**Background:** The human gastrointestinal tract harbours a dense and diverse microbial community, with obligate anaerobes from the phyla Firmicutes and Bacteroidetes predominating in the large intestine. Diet is a major driver of gut microbiota composition and metabolic activity, which in turn influences host physiology including nutrient acquisition, immune function, and protection against pathogens. The gut microbiota produces a vast array of metabolites, most notably short-chain fatty acids (SCFAs) from fermentation of dietary fibres, but also potentially deleterious compounds from protein fermentation. The authors aim to review the inter-relationships between diet, microbial composition and metabolism, and host health, with a focus on underlying mechanisms.
**Methods:** This is a narrative review synthesising evidence from human volunteer studies, in vitro experiments, animal models, and genomic/metagenomic analyses. The authors discuss findings from controlled dietary intervention studies (e.g., David et al. 2014), comparative analyses across populations with disparate dietary patterns, and mechanistic studies examining bacterial physiology, metabolite production, and host immune signalling. Key bacterial groups discussed include butyrate-producing Firmicutes (e.g., Faecalibacterium prausnitzii, Eubacterium rectale, Roseburia spp.), polysaccharide-degrading Bacteroides species, and lactate-producing Bifidobacterium.
**Key Results:** The review reports that the human gut metagenome contains many millions of unique genes, at least two orders of magnitude more complex than the human genome. Total SCFA levels in faeces reach approximately 50–150 mM, with up to 95% absorbed by the colonic mucosa. The colonic epithelium derives 60–70% of its energy needs from butyrate. Individuals consuming a typical Western diet produce 0.5–0.6 M of SCFAs per day, providing 5–10% of daily calorific requirements. Approximately 3–18 g of dietary proteins enter the large intestine daily. Switching from a plant-based diet to one high in animal fats and protein led to significant reduction in Firmicutes and increase in Bacteroides species. Diets low in total carbohydrate content result in low numbers of butyrate-producing Firmicutes and greatly reduced faecal butyrate levels. Ruminococcus bromii is identified as a keystone species for resistant starch degradation; individuals lacking this species cannot fully degrade resistant starch. Lactate cross-feeding by Anaerobutyricum and Anaerostipes spp. contributes approximately 20% to the butyrate pool. In patients with inflammatory bowel disease, butyrate-producing bacteria are often lower than in controls. Butyrate downregulates pro-inflammatory molecules (IL-6, IL-12, NOS2) via histone deacetylase inhibition and suppresses LPS-induced NF-κB activation through GPR109A signalling. High-protein diets increase levels of N-nitroso compounds, hydrogen sulphide, and trimethylamine (TMA), which is oxidised to TMAO in the liver and may contribute to cardiovascular disease. Colorectal cancer-associated bacteria include Escherichia coli (producing colibactin genotoxin), enterotoxigenic Bacteroides fragilis (producing BFT toxin), and Fusobacterium nucleatum (producing FadA adhesin).
**Clinical Implications:** The gut microbiota represents a modifiable target for health maintenance and disease prevention through dietary strategies. Increased dietary fibre consumption elevates colonic SCFA production, particularly butyrate, which supports regulatory T cell maintenance, epithelial barrier integrity, and anti-inflammatory signalling. Prebiotics (oligosaccharides, inulins) may selectively stimulate beneficial bacteria, though responses depend on baseline microbiota composition. High-protein and high-fat diets promote putrefactive fermentation and production of potentially carcinogenic and pro-inflammatory metabolites, suggesting dietary recommendations should consider both macronutrient composition and individual microbiota profiles. The authors note that personalised nutrition approaches may be necessary given inter-individual variation in microbiota composition and response to dietary interventions. Further research is needed to establish optimal intervention strategies for prebiotics, probiotics, and synbiotics at population and individual levels.