disabling physical and mental fatigue, poor concentration and memory, disturbed sleep and muscle and joint pain.
There is no cure and scientists don't know what causes it, but it affects around 17 million people worldwide.
Many sufferers say they think their illness started after a viral infection. But suggested links to a virus known as XMRV were shown in a scientific paper in 2010 to have been based on contaminated samples in a lab.
Recent research showing psychological treatments such as cognitive behavior therapy can help CFS sufferers become more active have also caused argument, with some patients complaining such results suggest they are just lazy or suffering from a condition that is all in the mind.
In the latest study, published in the Science Advances journal, researchers found that specific immune patterns in patients who had CFS for three years or less were not present in controls or in patients who had the disease for more than three years.
Short duration patients had higher amounts of many different types of immune molecules called cytokines, researchers found. The link was unusually strong with a cytokine called interferon gamma that has been linked to fatigue after viral infections.
"We now have evidence confirming what millions of people with this disease already know -- that ME/CFS isn't psychological," said Mady Hornig, who co-led the study.
The results should help speed diagnosis and the discovery of new treatments, she said.
However, other CFS specialists urged caution.
Michael Sharpe, a professor of psychological medicine, at Britain's Oxford University said the finding was "potentially interesting" but added: "This type of study (a case-control study) is notorious for producing findings that other researchers subsequently fail to replicate."
In a similar tone, Stephen Lawrie, an Edinburgh University psychiatry expert, said Hornig's team may well have found different immune profiles at different stages of the disease, but added this could be "down to chance and hence a false positive signal".