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

Background and Rationale

Gut microbes are crucial for health. They produce important substances like vitamines, metabolites, and gases, such as hydrogen and methane. Intestinal Methanogen Overgrowth (IMO) occurs when there is an overgrowth of methane-producing organisms called archaea. IMO can be measured with a non-invasive breath test is strongly associated with whole gut transit time and constipation.

Studies suggest that approximately half of people with Parkinson's disease (PD) test positive for IMO, and a positive test has been linked to worse movement control (motor performance) and motor fluctuations.

The presence of high methane levels is known to slow gut transit, cause constipation, and can hinder the absorption of levodopa, the main drug for PD symptom relief. This, in turn, may worsen inflammation and affect the gut lining. Crucially, constipation and body-wide inflammation are predictors of cognitive decline in PD, however, no study has yet examined the link between IMO and cognition.

Beyond their role in the gut, these methane-producing microbes might directly impact the body and the brain. They can produce their own signaling molecules, like neurotransmitters and metabolites, and may even trigger inflammatory pathways. While some research suggests methane may have neuroprotective effects in animals, other studies link high methane levels to metabolic issues in humans, highlighting the urgent need for human-focused investigation.

Probiotics such as Limosilactobacillus reuteri, also known as *L. reuteri* has long been used as a food supplement, with anti-inflammatory and gut-barrier-supporting effects. One study has previously reported that supplementation with the strain L. reuteri DSM17938 reduced methane production in constipated individuals. The strain L. reuteri DSM17938 is safe, colonises the gut, is available in chewable form, and has been associated with reduced inflammation and methane production. However, further studies are needed to investigate whether this strain reduces methane production and what impact it may have in people with PD.

Study design:

Study Description

Component 1 - Observational Study Despite the relationship between high methane breath levels and worse motor function in Parkinson's disease, the relationship between methane, cognition, and disease progression has not been investigated.

Component 1 of the Met-Pro study is an observational study designed to investigate the role of IMO in PD and related brain disorders.

A total of 200 participants will be enrolled across 4 groups of people:

50 people with Parkinson's disease, 50 people at high risk of developing Parkinson's disease (people with REM Sleep behaviour disorder), 50 people with other conditions affecting cognition (e.g. dementia, mild cognitive impairment), and 50 healthy controls.

Participants will be asked to attend 2 visits, 18 months apart, and will undergo:

• Breath methane testing
• Stool analysis to count the abundance of methane-producing archaea
• Blood tests to measure markers of the immune system and gut health
• Clinical and Cognitive Evaluations using standard neurological and mental function assessments (like the MDS-UPDRS, MoCA, and ACE-III), and questionnaires including: Gastrointestinal Dysfunction Scale for Parkinson's Disease (GIDS-PD), Hospital Anxiety and Depression Scale (HADS), Food Frequency Questionnaire (FFQ), and Short Form Health Survey 36 (SF-36)

Follow-up visits at 18 months will allow assessment of disease progression.

The primary aim is to assess whether breath methane can serve as a non-invasive biomarker of cognition, motor function, and progression in PD and related conditions, and to explore links between methane, archaeal abundance, systemic inflammation, and gut-brain interactions.

Component 2 - Experimental medicine substudy A small group of 15 participants with PD, who test positive for intestinal methanogen overgrowth (≥10 ppm of methane in the breath), will take the probiotic Limosilactobacillus reuteri as a daily chewable supplement for 18 months. The primary purpose is to see if taking the probiotic reduces breath methane levels and the number of archaea in the stool. Exploratory analyses will assess potential effects on cognition, motor symptoms, and gut function.

This part of the study will provide essential first evidence on whether targeting methane-producing microbes is a viable therapeutic strategy for PD and will inform the design of larger, more definitive clinical trials in the future.

Study Objectives

Primary objectives

To determine how intestinal methanogen overgrowth relates to inflammation and disease course, and whether breath methane testing could serve as a longitudinal and predictive biomarker in PD and other neurodegenerative disorders.

1.1. Measure and compare methane levels in exhaled breath among individuals with PD, prodromal PD, other conditions that affect cognition (e.g. AD, mild cognitive impairment), and healthy controls, and assess changes over an 18-month period.

1.2. Examine the relationship between breath methane concentrations, faecal archaeal populations, and peripheral immune markers.

1.3. Assess links between methane levels, cognitive and motor performance at baseline, and subsequent clinical progression over 18 months.

Secondary objectives

• To construct a prognostic model that incorporates breath methane levels and other gut-related biomarkers to predict neurodegenerative disease progression or the onset of disease in at-risk groups.

• To test whether probiotic supplementation with Lactobacillus reuteri lowers methane exhalation and faecal archaeal abundance, and to evaluate its impact on systemic inflammatory indices and gut function measures in PD.

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).