## 6 Conclusions  
This review presents evidence of the connection between the GBA and NDDs, which takes place via direct communication between the CNS and ENS through the vagus nerve and via indirect pathways like the immune system and microbial products [14,15]. Several studies suggest that gut microbiota play a crucial role in the neuropathogenesis of NDDs by modulating the activity of the GBA [3,4]. Dysbiosis can increase intestinal and BBB permeability and can lead to changes in intestinal mucus and the translocation of gut microbes and their metabolites. These alterations lead to a state of toxic inflammation [11]. Several factors can contribute to dysbiotic microbiomes, including a diet high in refined products with low fiber intake, excessive alcohol consumption, antibiotic use, bacterial or viral infections, and medical conditions such as diabetes and NDDs [12]. The correlation between gut dysbiosis and NDDs suggests that dietary interventions targeting gut dysbiosis could be an approach for treating symptoms and delaying the neuroinflammation/d regenerative processes in NDDs. Prebiotics are used by profitable gut microbes in the large bowel, ameliorating host health. During their fermentation, the production of by-products helps cross-feeding between microorganisms. Acidic fermentation products offer a favorable milieu for beneficial bacteria, like _Lactobacillus_ and _Bifidobacteria_, and inhibit pathogenic bacteria [63]. The most valuable by-products produced by prebiotics are SCFAs that can enter into the blood circulation. Therefore, prebiotics have an impact not only on the GI tract but also on distant organs, including the brain [45]. The probiotics mostly used are _Bifidobacterium_, _Lactobacillus_, _Enterococcus_, _Streptococcus_, and _Bacillus_[64]. Studies have shown that probiotics regulate gut microbiota and diminish  
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pathogen invasion and installation, promote epithelial cell development to fortify the intestinal barrier and diminish immunomodulation. Probiotics also produce healthy products like SCFAs, with anti-inflammatory and neuroprotective activity that enter the blood circulation and cross the BBB, and regulate CNS immune cell function, inflammatory cytokines, BBB integrity, and neurogenesis, thereby inducing brain health [64; 65]. They trigger the production and release of neurotransmitters, affecting neuronal activity in NDDs [66]. The limitations of the cited studies are the sample size (many studies use small samples, which may compromise the generalizability of results), a lack of standardization in interventions (different types of prebiotics and probiotics were used, making comparisons between studies difficult), the duration of treatment (some interventions were short-term and may not reflect long-term effects), individual factors (variations in gut microbiota among individuals may influence the results), and the absence of robust clinical trials (many cited studies are experimental or based on animal models, requiring more randomized clinical trials to validate findings). Continuous studies concerning the gut microbiome provide us with information about the connection between gut microbiota and its symbiotic correlation with humans. In the future, knowledge about the effects of gut microbiomes on NDDs like AD, PD, and MS will contribute to their prevention and therapy. Daily dietary interventions could have a therapeutic function. The development of functional foods containing prebiotics and probiotics holds potential, but further clinical trials are necessary to confirm their effectiveness. In addition, as our knowledge of gut microbiomes expands, predictive biomarkers for various NDDs and their outcomes can be developed. However, extensive studies are still required based on clinical evidence in humans. Moreover, new studies related to the modification of the human gut microbiome through dietary intervention should be performed. This research received no external funding. The data are available after upon request.