Intestinal Biomarkers and the Non-Motor Phenotype of Parkinson's Disease

What the study examined

This cross-sectional study by Chai and colleagues, published in the Journal of Parkinson's Disease in 2025, tested whether inexpensive, non-invasive intestinal readouts track with the clinical severity of Parkinson's disease (PD). The team enrolled 95 participants with established PD and profiled the gut across three complementary layers: inflammatory and metabolic biomarkers from stool, the composition of the gut microbiota, and inherited susceptibility to intestinal inflammation captured by polygenic risk scores.

Stool samples were assayed for fecal calprotectin (a marker of intestinal immune-cell activation) and for six short-chain fatty acids (SCFAs), the anti-inflammatory metabolites produced when gut bacteria ferment dietary fiber. Blood was used to compute polygenic risk scores for inflammatory bowel disease (PRS-IBD) and its Crohn's disease subtype (PRS-CD), alongside a PD polygenic score. Clinical severity was measured with the Unified Parkinson's Disease Rating Scale (UPDRS) and the Non-Motor Symptoms Questionnaire (NMSQ). Regression models were adjusted for age, sex, disease duration, and laxative use, and k-means clustering was used to stratify patients into subgroups.

Key findings: gut inflammation tracks non-motor severity

Constipation severity was linearly associated with greater overall disease activity, worse mentation and behavior, and more limited activities of daily living. Fecal calprotectin was elevated in participants with constipation and rose with longer disease duration, independent of age, pointing to persistent gut immune activation as PD progresses.

Patients with the worst non-motor burden showed the least favorable gut profile: lower microbial alpha-diversity (Chao1 index) and, in the clustered analysis, reduced fecal SCFA levels. Because SCFAs such as butyrate reinforce the intestinal barrier and restrain inflammation, their depletion is consistent with the elevated calprotectin. Finally, higher polygenic risk for Crohn's disease and IBD was linked to worse motor scores (UPDRS Part II), suggesting that an inherited tendency toward intestinal inflammation contributes to how PD manifests.

The mechanism: a leaky, inflamed gut feeding a gut-brain loop

The results fit the leading gut-first model of Parkinson's disease, in which pathology involving misfolded alpha-synuclein can begin in the enteric nervous system and ascend to the brain. A microbiota depleted of SCFA-producing bacteria makes less butyrate, weakening tight junctions and increasing intestinal permeability. Bacterial products then reach the mucosal immune system, recruiting neutrophils and macrophages that shed calprotectin into the lumen, which is why fecal calprotectin rises.

Chronic low-grade intestinal inflammation is thought to promote enteric alpha-synuclein aggregation and to prime systemic and neuroinflammation, plausibly accelerating dopaminergic decline and the non-motor features, constipation, cognitive and mood symptoms, that often precede or accompany the movement disorder. The genetic signal reinforces this: shared IBD risk loci imply overlapping immune biology between inflammatory bowel disease and PD, and comparative microbiome work has found the same butyrate-producing genera depleted in both conditions.

Calprotectin, nutritional immunity, and the metal-microbiome connection

The study's central biomarker is itself a metal-handling protein. Calprotectin (the S100A8/S100A9 heterodimer) is a cornerstone of nutritional immunity: neutrophils release it to chelate zinc and manganese, starving microbes of the trace metals they need to grow. A rising fecal calprotectin signal therefore does two things at once. It flags gut immune activation, and it marks a shift in the luminal metal economy, tightening the availability of zinc and manganese at the mucosal surface. This reshapes which microbes can thrive, favoring organisms with efficient metal-acquisition systems and disadvantaging metal-sensitive commensals, including some fiber-fermenting, SCFA-producing bacteria.

This is where the paper connects to the broader metal-microbiome-disease axis. Parkinsonism has a well-established environmental metal link in manganese: chronic manganese overexposure causes a Parkinson-like syndrome (manganism), and excess manganese also perturbs gut microbial communities. Iron dysregulation, meanwhile, is a hallmark of the Parkinsonian substantia nigra, and gut inflammation alters how iron is absorbed and made available to the microbiota. While this study measured inflammation and SCFAs rather than tissue metal levels, its findings sit squarely on the mechanistic pathway the axis describes: metal exposure and metal-dependent immune signaling reshape the microbiome, and a disrupted microbiome drives the inflammation associated with neurodegenerative disease.

Why it matters and what to read with caution

Because fecal calprotectin, SCFA panels, and polygenic scores are inexpensive and non-invasive, the authors frame them as candidate tools for stratifying PD, potentially identifying an inflammation-driven subtype that might respond to gut-targeted strategies such as dietary fiber, prebiotics, or resistant starch aimed at restoring SCFA production.

Important caveats apply. This is a cross-sectional study of 95 people, so it establishes associations, not causation, and cannot say whether gut inflammation drives PD severity or merely accompanies it. Fecal calprotectin is not specific to PD and can be raised by many gastrointestinal conditions. The metal-microbiome framing above is a mechanistic interpretation consistent with the wider literature, not a direct measurement made in this paper. Longitudinal and interventional studies are needed to confirm whether lowering gut inflammation or correcting metal-microbiome imbalance changes the course of Parkinson's disease.