Small Intestinal Bacterial Overgrowth as Potential Therapeutic Target in Parkinson’s Disease

Alpha-synucleinGut DysbiosisGut-brain axisHydrogen Breath TestMethane

Small Intestinal Bacterial Overgrowth as Potential Therapeutic Target in Parkinson’s Disease

Why this matters for your health

Parkinson’s disease (PD) affects more than 6 million people worldwide, and no treatment can yet prevent, slow or reverse it. Current drugs only ease symptoms, and their benefit fades as the disease advances. Pathology in sporadic PD may begin in the gut years before motor symptoms appear. Small intestinal bacterial overgrowth (SIBO) is common in PD, so it could be a modifiable target. The authors argue it may affect gut symptoms, motor fluctuations and absorption of levodopa, and may possibly influence disease progression.

Summary

This is a narrative review (no new patient data). It covers how SIBO is defined and tested (bacterial count versus hydrogen and methane breath tests) and how common it is in PD. It then discusses how SIBO might influence PD through gut inflammation, a leakier intestinal barrier and exposure to bacterial products such as lipopolysaccharide (LPS) that may promote alpha-synuclein misfolding. It also covers possible effects on gut motility, nutrition and levodopa absorption. The review looks at two gases made by gut microbes. Molecular hydrogen (H₂) may be neuroprotective by reducing oxidative stress, while methane from methanogenic archaea may be either protective or harmful. It also discusses antibiotic and other SIBO treatments and the expected symptomatic and possible disease-modifying effects of eradication. The authors conclude that SIBO eradication and microbiota modulation need further study as possible disease-modifying strategies in PD.

Key findings

The commonly accepted reference definition of SIBO is at least 10³ CFU/mL in small intestinal fluid. Some authors suggest 10⁵ CFU/mL because healthy people can have counts that high, so the authors say it is not a true gold standard.
Breath tests are widely used. A positive H₂ test is a rise of at least 20 ppm by 90 minutes after 75 g glucose or 10 g lactulose. A positive methane test is at least 10 ppm. Predictive values are about 70%, higher when H₂ and methane are both measured. The glucose test does not measure methane and may miss SIBO in people with fast gastric transit.
About half of people with PD test positive for SIBO, compared with up to a quarter of the general population. Reported prevalence ranges from 14% to 67% depending on population and test method.
The review cites a recent meta-analysis with a pooled SIBO prevalence in PD of 47% (95% CI 36–56). It was 52% in Western countries versus 33% in Eastern countries, with an odds ratio of 5.22 (95% CI 3.33–8.19) compared with healthy controls. The original meta-analysis abstract gave 46%.
That meta-analysis found no significant link between SIBO and constipation, bloating, diarrhea or disease duration. The authors suggest H₂-predominant and methane-predominant SIBO may have opposite effects, which could cancel out in pooled analyses.
Findings on motor complications are mixed. Some studies link SIBO to worse motor fluctuations (longer “off” time, delayed “on”), and eradication improved fluctuations in small studies. One suggested explanation is that Enterococcus bacteria with decarboxylases may break down levodopa before absorption, although SIBO eradication did not change levodopa pharmacokinetics.
No correlation between SIBO and weight loss was found in PD. Milder gut symptoms, especially constipation, were reported in PD patients with SIBO.
The gut microbiota in PD seems to produce less net H₂. One simulation estimated a more than 2-fold reduction. H₂ is described as antioxidant and anti-inflammatory, and it was neuroprotective in animal models.
Human H₂ trials were mixed. A 48-week pilot trial of hydrogen-enriched water improved motor scores, but a later multicenter trial of hydrogen water and a 16-week trial of H₂ gas inhalation found no benefit.
Methane producers (archaea) may have protective effects, such as clearing misfolded proteins, or harmful ones, such as promoting inflammation. The ACG guideline proposes the term intestinal methanogen overgrowth (IMO) to replace “methane-predominant SIBO”. The review notes that the evidence for these roles in PD is limited and largely experimental.
Symptomatic SIBO is usually treated with 7–14 days of antibiotics (rifaximin, metronidazole, ciprofloxacin, doxycycline, neomycin or amoxicillin-clavulanate), together with treating the cause, for example with prokinetics. Evidence that any one antibiotic works better is modest. Probiotics and prebiotics may help, but evidence is limited.
Small studies show SIBO eradication in PD is safe and achievable over up to 6 months. The authors propose that screening for and eradicating SIBO, even when it causes no symptoms, might be considered. This is a hypothesis, and trials of a disease-modifying effect are lacking.

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Citation

Dănău A, Dumitrescu L, Lefter A, Tulbă D, Popescu BO. Small Intestinal Bacterial Overgrowth as Potential Therapeutic Target in Parkinson’s Disease. Int J Mol Sci. 2021;22(21):11663. doi:10.3390/ijms222111663