The Small Intestinal Microbiota and the Gut–Brain Axis in Parkinson’s Disease: A Narrative Review

Researchers are increasingly focusing on understanding the microbiota’s influence on disease susceptibility and overall health. The vast number of microorganisms in our gastrointestinal tract and their extensive surface area underscore their undeniable impact on well-being. Viewing the gut microbiome as a distinct pool of microbial genetic information that interacts with the human genome highlights its pivotal role in genetically predisposed diseases. Investigating this complex crosstalk may lead to the development of novel therapeutic strategies—such as targeting dysbiosis—to complement conventional treatments and improve patient care. Parkinson’s disease (PD) is a multifactorial condition originating from a combination of genetic and environmental risk factors. Compelling evidence points to the enteric nervous system as an initial site of pathological processes that later extend to the brain—a pattern known as the ‘body-first’ model. Furthermore, most patients with PD exhibit both qualitative and quantitative alterations in the composition of the gut microbiota, including dysbiosis and small intestinal overgrowth. Nonetheless, the existing literature predominantly addresses fecal microbiota, while knowledge of upper intestinal sections, like the duodenum, remains scarce. Given the potential for microbiota modulation to impact both motor and gastrointestinal symptoms, further research exploring the therapeutic roles of balanced diets, probiotics, and fecal transplants in PD is warranted.