Gut-Brain Axis and Parkinson's Disease: The Role of Small Intestinal Microbiota from Symptom Onset to Therapeutic Perspectives

Researchers are increasingly focused on understanding how the microbiota influences disease susceptibility and contributes to overall health. Given the vast number of microorganisms inhabiting our gastrointestinal tract and the extensive surface area they occupy, their impact on our well-being is undeniable. For example, when we consider the gut microbiota's collective genetic information—referred to as the microbiome—and view our genetic profile as a blend of both microbial and human genes, it becomes evident that the microbiota may play a pivotal role in the development of genetically predisposed diseases. Investigating these complex interactions could pave the way for new therapeutic strategies, such as targeting dysbiosis, to complement conventional treatments and enhance patient care. Parkinson's disease (PD) is a multifactorial condition characterized by various genetic and environmental factors that collectively increase the risk of developing the disease. There is strong evidence of the involvement of the enteric nervous system where the pathological processes may start initially and to proceed later to the brain. Moreover, it has been observed that most of PD patients exhibit qualitative and quantitative alterations in the composition of the intestinal microbiota, such as dysbiosis and increased proliferation in the small intestine. Despite this evidence, the available literature largely focuses on information regarding the fecal microbiota, while knowledge of the microbiota in the upper sections of the intestine, such as the duodenum, remains limited. Since modulation of the microbiota may have an effect on both motor and gastrointestinal symptoms, further research exploring how a balanced diet, probiotics, and/or fecal transplants may have a role in PD therapy is warranted.