There is a lot of evidence establishing that nervous system development is related to the composition and functions of the gut microbiome. In addition, the central nervous system (CNS) controls the imbalance of the intestinal microbiota, constituting a bidirectional communication system. At present, various gut-brain crosstalk routes have been described, including immune, endocrine and neural circuits via the vagal pathway. Several empirical data have associated gut microbiota alterations (dysbiosis) with neuropsychiatric diseases, such as Alzheimer's disease, autism and Parkinson's disease, and with other psychological disorders, like anxiety and depression. Fecal microbiota transplantation (FMT) therapy has shown that the gut microbiota can transfer behavioral features to recipient animals, which provides strong evidence to establish a causal-effect relationship. Interventions, based on prebiotics, probiotics or synbiotics, have demonstrated an important influence of microbiota on neurological disorders by the synthesis of neuroactive compounds that interact with the nervous system and by the regulation of inflammatory and endocrine processes. Further research is needed to demonstrate the influence of gut microbiota dysbiosis on psychiatric and psychological disorders, and how microbiota-based interventions may be used as potential therapeutic tools.

Diet is one of the most important external factor shaping the composition and metabolic activities of the gut microbiome. The gut microbiome plays a crucial role in host health, including immune system development, nutrients metabolism, and the synthesis of bioactive molecules. In addition, the gut microbiome has been described as critical for the development of several mental disorders. Nutritional psychiatry is an emerging field of research that may provide a link between diet, microbial function, and brain health. In this study, we have reviewed the influence of different diet types, such as Western, Mediterranean, vegetarian, and ketogenic, on the gut microbiota composition and function, and their implication in various neuropsychiatric and psychological disorders.

The homeostasis of the skin microbiome can be disrupted by both extrinsic and intrinsic factors, leading to a state of dysbiosis. This imbalance has been observed at the onset of persistent skin diseases that are closely linked to mental health conditions like anxiety and depression. This narrative review explores recent findings on the relationship between the skin microbiome and the pathophysiology of specific skin disorders, including acne vulgaris, atopic dermatitis, psoriasis, and wound infections. Additionally, it examines the psychological impact of these skin disorders, emphasizing their effect on patients’ quality of life and their association with significant psychological consequences, such as anxiety, depression, stress, and suicidal ideation in the most severe cases.

An estimated 2.6 billion individuals are currently living with overweight or obesity, and this number is projected to exceed 4 billion by 2035. Consequently, unless this increasing trajectory is effectively addressed, the trend is expected to continue in the coming years. The gut microbiome has emerged as a central regulator of host metabolism and energy homeostasis, making its detailed characterization crucial for the advancement of innovative therapeutic strategies and for elucidating mechanisms underlying metabolic health and disease. This review examines human obesity through the lens of the gut microbiome, providing a comprehensive overview of its role by addressing gut microbiome alterations, microbiome-driven mechanisms, dietary influences, prebiotic effects, microbiome-based therapeutics, and other approaches in the treatment of obesity and related metabolic disorders. The composition of the gut microbiome is altered in obesity and characterized by reduced microbial diversity and inconsistent shifts in dominant bacterial phyla, which collectively contribute to metabolic dysregulation. The gut microbiome influences obesity through multiple mechanisms. These include regulating energy balance and insulin sensitivity via short-chain fatty acids, inducing chronic inflammation, modulating metabolic and appetite genes, altering bile acid signaling, and promoting fat storage by inhibiting fasting-induced adipose factor. Dietary patterns exert a profound influence on gut microbiome composition and function, with plant-based diets conferring protective effects against obesity and its comorbidities. Microbiome-based therapeutics, including probiotics, synbiotics, and fecal microbiota transplantation, have demonstrated potential in modulating key metabolic and inflammatory pathways associated with obesity. As the scientific understanding of the human gut microbiome continues to advance, the integration of microbiome-based therapies into standard clinical practice is poised to become increasingly feasible and therapeutically transformative, particularly for obesity, a complex condition that demands innovative and customized interventions.

The formation of the human gut microbiome initiates in utero, and its maturation is established during the first 2–3 years of life. Numerous factors alter the composition of the gut microbiome and its functions, including mode of delivery, early onset of breastfeeding, exposure to antibiotics and chemicals, and maternal stress, among others. The gut microbiome–brain axis refers to the interconnection of biological networks that allow bidirectional communication between the gut microbiome and the brain, involving the nervous, endocrine, and immune systems. Evidence suggests that the gut microbiome and its metabolic byproducts are actively implicated in the regulation of the early brain development. Any disturbance during this stage may adversely affect brain functions, resulting in a variety of neurodevelopmental disorders (NDDs). In the present study, we reviewed recent evidence regarding the impact of the gut microbiome on early brain development, alongside its correlation with significant NDDs, such as autism spectrum disorder, attention-deficit/hyperactivity disorder, Tourette syndrome, cerebral palsy, fetal alcohol spectrum disorders, and genetic NDDs (Rett, Down, Angelman, and Turner syndromes). Understanding changes in the gut microbiome in NDDs may provide new chances for their treatment in the future.

The initial gut colonization of the infant plays a pivotal role in shaping the immune system, developing the intestinal tract, and influencing host metabolism, all of which are strongly influenced by several determinants, such as gestational age at birth, mode of delivery, neonatal feeding practices, early-life stress (ELS), and exposure to perinatal antibiotics. However, resulting gut microbiome (GM) dysbiosis may alter this developmental programming, leading to long-term adverse health outcomes. This narrative review synthesizes current knowledge on early-life GM development and its long-term impact on health. Specifically, it addresses how early-life GM dysbiosis may affect the trajectory of physiological processes, predisposing individuals to conditions such as allergic diseases, metabolic disorders, type 1 diabetes, inflammatory bowel disorders, and atherosclerotic cardiovascular diseases. In addition, it examines the influence of probiotic and prebiotic supplementation during pregnancy and early life in shaping infant GM composition, as well as the impact of ELS-induced GM dysbiosis on mental health. Recent research suggests that the early-life microbiota initiates long-lasting effects, and inadequate or insufficient microbial exposure triggers inflammatory responses associated with several physiological conditions. Although several studies have reported a connection between ELS and the GM during both prenatal and postnatal periods, a unified microbiome signature linked to either prenatal or postnatal stress remains to be fully elucidated. Thus, future studies are needed to establish causality and determine whether modifiable factors affecting the GM could be targeted to improve gut health, especially in children exposed to contextual stress or adverse conditions.