# Gut microbial community composition as a primary systemic cause: research update

Dated 5 October 2026. Local prepublication addendum; archived grade unchanged.

Microbiota can causally influence systemic phenotypes in defined models and selected human settings. Engraftment, clinical benefit and disease initiation remain different claims; positive and null FMT studies prevent a universal intervention narrative.

## Research timeline

Electronic year is used where recorded; issue year is retained in each source card. The chronology is selected, not exhaustive, and does not establish priority or causation.

| Year | Primary study | Design and finding | Inference limit |
|---|---|---|---|
| 2010 | [Gut-residing segmented filamentous bacteria drive autoimmune arthritis via T helper 17 cells.](https://pubmed.ncbi.nlm.nih.gov/20620945/) | Gnotobiotic arthritis model. A defined commensal species restored Th17-related autoimmunity and arthritis in susceptible mice. | Host genotype and immune context are part of the demonstrated causal system. |
| 2012 | [Transfer of intestinal microbiota from lean donors increases insulin sensitivity in individuals with metabolic syndrome.](https://pubmed.ncbi.nlm.nih.gov/22728514/) | Randomized lean-donor transfer in metabolic syndrome. Insulin sensitivity improved at six weeks after allogenic transfer. | Short-term metabolic response is not proof that dysbiosis initiated common metabolic disease. |
| 2013 | [Gut microbiota from twins discordant for obesity modulate metabolism in mice.](https://pubmed.ncbi.nlm.nih.gov/24009397/) | Obesity-discordant twin microbiota in mice. Donor-related adiposity phenotypes and diet-dependent rescue were transmissible. | Recipient environment and diet condition the transfer phenotype. |
| 2015 | [Disentangling type 2 diabetes and metformin treatment signatures in the human gut microbiota.](https://pubmed.ncbi.nlm.nih.gov/26633628/) | Human metagenomic medication analysis. Metformin-associated signals were separated from diabetes-associated microbial signatures. | Observed disease signatures can partly reflect treatment; correction notice retained. |
| 2017 | [Improvement of Insulin Sensitivity after Lean Donor Feces in Metabolic Syndrome Is Driven by Baseline Intestinal Microbiota Composition.](https://pubmed.ncbi.nlm.nih.gov/28978426/) | Metabolic-syndrome FMT trial. Short-term insulin-sensitivity response depended on baseline microbiota and was not sustained at 18 weeks. | Responder prediction and durability require independent prospective testing. |
| 2020 | [Fecal microbiota transplantation for the improvement of metabolism in obesity: The FMT-TRIM double-blind placebo-controlled pilot trial.](https://pubmed.ncbi.nlm.nih.gov/32150549/) | FMT-TRIM randomized placebo-controlled trial. Engraftment occurred without clinically significant metabolic effects during follow-up. | Microbial target engagement is insufficient to assume systemic efficacy. |
| 2024 | [Safety and efficacy of faecal microbiota transplantation in patients with mild to moderate Parkinson's disease (GUT-PARFECT): a double-blind, placebo-controlled, randomised, phase 2 trial.](https://pubmed.ncbi.nlm.nih.gov/38686220/) | GUT-PARFECT Parkinson trial. A mild long-term motor benefit was reported after donor FMT. | Small phase-2 study and specific comparator do not establish universal efficacy. |
| 2024 | [Fecal Microbiota Transplantation for Treatment of Parkinson Disease: A Randomized Clinical Trial.](https://pubmed.ncbi.nlm.nih.gov/39073834/) | Independent Parkinson FMT randomized trial. FMT did not produce clinically meaningful improvement. | Donor, bowel preparation and outcome differences need direct comparison, not selective citation. |
| 2025 | [Microbiota transplant for hepatic encephalopathy in cirrhosis: The THEMATIC trial.](https://pubmed.ncbi.nlm.nih.gov/39800192/) | THEMATIC cirrhosis randomized trial. FMT was tolerated; recurrence patterns suggested benefit on a secondary outcome. | Secondary and post hoc contrasts must retain their analysis status. |
| 2025 | [Fecal transplantation from humans with obesity to mice drives a selective microbial signature without impacting behavioral and metabolic health.](https://pubmed.ncbi.nlm.nih.gov/40316655/) | Human obesity-donor transfer to chow-fed mice. Microbial/metabolite differences appeared without reproducing metabolic or behavioural obesity phenotypes. | A bounded transfer null does not erase effects under other diets or host conditions. |

## Research gaps

### G1: Which host, donor and dietary context determines efficacy?

Evidence anchors: [P24009397](https://pubmed.ncbi.nlm.nih.gov/24009397/), [P28978426](https://pubmed.ncbi.nlm.nih.gov/28978426/), [P32150549](https://pubmed.ncbi.nlm.nih.gov/32150549/), [P40316655](https://pubmed.ncbi.nlm.nih.gov/40316655/).

Competing explanation: Microbial functions may require permissive host ecology or dietary substrate.

Discriminating test and controls: Factorial donor/function-by-diet testing with colonization measures; prespecified host phenotypes and independent recipient replication.

### G2: Does engraftment deliver a durable systemic effect?

Evidence anchors: [P22728514](https://pubmed.ncbi.nlm.nih.gov/22728514/), [P28978426](https://pubmed.ncbi.nlm.nih.gov/28978426/), [P32150549](https://pubmed.ncbi.nlm.nih.gov/32150549/).

Competing explanation: Stool compositional change may lack functional relevance or fade before clinical benefit.

Discriminating test and controls: Longer randomized studies with function/metabolites, metabolic outcomes and post-treatment follow-up; define target engagement before outcome analysis.

### G3: What explains opposing Parkinson trial results?

Evidence anchors: [P38686220](https://pubmed.ncbi.nlm.nih.gov/38686220/), [P39073834](https://pubmed.ncbi.nlm.nih.gov/39073834/).

Competing explanation: Comparator activity, bowel cleansing, donor properties or measurement choices may create apparent disagreement.

Discriminating test and controls: Harmonized multisite trial with matched preparation, sham expectations, donor characterization and common motor/nonmotor endpoints.

### G4: Does a microbial change initiate disease or modify an established disease?

Evidence anchors: [P20620945](https://pubmed.ncbi.nlm.nih.gov/20620945/), [P26633628](https://pubmed.ncbi.nlm.nih.gov/26633628/), [P39800192](https://pubmed.ncbi.nlm.nih.gov/39800192/), [P32150549](https://pubmed.ncbi.nlm.nih.gov/32150549/).

Competing explanation: Medication, transit, inflammation or disease can generate the microbial signature.

Discriminating test and controls: Prospective pre-onset cohorts plus randomized disease-stage interventions and time-ordered mediator analysis; control treatment, diet and transit.

## Methods and access

Case-specific Consensus records assisted gap discovery, chronology and targeted paper questions. Shared template wording about an amyloid-specific Deep follow-up is not evidence of a Deep run for this topic; the dated source records establish the actual use and access limits. Primary identifiers, selected abstract passages, correction notices and available primary-body passages were cross-checked. Generated absence cells, duplicate milestones and unsupported access claims were rejected.

This focused addendum does not establish 50–70 complete primary Methods/Results examinations per review. Baseline reading receipts and scores retain their original scope.

Checked timeline sources: 10; selected full-text checks: 1; abstract-excerpt checks: 9; metadata-only historical anchors: 0. Complete papers examined in this update: 0.

Historical scorecards declare schema 5.4. This enrichment uses HSEF v5.3 causal distinctions without retrospectively relabelling or rescoring those files.

Targeted paper-check scope: Methods and engraftment Results: clamp M-value was the primary metabolic measure at six weeks; engraftment varied and persisted. Primary abstract reports no clinically significant metabolic effect.