Background: The gut microbiome functions as a dynamic metabolic system that interacts continuously with host nutrition and physiology, influencing energy balance, immune function, and disease risk. Objective: This review examines the gut microbiome as a metabolic organ, focusing on biochemical and nutritional evidence linking microbial activity to human health and disease. Methods: A narrative review approach was considered, integrating recent peer-reviewed literature on host–microbiome metabolic interactions, dietary modulation of gut microbial composition, and microbial metabolite signaling pathways. Results: Evidence indicates that gut microbes contribute to host metabolism through fermentation of dietary fibers into short-chain fatty acids, transformation of bile acids, and metabolism of amino acids and polyphenols. These microbial products act as signaling molecules influencing glucose homeostasis, lipid metabolism, inflammatory pathways, and gut barrier integrity. Dysbiosis has been associated with metabolic disorders including obesity, type 2 diabetes, non-alcoholic fatty liver disease, and cardiovascular disease. Dietary patterns rich in fiber and plant-derived compounds are consistently linked with beneficial microbial profiles and improved metabolic outcomes. Conclusion: The gut microbiome operates as a metabolically active organ that integrates dietary inputs with host biochemical pathways. Understanding these interactions provides a basis for nutritional strategies aimed at preventing and managing metabolic diseases.