Changes in the gut’s bacterial makeup may signal a person’s risk of developing Parkinson’s disease long before any symptoms emerge, according to a new study by researchers at University College London.

The findings suggest that analyzing the gut microbiome, which is the assortment of microorganisms such as bacteria that live in the digestive tract, could help researchers develop tests to identify people at elevated risk for Parkinson’s before symptoms appear.

“Parkinson’s disease is a major cause of disability worldwide, and the fastest growing neurodegenerative disease in terms of prevalence and mortality,” said Professor Anthony Schapira of UCL’s Queen Square Institute of Neurology, who led the research. “There is an urgent need to develop treatments that can stop or slow the disease’s progression.”

To conduct the study, Mr. Schapira’s team, working with researchers at INRAE, France’s national agricultural research institute, analyzed stool and clinical data from participants in the United Kingdom and Italy. The group included 271 people already diagnosed with Parkinson’s, 43 carriers of a gene variant called GBA1 that can raise Parkinson’s risk by up to 30-fold but who had no clinical symptoms, and 150 healthy participants used as a comparison.

The researchers found that more than a quarter of all gut microbe species (176 in total) were present at different levels in people with Parkinson’s compared with healthy controls. Some species were more abundant in those with the disease; others were more common in healthy participants. The differences were most pronounced in people with more advanced Parkinson’s.

More striking was what the team found in the symptom-free GBA1 gene carriers. Of those 176 species, 142 also showed the same pattern of altered abundance in the at-risk group. The gut microbial profile of those carriers showed an intermediate pattern between healthy individuals and those with Parkinson’s. It’s a finding that researchers say could point to detectable changes in the gut before the disease fully develops.

“For the first time we identify bacteria in the gut of people with Parkinson’s that can also be found in those with a genetic risk for the disease, but before they develop symptoms,” Mr. Schapira said. “Importantly, these same changes can be found in a small proportion of the general population that may put them at increased risk for Parkinson’s.”

The team checked their results against a separate group of more than 950 additional participants from the United Kingdom, Korea, and Turkey, which included 638 people with Parkinson’s and 319 healthy controls. The patterns held.

The study also collected dietary data from participants. Those who reported a more varied and balanced diet were less likely to have gut microbiomes that suggested an elevated risk of Parkinson’s. The researchers note that more work is needed to understand what genetic or environmental factors determine whether someone ultimately develops the disease, but say the dietary finding may suggest that diet could influence gut microbial patterns associated with Parkinson’s risk.

Co-lead author Professor Stanislav Dusko Ehrlich, an honorary professor at UCL’s Queen Square Institute of Neurology, said the findings point toward a practical application. “Gut microbiome analysis can enable us to identify individuals who are at risk of developing Parkinson’s disease, so that we can suggest ways for them to reduce their own risk, such as through dietary adjustments,” Mr. Ehrlich said.

Mr. Schapira said the results open two distinct lines of follow-up work: developing microbiome-based tests to identify people at risk before symptoms begin, and investigating whether changing the bacterial population could reduce a person’s chances of developing the disease.

“This discovery opens the way not only to see if the bacteria are a way to identify those at risk of Parkinson’s, but also to see if changing the bacterial population, through dietary changes or medication, can reduce a person’s risk for Parkinson’s,” he said.

Parkinson’s disease affects the nervous system, causing progressively worsening movement problems including tremor, stiffness, and difficulty with balance. It has no cure. Several parallel UCL-led studies are testing potential treatments, including a trial examining a common cough medicine as a candidate therapy and a large-scale international trial of drugs designed to slow or stop the disease’s progression.

The study, published in Nature Medicine, was supported by the Michael J. Fox Foundation for Parkinson’s Research and the Medical Research Council.