Autism Spectrum Disorder (ASD) is a complex neurodevelopmental disorder characterized by deficits in social interaction, communication, and the presence of restricted, repetitive behaviors. The rising global prevalence of ASD suggests a multifactorial etiology involving genetic, environmental, and neurodevelopmental factors. This review explores the establishment of the early-life microbiome, highlighting rapid microbial colonization from maternal and environmental sources. Emerging evidence indicates that delivery mode and infant feeding practices may influence ASD susceptibility. Although the concept of a sterile intrauterine environment remains debated, its investigation is valuable. The bidirectional "microbiota-gut-brain axis" has emerged as a critical pathway linking gut microbiota and brain function, offering potential therapeutic targets for ASD. Dietary patterns in children with ASD are often characterized by selectivity and restriction, which may disrupt gut microbiota composition and exacerbate gastrointestinal symptoms, thereby increasing ASD risk. Nutritional interventions and early behavioral therapies are thus essential. The gluten-free, casein-free (GFCF) diet remains controversial, with inconsistent evidence regarding its efficacy. Probiotic supplementation shows strain-specific effects, necessitating rigorous evaluation before clinical application. Given the heterogeneity of ASD, pharmacological treatments have shown limited universal efficacy. This review provides a comprehensive analysis of the interplay between diet, gastrointestinal symptoms, and ASD, evaluates the gut-brain axis as a mechanistic framework, and assesses the therapeutic potential of microbial interventions, including probiotics, prebiotics, and fecal microbiota transplantation (FMT). While promising findings have emerged, further well-designed clinical studies are needed to elucidate the complex etiology of ASD and validate therapeutic strategies.
