A study shows that changes in the gut microbiome can identify people at risk of developing Parkinson's disease

This study aligns with and strongly supports the “body-first” model of Parkinson’s disease, which suggests that the disease may begin with alterations in the enteric and autonomic nervous systems before spreading to the brain. Evidence of changes in the microbiome had already been observed in patients with diagnosed Parkinson’s disease and in animal models. The major contribution demonstrated here is that a significant component of the microbiome (approximately 25%) already shows alterations in individuals with genetic risk but no symptoms (GBA-NMC), representing an intermediate state between healthy controls and patients with Parkinson’s disease.

From a methodological standpoint, this study combines comprehensive clinical data and fecal metagenomics from a primary cohort of 464 individuals and employs a novel analysis based on the consistency of bacterial abundance variation across different groups (assessed using Cliff’s delta). This allows for the detection of subtle yet consistent changes across a large proportion of the microbiome, rather than focusing solely on extreme variations in a few species. Thus, the researchers have taken a step further by creating a tool based on 16 bacterial species (PDMS-16), capable of identifying individuals in the general healthy population who exhibit a clinical profile closer to that of Parkinson’s patients. Equally important, the main findings of this study were not limited to the original cohort but were validated in three independent and geographically diverse cohorts (the United States, Korea, and Turkey), lending great robustness to the results obtained.

However, as the authors themselves note, the study design was cross-sectional (a snapshot at a given point in time) and included a small number of asymptomatic individuals at genetic risk (43 GBA-NMC individuals), so it cannot be confirmed which at-risk individuals will actually go on to develop the disease. Longitudinal studies over time are needed to confirm conversion to clinical Parkinson’s disease.

Although alterations in the microbiome or digestive disorders have previously been associated with neurodegenerative diseases such as Parkinson’s, several highly relevant conclusions can be drawn from this study that shift the perspective on the disease. Alterations in the gut microbiome evolve progressively with the development of the disease and are not merely a response to pharmacological treatment or to the late-stage symptoms of Parkinson’s. Furthermore, changes in certain bacterial species are strongly correlated with prodromal symptoms (those that precede motor symptoms by years), such as autonomic dysfunction, chronic constipation, REM sleep disorders, or depression. This is observed both in individuals with genetic risk and in the “healthy” population with a high vulnerability to developing the disease. The main conclusion is that the gut microbiome has real potential to serve as a non-invasive early marker. Assessing the composition of the microbiome could help identify, within the general population (with or without known genetic risk), those individuals who are in the preclinical phase and progressing toward Parkinson’s, opening a crucial window of opportunity for future preventive neuroprotective therapies.

The study presents several elements that suggest reasonable methodological quality, such as the use of a relatively large sample size, the inclusion of different groups (patients with Parkinson’s disease, carriers of GBA1 variants without clinical manifestations, and healthy controls), and the validation of the findings in independent cohorts from different countries. Furthermore, the incorporation of fecal metagenomic data and the attempt to apply quantitative metrics such as Cliff’s delta suggest a sophisticated analytical approach. However, the overall quality of the study should be interpreted with caution, as the robustness of its conclusions depends not only on sample size or replication but also on the study design and control of external variables.

Among the main limitations, the fundamentally cross-sectional nature of the study stands out, which prevents the establishment of clear causal relationships. That is, it cannot be determined whether alterations in the gut microbiota contribute to the development of Parkinson’s disease or whether, on the contrary, they are a consequence of pathophysiological processes already underway, including in subclinical phases. Furthermore, the gut microbiota is strongly influenced by multiple confounding factors, such as diet, medication use, lifestyle, or geographic environment, and it is not entirely clear to what extent these have been rigorously controlled. Also relevant is the smaller size of the group of asymptomatic GBA1 carriers, which could affect the robustness of the comparisons and the interpretation of the concept of an “intermediate” microbiota. On the other hand, the use of novel analytical methods, while potentially valuable, may hinder reproducibility and comparison with other studies if not sufficiently standardized.

Regarding the implications, the study reinforces the hypothesis that there is a close relationship between the gut microbiota and Parkinson’s disease, and suggests that certain microbial profiles may be associated not only with established disease but also with risk states or prodromal phases. This aligns with a growing body of evidence linking the gut-brain axis to neurodegenerative diseases, including previous findings on early gastrointestinal alterations and changes in bacterial composition in patients with Parkinson’s. However, although these results are consistent with previous studies, they do not yet allow the microbiota to be established as a reliable clinical biomarker or as a proven causal factor.

In light of previous evidence, the fact that associations between digestive disorders, microbiota alterations, and diseases such as Parkinson’s had already been described suggests that we are dealing with a consistent phenomenon, albeit one that is still incompletely understood. Based on studies such as this one, we can begin to conclude that the gut microbiota may play a significant role in the early stages of the disease or in modulating risk, especially in individuals with a genetic predisposition. However, conclusions must be cautious: rather than a definitive marker of disease progression, the microbiota appears, for now, to be just one component within a complex system in which genetic, environmental, and physiological factors interact. Longitudinal and better-controlled studies will be necessary to determine whether these alterations have real predictive value or therapeutic potential.

Is the study of high quality?

“This study is methodologically sound. In addition to involving a large number of patients and controls, it includes, for the first time, individuals who, despite having the genetic mutation most closely associated with Parkinson’s, do not develop the disease. It also employs an innovative analysis of the microbiota, although I feel it should be supplemented with metabolomics—that is, not just identifying which bacteria are present, but also determining which metabolites they produce.”

Are there any limitations to consider?

“In the field of the microbiome, we can only compare patients and healthy individuals and observe the differences statistically, but we don’t know if this is a cause or a consequence. The authors themselves mention this, and now the next step is to validate these changes prospectively, as well as to incorporate metabolomic studies to identify what those bacteria are doing differently compared to healthy controls.”

What are the implications, and how does this fit with existing evidence?

“The biggest implication is that in individuals with mutations that predispose them to Parkinson’s, their gut microbiome resembles that of patients more than that of healthy individuals, suggesting that we should study these healthy mutation carriers with similar bacteria (to understand why they do not develop the disease or why it is delayed). This leads to the development of new lines of research, as well as predictive tests based on human and microbial genomics to assign the probability of developing Parkinson’s to a subject while they are still healthy.”

Alterations in the microbiota or digestive disorders have previously been associated with neurodegenerative diseases such as Parkinson’s. What conclusions can we begin to draw?

“This is the most difficult question to answer. Indeed, the focus has long been on the gut in relation to degenerative neurological diseases. Significant differences have been found between patients and controls, but no specific microorganism has been identified. This is not a microorganism infection, but rather a pathogenic colonization of an entire ecosystem, where it is not easy to pinpoint the specific cause. Despite changes in composition, we all believe that metabolic changes are more important, especially because these bacteria produce substances that can be neurotoxic, but they can also degrade others that are neuroprotective. There is much discussion about how the microbiota modifies drugs, and in the case of Parkinson’s disease, this is linked to microorganisms that degrade dopamine, which is why some patients stop responding to medication. Despite all the existing literature on the gut-brain axis, the specific mechanism associated with each disease has not yet been pinpointed, but we have no doubt that the microbiota contributes to neurodegenerative diseases, and specifically that Parkinson’s begins in the gut. One factor common to all of them is severe constipation, which prolongs bacterial fermentation of food; improving intestinal transit is key to preventing this overfermentation.”

The main problem with studies examining the association between the microbiota and specific diseases is that they typically lack a design suitable for understanding their clinical implications or determining whether the results are generalizable. This study, however, features an impeccable design (within the limitations of observational studies) in which it not only describes a healthy population and a diseased population (Parkinson’s patients), but also studies patients with a genetic variant that carries a high risk of developing the disease (somewhere between healthy and diseased individuals) using a large sample size. Consistently, they describe a subgroup of bacteria with predictive value for the development of the disease in its early stages and successfully validate it in independent cohorts from other populations. The metabolic functions of these bacteria allow us to explain—or at least generate plausible hypotheses—about how Parkinson’s disease begins in the gut rather than in the brain years before the characteristic neurological symptoms of Parkinson’s develop.

Intestinal symptoms, such as constipation, are characteristic of the gut in Parkinson’s patients, but the described intestinal dysbiosis appears to precede the development of the disease by years; therefore, the implications of the study are highly significant from the perspective of developing early intervention, whether in the form of dietary recommendations for high-risk individuals or the possibility of replacing damaged microbiota with healthy microbiota, such as through fecal microbiota transplantation.

The identified microbial signature must be validated prospectively, but this article undoubtedly presents very promising results.