The human gut microbiota plays a crucial role in the metabolism of nutrients and other food components, complementing the activity of mammalian enzymes in the liver and gut mucosa. It contributes essential functions for host digestion and influences the biochemical profile of the diet, thereby impacting host health and disease. The review discusses the main gut microorganisms, particularly bacteria, and their associated metabolic pathways for dietary carbohydrates, proteins, plant polyphenols, bile acids, and vitamins. It also explores methodologies for studying gut microbial pathways, including mathematical models, omics techniques, isolated microbes, and enzyme assays. Carbohydrates are mainly fermented by gut bacteria, producing short-chain fatty acids (SCFAs) and gases. The three most abundant SCFAs in feces are acetate, propionate, and butyrate, which have distinct roles in human health. Butyrate is crucial for colonocyte energy and has anti-cancer properties. Propionate is an energy source for epithelial cells and is involved in satiety signaling. Acetate supports bacterial growth and is involved in cholesterol metabolism and appetite regulation. Bacterial cross-feeding influences SCFA production, with some bacteria utilizing intermediates produced by others. The specificity of SCFA production by intestinal species varies, with certain bacteria producing propionate and butyrate. The production of SCFAs is not defined by phylogeny, requiring targeted gene methods for enumeration. Gas production in the intestinal tract is a byproduct of microbial fermentation, with hydrogen, carbon dioxide, and methane being the main gases. Hydrogen is primarily produced by Bacteroides and Clostridium, and its removal via dissimilatory sulfate reduction, methanogenesis, and acetogenesis helps reduce gas accumulation. However, hydrogen sulfide, a byproduct of sulfate reduction, is toxic to colonic cells. Proteins are metabolized by the gut microbiota, producing peptides, amino acids, and gases. Aromatic amino acids can be fermented to phenylpropanoid metabolites, which are abundant in feces. The metabolism of amino acids is complex and influenced by bacterial utilization, affecting overall amino acid availability. Vitamin synthesis by the gut microbiota is well-documented, with bacteria producing vitamins such as vitamin K and B-group vitamins. These vitamins are essential for bacterial metabolism and have physiological significance in mammals. The gut microbiota is responsible for the majority of vitamin diversity, with certain bacteria playing key roles in their synthesis. Bile acids are modified by the gut microbiota, influencing their structure and properties. These modifications can affect the antimicrobial properties of bile acids and their impact on the gut microbiota. Bile acids also serve as signaling molecules, influencing host metabolism and gene expression. Polyphenols and polyphenol-derived compounds are extensively metabolized by the gut microbiota, affecting their bioactivity. The metabolism of polyphenols varies among individuals due to differences in gut microbiota composition, leading to inter-individualThe human gut microbiota plays a crucial role in the metabolism of nutrients and other food components, complementing the activity of mammalian enzymes in the liver and gut mucosa. It contributes essential functions for host digestion and influences the biochemical profile of the diet, thereby impacting host health and disease. The review discusses the main gut microorganisms, particularly bacteria, and their associated metabolic pathways for dietary carbohydrates, proteins, plant polyphenols, bile acids, and vitamins. It also explores methodologies for studying gut microbial pathways, including mathematical models, omics techniques, isolated microbes, and enzyme assays. Carbohydrates are mainly fermented by gut bacteria, producing short-chain fatty acids (SCFAs) and gases. The three most abundant SCFAs in feces are acetate, propionate, and butyrate, which have distinct roles in human health. Butyrate is crucial for colonocyte energy and has anti-cancer properties. Propionate is an energy source for epithelial cells and is involved in satiety signaling. Acetate supports bacterial growth and is involved in cholesterol metabolism and appetite regulation. Bacterial cross-feeding influences SCFA production, with some bacteria utilizing intermediates produced by others. The specificity of SCFA production by intestinal species varies, with certain bacteria producing propionate and butyrate. The production of SCFAs is not defined by phylogeny, requiring targeted gene methods for enumeration. Gas production in the intestinal tract is a byproduct of microbial fermentation, with hydrogen, carbon dioxide, and methane being the main gases. Hydrogen is primarily produced by Bacteroides and Clostridium, and its removal via dissimilatory sulfate reduction, methanogenesis, and acetogenesis helps reduce gas accumulation. However, hydrogen sulfide, a byproduct of sulfate reduction, is toxic to colonic cells. Proteins are metabolized by the gut microbiota, producing peptides, amino acids, and gases. Aromatic amino acids can be fermented to phenylpropanoid metabolites, which are abundant in feces. The metabolism of amino acids is complex and influenced by bacterial utilization, affecting overall amino acid availability. Vitamin synthesis by the gut microbiota is well-documented, with bacteria producing vitamins such as vitamin K and B-group vitamins. These vitamins are essential for bacterial metabolism and have physiological significance in mammals. The gut microbiota is responsible for the majority of vitamin diversity, with certain bacteria playing key roles in their synthesis. Bile acids are modified by the gut microbiota, influencing their structure and properties. These modifications can affect the antimicrobial properties of bile acids and their impact on the gut microbiota. Bile acids also serve as signaling molecules, influencing host metabolism and gene expression. Polyphenols and polyphenol-derived compounds are extensively metabolized by the gut microbiota, affecting their bioactivity. The metabolism of polyphenols varies among individuals due to differences in gut microbiota composition, leading to inter-individual