Gut Microbiota and Tryptophan Metabolites in Parkinson's Disease

Inclusion Criteria: * Participants aged 45 to 90 years, of any sex. Participants able to understand the study, provide written informed consent, and provide an acceptable stool sample according to the study procedures. For the Parkinson disease group: clinically established or clinically probable Parkinson disease diagnosed by a neurologist according to the Movement Disorder Society Clinical Diagnostic Criteria. For the Parkinson disease group: preferably a disease duration of 5 years or less and Hoehn-Yahr stage I to III. For the Parkinson disease group: stable antiparkinsonian medication regimen for at least 4 weeks before enrollment. For the healthy control group: no history of Parkinson disease, parkinsonism, or another neurodegenerative disorder and no evident parkinsonian symptoms at screening. Healthy controls matched individually to participants with Parkinson disease by sex, age within 5 years, and body mass index within 3 kg/m\^2 Exclusion Criteria: * Use of systemic antibiotics within 3 months before stool collection. Use of probiotics, prebiotics, tryptophan, 5-hydroxytryptophan, live biotherapeutic products, or other microbiome-related supplements within 4 weeks before stool collection. Acute infection, fever, acute diarrhea, gastroenteritis, colonoscopy preparation, or a major dietary change within 4 weeks before stool collection. Active inflammatory bowel disease, celiac disease, short-bowel syndrome, gastrointestinal malignancy, or major gastrointestinal surgery within 6 months. Active cancer, decompensated liver or kidney disease, severe cardiovascular or hematological disease, active autoimmune disease, or current systemic glucocorticoid or immunosuppressive treatment. Pregnancy or breastfeeding. Long-term exclusive enteral or parenteral nutrition. Inability to provide an acceptable stool sample or essential clinical information. Any other condition considered by the investigator to compromise participant safety, study adherence, or interpretation of the results.