Systematic review of dietary effects on gut microbiota in individuals at elevated colorectal cancer risk
With only 7 included RCTs and marked heterogeneity, current evidence is insufficient to establish consistent effects of dietary interventions on gut microbiota in populations at elevated risk for colorectal cancer.
| Population | Adults at elevated CRC risk (personal or family history of adenoma/CRC, hereditary syndrome, or other risk factors as defined by primary studies) |
|---|---|
| Intervention | Dietary interventions (supplements, functional foods, whole foods, or dietary patterns), including navy bean powder, rice bran, and a general healthy eating pattern |
| Comparator | Control (placebo, habitual diet, or active comparator, varying by study) |
| Outcome | Alpha microbial diversity; Genus-level taxonomic composition; Short-chain fatty acids (SCFAs); Secondary bile acids; Beta microbial diversity; Plant-derived metabolites; P-cresol sulfate and branched-chain bacterial fatty acids |
Summary of findings
| Outcome | Effect | 95% CI | Certainty | Clinical relevance | Notes |
|---|---|---|---|---|---|
| Alpha microbial diversity | No pooled effect size calculable; minimal effects observed in 3 of 7 studies | — | Low | — | 7 studies |
| Genus-level taxonomic composition | No pooled effect size; direction inconsistent across studies and genera | — | Low | — | 7 studies |
| Short-chain fatty acids (SCFAs) | No pooled effect size; increases reported in some trials, in the change in others | — | Low | — | 4 studies |
| Secondary bile acids | No pooled effect size; reductions reported in subset of 4 metabolite-assessing trials | — | Low | — | 4 studies |
| Beta microbial diversity | No pooled effect size calculable; inconsistent findings across 7 studies | — | Low | — | 7 studies |
| Plant-derived metabolites | No pooled effect size; increases reported in some trials only | — | Low | — | 4 studies |
| P-cresol sulfate and branched-chain bacterial fatty acids | No pooled effect size; reductions reported in subset of 4 metabolite-assessing trials | — | Low | — | 4 studies |
Context
Colorectal cancer (CRC) is among the most incident cancers globally, with projected increases of up to 124% among adults aged 20–34 by 2030. Diet is the primary modifiable factor interacting with gut microbiota in CRC risk modulation. Identifying which dietary interventions consistently alter the microbiome in this population is clinically relevant.
What the study showed
Of 5,558 identified records, 7 RCTs met inclusion criteria. Effects on microbial diversity metrics were minimal, observed only with a healthy eating dietary pattern, navy bean powder, and one rice bran intervention; no consistent pattern emerged by intervention type, duration, or diversity metric. Enrichment of Faecalibacterium, Bifidobacterium, and Lactobacillus was reported in some rice bran and navy bean studies, while reductions in Akkermansia, Ruminococcus, Bacteroides, Prevotella, and Porphyromonas were observed in others. Among 4 trials assessing metabolites, some showed increased SCFAs and plant-derived metabolites; others reported reductions in secondary bile acids, p-cresol sulfate, and branched-chain bacterial fatty acids. No pooled effect sizes or 95% CIs were calculable.
How it was done
Systematic review (PROSPERO CRD42024530038) searching PubMed, Scopus, and Web of Science, supplemented by manual screening of systematic reviews and reference lists. Only RCTs of dietary interventions in elevated-CRC-risk individuals assessing gut microbiome outcomes were included. Of 5,558 records, 7 studies were included after screening; quantitative meta-analysis was not feasible due to heterogeneity.
Effect magnitude
No pooled effect size (RR, OR, SMD, or MD with 95% CI) was calculated; the review is qualitative only. Individual study effect magnitudes cannot be aggregated.
Risk of bias
The number of included RCTs is critically low (n=7), with substantial heterogeneity across populations, interventions, microbiome analysis methods, and reported outcomes, precluding quantitative synthesis. The risk-of-bias tool applied is not explicitly named in the provided text (RoB 2 not confirmed). Generalizability is limited by variable definitions of 'elevated risk' across primary studies.
What this study does NOT prove
This study does not prove that dietary interventions prevent CRC through microbiome modulation, nor that observed taxonomic shifts translate into clinically relevant outcomes. Findings are not generalizable beyond the specific populations and interventions of the 7 included trials.
In clinical practice
Current evidence does not support recommending a specific dietary intervention to modulate gut microbiota for CRC prevention in high-risk populations. Clinicians should advise evidence-based dietary patterns for CRC prevention (high fiber, low processed meat) without attributing specific microbiome-mediated effects. Microbiome assessment as a response biomarker to diet remains experimental in this population.
Limitations
The number of included RCTs is critically low (n=7), with substantial heterogeneity across populations, interventions, microbiome analysis methods, and reported outcomes, precluding quantitative synthesis. The risk-of-bias tool applied is not explicitly named in the provided text (RoB 2 not confirmed). Generalizability is limited by variable definitions of 'elevated risk' across primary studies.
What is still missing
Larger RCTs with standardized microbiome analysis methods (shotgun metagenomics, metatranscriptomics, metabolomics), clearly defined at-risk populations, and long-term follow-up are required to establish causality and clinical relevance.
Technical appendix
Version history
- 1.0 · 2026-11-02 — Auto-generated under Evidence Standard v1.0