With CFS affecting roughly 2.5 million people in the US alone - with no known cause or cure - these findings could help researchers to better understand the mechanisms at play in the condition, leading to better treatments and diagnosis in the future.

"CFS is a very challenging disease," said lead researcher Robert K. Naviaux from the University of California San Diego.

"It affects multiple systems of the body. Symptoms vary and are common to many other diseases. There is no diagnostic laboratory test. Patients may spend tens of thousands of dollars and years trying to get a correct diagnosis."

Also known as myalgic encephalomyelitis (ME), CFS is a condition that causes severe fatigue that lasts more than six months. During this time, a patient can experience headaches, sleep problems, and memory issues.

Women are generally more at risk than men, though it affects both, usually between the ages of 30 and 50.

To investigate the biological processes involved in the disease, Naviaux and his team recruited 84 subjects - 45 men and women with CFS symptoms, and 39 as a control group.

After taking samples from each of the volunteers, they identified 612 metabolites - substances that are produced by cells as they break down molecules - and observed how they interacted with 63 chemical pathways within the blood plasma.

After analysing the differences between the CFS group and the control, the team found that those with CFS symptoms had abnormalities in 20 of these pathways, and 80 percent of the studied metabolites had decreased.

This suggests that those with CFS metabolise much slower than those without it. In fact, the team says this action mimics the `dauer state` - a form of hibernation practised by certain types of worms when they experience harsh environmental conditions.

"Despite the heterogeneity of CFS, [and] the diversity of factors that lead to this condition, our findings show that the cellular metabolic response is the same in patients," said Naviaux.

"And interestingly, it’s chemically similar to the dauer state you see in some organisms, which kicks in when environmental stresses trigger a slow-down in metabolism to permit survival under conditions that might otherwise cause cell death. In CFS, this slow-down comes at the cost of long-term pain and disability."

Understanding the similarities between dauer and CFS could help to explain what causes the condition in the first place. Since the dauer state is normally a response to environmental pressures, the team says CFS might operate in much the same way.

"All animals have ways of responding to changes in environmental conditions that threaten survival," Naviaux told The Telegraph.

"Historical changes in the seasonal availability of calories, microbial pathogens, water stress, and other environmental stresses have ensured that we all have inherited hundreds to thousands of genes that our ancestors used to survive all of these conditions."

While the possible link between CFS and the dauer state is intriguing, the real win here is that the team is on the cusp of understanding how to test patients for CFS by analysing the number metabolites in their bodies and defects inside blood plasma.

Since the current method of diagnosis is only a little better than guesswork, the new findings offer a glimmer of hope for those who are looking for more definitive diagnoses.

This isn’t the first time researchers have found a biomarker for CFS. Back in June, a team from Cornell University used biomarkers in gut bacteria to diagnose 87 patients with the condition.

Both studies have small sample sizes, so hopefully larger investigations in the future will help us get to the bottom of this difficult and life-altering disease that is so much more than `just psychological`.

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