## 1 Introduction  
Recent studies emphasize the impact of the gut microbiota via the gut-brain axis (GBA) on neurodegenerative diseases (NDDs), such as Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS) [1, 2, 3, 4]. NDDs are a heterogeneous group of diseases; their main clinical characteristic is the impairment of the central and peripheral nervous systems [3, 5]. Older people are mostly affected by NDDs, which are an important cause of mortality and comorbidities [1].  
The World Health Organization (WHO) reports that by 2040, NDDs will become the second cause of mortality globally [5, 6]. NDDs present a pathological aggregation of deficient proteins in the brain, like toxic Tau proteins, extracellular amyloid \(\beta\)-protein (A\(\beta\)), and \(\alpha\)-synuclein (\(\alpha\)-Syn), that lead to the progression of AD and PD, respectively [3, 7]. These accumulated proteins, stimulating astrocytes and microglia, cause damage and influence synaptic activity [3]. These pathogenic proteins enter the brain through the vagus nerve. The gut microbiota can influence vagus nerve activation, potentially leading to the accumulation of these toxic proteins in the brain [3].  
The GBA is a two-way communication pathway between the gut and brain [2; 3; 5; 8; 9]. NDDs present as a dysfunction of the GBA, suggesting the probable involvement of GBA in disease pathogenesis [3]. In this bidirectional pathway, the gut microbiota interacts with the immune system, enteric nervous system (ENS), and enteroendocrine system, supporting signal transmission via the vagus nerve and the blood circulation to the central nervous system (CNS) [3]. This involves various microorganism metabolites, neurotransmitters, hormones, and cytokines, indicating the crucial role of the gut microbiota in keeping this communication in equilibrium [3; 5; 10].  
The gut microbiota interacts with its host in a symbiotic relationship [3]. The intestinal microbiota has crucial functions, such as the development and maturation of the immune system [1], the integrity of the gut barrier, the permeability of the blood-brain barrier (BBB), the homeostasis of the endocrine system, and the development of astrocyte cells and microglia [1; 2; 3; 5]. The gut microbiota is a very important constituent of the GBA, and an unbalanced gut microbiota, known as dysbiosis, contributes to the increased permeability of both the intestinal barrier and the BBB. Furthermore, dysbiosis leads to a state of toxic inflammation [11] that causes oxidative stress, pathological protein accumulation, neuroinflammation, neuronal death, and alteration of the brain architecture, contributing to the etiopathogenesis of NDDs [12].  
The type of diet is a very important factor that can modify the constitution of the gut microbiota [9; 13]. In this way, dietary interventions modulating gut microbiota could offer probable therapeutic strategies for controlling the pathology of NDDs [1]. This review aims to investigate the most recent literature regarding the impact of gut microbiota through GBA on NDDs. In addition, it examines potential therapeutic approaches through nutritional interventions targeting gut microbiota. The review also investigates the potential role of factors that ameliorate the constitution of the gut microbiota, such as prebiotics and probiotics.