Small Intestinal Bacterial Overgrowth: Microbiome Dysregulation, Gut–Brain Axis Disruption, and Systemic Consequences

Small intestinal bacterial overgrowth (SIBO) is a gastrointestinal disorder characterized by excessive bacterial colonization in the small intestine, leading to impaired digestion and nutrient malabsorption. Increasing evidence indicates that SIBO exerts systemic effects beyond the gut, contributing to metabolic, neurological, cardiovascular, dermatological, and autoimmune conditions. Current management strategies include nonsystemic antibiotics such as rifaximin, dietary interventions (low-FODMAP and biphasic diets), and nutraceuticals including berberine, oregano oil, peppermint oil, garlic derivatives, vitamins, and magnesium. Despite demonstrated clinical efficacy, challenges persist, including high recurrence rates, antimicrobial resistance, and long-term disruption of gut microbiota. Nutraceutical and dietary approaches offer promising patient-centered alternatives but require stronger clinical validation. This review critically examines the multifactorial pathophysiology of SIBO, emphasizing gut-brain axis dysregulation, microbial dysbiosis, oxidative stress, impaired intestinal motility, anatomical abnormalities, hypochlorhydria, bile acid malabsorption, and immune dysfunction. It synthesizes current diagnostic and therapeutic strategies, highlighting antibiotics, dietary approaches, and nutraceuticals, while distinguishing robust evidence from observational or preclinical findings. Finally, it identifies key research priorities, including improved diagnostics, global clinical trials, and microbiome- and genetics-based personalized treatments for sustained remission.