Physiological Principles of Bariatric Surgery: The Good and the Dark Side

Obesity is a chronic, relapsing, and multifactorial disease arising from the complex interaction of neuroendocrine, genetic, and environmental factors. Traditional anthropometric measures, such as body mass index, inadequately capture the underlying pathophysiology of adipose tissue dysfunction and energy dysregulation. Metabolic and bariatric surgery remains the most effective and durable therapy for obesity and its metabolically related diseases; however, its benefits extend far beyond mechanical restriction and/or hypo- or malabsorption. This review synthesizes current evidence on the physiological mechanisms underlying metabolic and bariatric surgery, focusing on how distinct procedures modulate gut- brain signaling, energy balance, and metabolic homeostasis. Key pathways influenced by metabolic and bariatric surgery- including alterations in gut hormone secretion, entero- insular axis activity, bile acid metabolism, gut microbiota composition, vagal signaling, and adipose tissue remodeling- are discussed. The review also addresses the metabolic outcomes associated with the most common surgical techniques and explores the "dark side" of bariatric surgery, encompassing dumping syndrome, post- bariatric hypoglycemia, and weight regain. Metabolic and bariatric surgery induces systemic physiological reprogramming that transcends anatomical modification, acting primarily through neuroendocrine networks governing appetite, metabolism, and energy expenditure. A deeper understanding of these mechanisms is essential for optimizing procedure selection, improving long-term outcomes, and advancing the precision management of obesity as a chronic disease.