One of the key indicators of Parkinson's disease, a degenerative brain disorder, is “REM sleep behavior disorder (RBD),” characterized by kicking, shouting, and other severe sleep disturbances.

Patients with RBD prior to Parkinson's disease diagnosis exhibited impaired gut health from the early stages, while those without RBD before diagnosis showed balanced gut health initially but later developed RBD, similar to the group with RBD two years after diagnosis.

Parkinson's disease is a degenerative brain disorder caused by the abnormal accumulation of the protein “alpha-synuclein,” which plays a crucial role in dopamine regulation, leading to damage to neurons. However, cases where protein aggregation first appeared in the gut nervous system or peripheral nerves rather than the brain have been identified, leading to the classification of the disease into two subtypes based on the onset pathway: brain-first (starting in the brain); and gut-first (starting in the gut or peripheral nervous system and transmitting signals to the brain).

Patients with the brain-first type often exhibit delayed or absent REM sleep behavior disorders, while those with the gut-first type frequently show REM sleep behavior disorders before Parkinson's disease symptoms. Although previous studies have reported similar changes in gut microbiota between Parkinson's disease patients and those with REM sleep behavior disorders, the differences in gut environment based on the disease's onset pathway remain unclear.

Asan Medical Center announced on Monday that a research team led by Professors Chung Sun-ju and Jo Sung-Yang of the Department of Neurology had confirmed this through a study comparing and analyzing changes in gut microbiota according to the presence or absence of REM sleep behavior disorder and disease progression in 104 Parkinson's disease patients and 85 control subjects treated at the hospital from 2019 to 2024.

Among the 104 Parkinson's disease patients, 57 had experienced REM sleep behavior disorder before being diagnosed with Parkinson's disease, while 47 had not experienced RBD before being diagnosed with Parkinson's disease. The 85 control subjects were recruited from among the spouses of patients with Parkinson's disease and were compared under similar living conditions.

The results showed that patients who experienced RBD before being diagnosed with Parkinson's disease had higher levels of harmful bacteria, such as Akkermansia and Escherichia, which break down the intestinal mucus layer and form a bacterial membrane in the intestines, from the early stages of the disease. Gene expression related to protecting the intestinal barrier was significantly reduced, creating an environment where harmful bacteria could attach to the barrier and inflammation was easily triggered. This condition persisted with little change even as the disease progressed.

In contrast, Parkinson's disease patients without RBD had high levels of fiber-related beneficial bacteria, such as Prevotella and Faecalibacterium, which help protect the intestinal mucosa, similar to healthy control groups at the time of diagnosis. However, two years after diagnosis, the gut microbiota composition showed a similar trend to that of patients with RBD.

Despite consuming 34–36 grams of dietary fiber per day, which exceeds the general recommended intake of 25 grams, the study participants exhibited a clear imbalance in their gut microbiota. This suggests that restoring gut microbiota balance through dietary adjustments alone is a challenging task.

“As we enter an ultra-aged society, the number of Parkinson’s disease patients in Korea is rapidly increasing. Parkinson’s disease is difficult to detect in its early stages as its symptoms resemble those of general aging processes. This study suggests the possibility that gut microbiota could serve as an indicator for the early diagnosis of Parkinson’s disease,” Professor Chung said.

Professor Jo remarked, “The composition of gut microbiota showed significant differences depending on the presence or absence of REM sleep behavior disorder. By focusing on the gut microbiota environment of Parkinson’s disease patients with RBD and developing customized treatment strategies, we expect to significantly improve patients’ quality of life.”