In 2016, years before long COVID was a thing, the US National Institutes of Health, the largest single public funder of medical research in the world, launched a study into a long-neglected and puzzling condition: chronic fatigue syndrome (CFS), also known as myalgic encephalomyelitis, or ME/CFS.

Eight years later, the results of that study are finally out. In one of the most thorough investigations to date, researchers took a deep dive into a small group of 17 people who developed ME/CFS after an infection and found distinct biological differences compared to 21 healthy controls.

"Overall, what we show is that ME/CFS is unambiguously biological, with multiple organ systems affected," neurologist Avindra Nath, lead researcher of the study and clinical director of NIH's National Institute of Neurological Disorders and Stroke (NINDS), said in an interview with JAMA.

For decades, many doctors had dismissed ME/CFS as a psychosomatic condition that was 'all in patients' heads'. Now there is little doubt: a host of biological changes underpin ME/CFS.

"It's a systemic disease," Nath continued, "and the people living with it deserve to have their experiences taken seriously."

In a week of extensive tests, study participants underwent brain scans, sleep studies, muscle strength and cognitive performance tests, skin and muscle biopsies, blood tests, and gut microbiome and spinal fluid analyses. Participants were also put on a controlled diet and spent time in metabolic chambers, where their energy and nutrient consumption was measured under stable conditions.

Similar to previous studies, people with ME/CFS had higher resting heart rates, markers of a drawn-out and overstimulated immune response that exhausts T cells, and less diverse gut bacteria than controls.

The ME/CFS group showed no signs of muscle fatigue, and performed normally on cognitive tests, despite reporting greater cognitive symptoms.

And yet the immune and gut microbiome changes clearly impacted the central nervous system in several ways. People with ME/CFS had lower levels of chemicals called catechols, which help regulate the nervous system, in their cerebrospinal fluid and less activity in a brain region called the temporal-parietal junction (TPJ) during motor tasks.

The TPJ drives the motor cortex, a brain region in charge of telling the body to move, so its dysfunction might disrupt how the brain decides to exert effort, the researchers think. In turn, these changes might alter patients' tolerance for exertion and their perception of fatigue.

"We may have identified a physiological focal point for fatigue in this population," says Brian Walitt, lead author and medical scientist studying ME/CFS at NINDS.

"Rather than physical exhaustion or a lack of motivation, fatigue may arise from a mismatch between what someone thinks they can achieve and what their bodies perform."

While welcoming the research effort, ME/CFS advocacy groups have questioned the use of some assessments of fatigue in the study, and the exclusion of other core features of the condition, namely post-exertional malaise.

Some might also query how the initial group of 217 patients who were screened for the study were whittled down to just 17 people who a panel of clinicians unanimously confirmed had post-infection ME/CFS.

In selecting a small group of patients, the study aimed to complete the most rigorous assessment practically possible to land the best chance of pulling out meaningful differences that researchers could then investigate in larger groups. Otherwise, as with long COVID and Alzheimer's disease, it can be hard to get to the bottom of what causes these conditions, which in turn makes it difficult to find a treatment that works.

The researchers had wanted to recruit 40 patients with ME/CFS but the COVID-19 pandemic cut their efforts short. They also opted not to include people who had been sick for more than 5 years or were too unwell to travel; and avoided asking participants to complete days-long physical tests because they were concerned about harming patients through overwork.

Only with more research will it become clear whether the changes detected in these 17 patients translate to more people with ME/CFS, but this study lays the foundation for that future work.

The study has been published in Nature Communications.