A Systematic Review of the Effects of Diet on the Gut Microbiota in Individuals at Risk for Colorectal Cancer

Colorectal cancer (CRC) poses a major public health concern, with emerging evidence highlighting the critical role of the gut microbiota in CRC progression. Considering that diet is a major modifiable factor influencing CRC risk, partly through its interaction with the gut microbiome, this systematic review evaluated the effects of dietary intervention RCTs, including supplements, functional foods, whole foods, and dietary patterns, on the gut microbiome in at-risk individuals (PROSPERO: CRD42024530038). A search of PubMed, Scopus, and Web of Science identified 4,746 records, with 20 additional records sourced manually. Following screening, six studies met inclusion criteria, focusing on microbial diversity, taxonomic composition, and metabolites. Findings demonstrate that whole food interventions like navy beans and functional foods like rice bran increased microbial diversity over 4-8 wk; with no effects of rice bran over 24 wk. Navy beans (8 wk) enriched beneficial taxa (Faecalibacterium, Bifidobacterium), while a shorter 4-wk intervention using bean powder showed no taxonomic shifts. Both forms of navy bean increased amino acid derivatives and anti-inflammatory phenolic metabolites. Rice bran increased Lactobacillus over 24 wk but showed differing effects on Firmicutes over short (2 wk) versus longer (24 wk) durations. Rice bran also increased SCFAs (acetate, propionate) over 2 wk and phenolic metabolites (e.g., diosmin) while reducing pro-carcinogenic byproducts (p-cresol sulfate, glycodeoxycholate) over 4 wk. {beta}-glucan-enriched bread increased acetate over 12 wk but had minimal effects on microbial composition. Healthy Eating and Mediterranean dietary patterns did not alter taxonomic composition over 26 wk but reduced branched-chain bacterial fatty acids, indicating reduced proteolytic fermentation. This review underscores the potential of dietary strategies to modulate the gut microbiome in CRC-risk populations. However, limited RCTs and heterogeneity limit generalizability. Future research should conduct rigorous RCTs across the lifespan, using advanced microbiome and metabolite analyses and examining understudied dietary patterns to guide CRC prevention.