Obesity has reached pandemic levels worldwide and is increasingly recognized as a multifactorial condition beyond excess caloric intake and sedentary lifestyle. Accumulating evidence emphasizes that the gut microbiota (GM), primarily composed of Firmicutes and Bacteroidetes, plays a crucial role in regulating energy balance, immune response, and host metabolism. Gut dysbiosis, characterized by reduced microbial diversity and altered phylum-level composition and shifts toward commonly observed higher Firmicutes-to-Bacteroidetes ratios (although this finding is inconsistent across studies), contributes to enhanced energy harvest, systemic inflammation, and metabolic dysfunction. Key mechanisms involve GM production of short-chain fatty acids (SCFAs) and modulation of hormonal signals, including leptin, ghrelin, insulin, GLP-1, and PYY, alongside interactions via the gut-brain axis. These pathways link microbial composition to appetite regulation, fat storage, and energy balance. Emerging microbiome-targeted therapies, such as probiotics, prebiotics, dietary modulation (e.g., fiber-rich diets), fecal microbiota transplantation, and bacteriophage therapy, show promise in restoring GM balance, promoting weight loss, and improving metabolic health, though results vary and require further validation. Despite advances in metagenomics and metabolomics, gaps persist in establishing causality and long-term efficacy. The integration of GM data with host genetics, diet, and environmental factors through systems biology has the potential to facilitate personalized management of obesity. This review synthesizes the GM's role in obesity pathogenesis and hormonal regulation, highlighting therapeutic potential and research directions for microbiota-based prevention and treatment.
