The Role of METhanogens in the PROgression Of Parkinson's Disease and Related Neurological Conditions

Inclusion Criteria: People with PD: * 55 years of age or above; * MDS criteria for Idiopathic PD; * H\&Y\<3. People at high risk of developing PD (people with REM Sleep behaviour disorders): * 55 years of age or above; * RBD diagnosis confirmed by polysomnography. Other conditions affecting cognition (e.g. dementia, mild cognitive impairment): * 55 years of age or above; * Diagnosis of non-PD dementia or MCI, or, MoCA score ≤25 Healthy Controls: * 55 years of age or above * MoCA total score ≥26. Exclusion Criteria: * Presence of other neurological disorder, chronic inflammatory/autoimmune disorder, active cancer, active metabolic disease, diabetes type I and II, and active or latent infection; * Use of immunosuppressive drugs within the preceding 12 months; * Use of oral/intravenous steroids within the preceding 3 months; * Regular use (more than twice per week) of non-steroidal anti-inflammatory drugs (e.g. ibuprofen, naproxen, diclofenac, meloxicam) or over 75mg aspirin; * Participation in other interventional studies, within the preceding 3 months; * Consumption of laxatives, stool softeners, stool bulking agents, motility agents, iron supplements and probiotics within the preceding 3 months; * Current smoker * Inability to understand or speak English fluently. For participants in Component 2 of the study (Experimental Medicine Study), additional exclusion criteria will be in place, namely, known allergy to any of the probiotic's ingredients: Bulking agent (isomalt), sweetener (xylitol), L. reuteri DSM 17938, strawberry flavouring and flavour enhancer (citric acid).