Consequences of Western and Mediterranean Diets’ Nutrients on the Microbiota–Gut–Brain Axis

Background: The prevalence of neurodegenerative diseases like Alzheimer’s and mental disorders like depression or anxiety appears higher in patients with gastrointestinal tract diseases like inflammatory bowel disease (IBD). Conversely, depressed patients have higher rates of gastrointestinal disorders. These observations suggest bidirectional communication between the brain and the gastrointestinal tract, the so-called “gut–brain axis”. Moreover, an altered microbiota, called “dysbiosis”, has been reported in these diseases, highlighting the network between gut microbes and their host. The emergence of the microbiota as a key regulator of the gut–brain dialog has led to the establishment of the concept of the “microbiota–gut–brain axis”. Objectives: In this narrative review, we outline the main interaction channels between the gastrointestinal tract and the brain. Then, we summarize current knowledge of two major diets (i.e., Western and Mediterranean diets) and the principal dietary components that modulate the microbiota–gut–brain axis to discuss the mechanisms putatively involved in intestinal, psychiatric, and neurological disorders. Conclusions: Diet is a major factor influencing the gut microbiota, and consequently, also putatively systemic mechanisms through the microbiota–gut–brain axis. Indeed, the composition of the diet is crucial for health and disease. Despite the main role of diet, the physiological, cellular, or molecular mechanisms involved in the complex communication between the microbiome, gut, and brain are still poorly understood.