Using tongue coating to spot early signs of Parkinson’s disease

By Dr. Sanchari Sinha Dutta, Ph.D.Reviewed by Lauren HardakerJul 31 2025

By analyzing what’s on the surface of the tongue, scientists uncover powerful clues to detecting Parkinson’s disease: no needles, no scans, just a simple swab.

Study: Microbiome and metabolome integrated analysis: exploring potential diagnostic approaches for Parkinson’s disease using tongue coating samples. Image credit: Andrey_Popov/Shutterstock.com

Researchers at the Shanghai Jiao Tong University School of Medicine, China, explored the diagnostic potential of tongue coating samples for the early detection and monitoring of Parkinson’s disease. In their study, which is published in Frontiers in Microbiology, tongue coating sampling emerges as a promising tool for disease diagnosis, highlighting the involvement of microbial communities and metabolic compounds in the disease pathogenesis.

Background

Parkinson’s disease is the second most prevalent neurodegenerative disease, characterized by resting tremors, muscle rigidity, and slow movement. In high-income countries, the incidence rate of this disease is approximately 14 cases per 100,000 individuals.

The disease pathogenesis is associated with mitochondrial dysfunction, oxidative stress, protein aggregation, impaired autophagy, and neuroinflammation. Such complex pathogenesis limits diagnostic and therapeutic options for Parkinson’s disease.

Conventional methods currently used for Parkinson’s disease diagnosis require cerebrospinal fluid or blood sample collection, which are invasive procedures and thus can impose significant psychological stress and financial burden on patients, and can increase the risk of infection due to invasive procedures. Moreover, these methods are costly and often involve complex procedures.

These shortcomings highlight the need for developing more convenient, accurate, and early diagnostic methods. The current study was designed to meet this requirement. The basic idea was initiated when the researchers noticed a distinct type of layer on the tongue surface (tongue coating) in patients with Parkinson’s disease. This tongue coating is characterized by a notable thick and greasy appearance, with some presenting abnormal white or yellow discoloration.

Tongue coating is a critical observation target in the “visual diagnosis” of traditional Chinese medicine. It typically contains a mixture of bacteria, epithelial cells, saliva, blood metabolites, and food residues.

Tongue coating analysis is gaining significant attention in the biomedical field, with its applications gradually increasing for diagnosing oral, gastrointestinal, and many other health conditions.

Given the easy, noninvasive, low-risk, and cost-effective nature of tongue coating sampling, researchers at the Shanghai Jiao Tong University School of Medicine conducted a compositional analysis to improve the diagnosis and prognosis of Parkinson’s disease.

The study

The researchers conducted compositional analyses of microorganisms and metabolic compounds in tongue coating samples collected from 36 Parkinson’s disease patients and 31 healthy individuals.

By integrating microbiome data with metabolomics data, the researchers aimed to identify reliable biomarkers that could serve as non-invasive diagnostic tools and provide new therapeutic targets. They also applied a Random Forest machine learning model to evaluate the predictive value of identified biomarkers.

Key findings

The microbiome analyses of tongue coating samples revealed significant differences in the abundance of several bacterial communities between patients with Parkinson’s disease and healthy controls, including Firmicutes, Bacteroidetes, and Actinobacteria. These differential bacterial communities may serve as potential biomarkers for the pathophysiological changes in Parkinson’s disease. 

Similarly, the metabolomics analyses of tongue coating samples identified noticeable differences between the two study groups. The findings revealed a reduction in palmitoylethanolamide levels in Parkinson’s disease patients and a decrease in carnitine levels in advanced-stage patients with severe disability and mild cognitive impairment. Palmitoylethanolamide is an endogenous fatty acid amide that has been widely studied in the context of neurodegenerative diseases.

The study also identified elevated levels of docosanamide, though this compound is likely due to external contamination and is not considered a reliable biomarker.

Study significance

The study identifies a potential link between specific changes in microbial composition and metabolic profile within the tongue coating, which may substantially contribute to the pathogenesis and progression of Parkinson’s disease.

Visual characteristics of tongue coating have been widely explored as a tool to diagnose oral and gastrointestinal diseases, as well as several other healthy conditions, including rheumatoid arthritis, pneumonia, lung cancer, chronic kidney disease, heart disease, osteoporosis, and ischemic stroke. For the first time, the current study focused on the compositional changes in tongue coating samples from patients with Parkinson’s disease, offering insights into disease pathogenesis at the molecular level.    

The observed microbial alterations in tongue coating are similar to those reported by other studies analyzing fecal samples from Parkinson’s disease patients. Tongue coating sampling is more convenient, clean, and safe than fecal samples, further highlighting the benefits of considering tongue coating samples for diagnostic purposes.

Among the metabolic alterations identified, the most prominent ones were the upregulation of certain externally derived compounds and the downregulation of palmitoylethanolamide, which is primarily derived from the body itself. These findings suggest that the disease state of patients with Parkinson’s disease is influenced by both environmental and endogenous factors.       

Significantly reduced levels of palmitoylethanolamide in tongue coating samples suggest that this metabolite could be a promising biomarker for differentiating between patients with and without Parkinson’s disease. Palmitoylethanolamide is produced and released in the central nervous system from neurons and glial cells. This metabolite is vital in maintaining gut barrier integrity, mitigating inflammation and pain, and regulating energy metabolism.    

Carnitine is another vital metabolite identified in the study. Its levels were reduced in advanced-stage patients and those with mild cognitive impairment. This observation is similar to previous studies reporting neuroprotective and cognition-improving effects of this metabolite.

Notably, the study emphasizes that these findings are preliminary and based on a limited sample size. Further validation is needed before these biomarkers can be adopted in clinical diagnostics.
The application of a machine learning classification model (Random Forest) achieved nearly 89% accuracy in distinguishing Parkinson’s disease patients from healthy individuals, supporting the diagnostic potential of the identified metabolomic profile.

Overall, the study findings underscore the potential of tongue coating samples as a non-invasive, cost-effective tool for diagnosing and monitoring Parkinson’s disease. While promising, the results require further large-scale studies to confirm their clinical applicability.

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