# Video research-update insert: Gut microbial composition and human psychiatric symptoms

Editorial script and storyboard insert, 5 October 2026. No rendered video. Read alongside the original review; no new grade is announced.

## Narration

Human associations, animal transfer experiments and clinical microbiome interventions address different causal steps. Recent adjunctive FMT findings strengthen an intervention line in depression, while human mediator specificity and generalization across psychiatric disorders remain unresolved.

In the 2016 study, depression-donor microbiota produced depressive-like behaviour and metabolic changes. Germ-free host conditions and cross-species transfer bound applicability. [P27067014]

In the 2022 study, several taxa were associated with depressive symptoms across cohorts. Association and inferred neuroactive potential do not establish effector delivery to the brain. [P36473852]

In the 2026 study, the report linked improved adjunctive outcomes with engraftment, bile acids and mediation analysis. Statistical mediation does not directly randomize the proposed bile-acid mechanism. [P42309058]

The next discriminating question is: Which microbial function carries a human depression-treatment effect? Taxa can be markers of diet, medication, inflammation or a broader microbial functional state. Replicated, strain/function-resolved randomized interventions with repeated metabolites, symptoms and prospective mediator analysis; stabilize medication and diet.

The timeline shows selected changes in evidence, not inevitable progress or proof of a single cause. The review’s archived grade remains unchanged; the depth of source reading is documented in the update.

## Storyboard and source locks

| Insert | Visual | Narration anchor | Sources |
|---|---|---|---|
| Opening | Claim and bounded update statement | First paragraph | Review claim and source ledger |
| Timeline 2016 | Dated study card with design, result and limit | Depression-donor microbiota produced depressive-like behaviour and metabolic changes. | [P27067014](https://pubmed.ncbi.nlm.nih.gov/27067014/) |
| Timeline 2022 | Dated study card with design, result and limit | Several taxa were associated with depressive symptoms across cohorts. | [P36473852](https://pubmed.ncbi.nlm.nih.gov/36473852/) |
| Timeline 2026 | Dated study card with design, result and limit | The report linked improved adjunctive outcomes with engraftment, bile acids and mediation analysis. | [P42309058](https://pubmed.ncbi.nlm.nih.gov/42309058/) |
| G1 | Question → competing explanation → discriminating test | Which microbial function carries a human depression-treatment effect? | [P36473852](https://pubmed.ncbi.nlm.nih.gov/36473852/), [P42309058](https://pubmed.ncbi.nlm.nih.gov/42309058/), [P27491067](https://pubmed.ncbi.nlm.nih.gov/27491067/) |
| G2 | Question → competing explanation → discriminating test | Is the vagus a necessary human route? | [P37131071](https://pubmed.ncbi.nlm.nih.gov/37131071/), [P28483500](https://pubmed.ncbi.nlm.nih.gov/28483500/), [P42309058](https://pubmed.ncbi.nlm.nih.gov/42309058/) |
| G3 | Question → competing explanation → discriminating test | When do positive and null clinical effects occur? | [P28483500](https://pubmed.ncbi.nlm.nih.gov/28483500/), [P28068788](https://pubmed.ncbi.nlm.nih.gov/28068788/), [P39504580](https://pubmed.ncbi.nlm.nih.gov/39504580/), [P42309058](https://pubmed.ncbi.nlm.nih.gov/42309058/) |
| G4 | Question → competing explanation → discriminating test | Does benefit persist after dosing and engraftment changes? | [P42309058](https://pubmed.ncbi.nlm.nih.gov/42309058/), [P28068788](https://pubmed.ncbi.nlm.nih.gov/28068788/), [P36473852](https://pubmed.ncbi.nlm.nih.gov/36473852/) |

## Editorial constraints

- Use the primary-source links shown above and keep the inference limit on screen with each milestone.
- Do not narrate an abstract check as complete-paper examination, a model result as human proof, or an unresolved inference as absence of all research.
- Preserve trial endpoint hierarchy, cohort reuse, correction notices and online/issue dates.
- Use original graphics made from this ledger. Publisher figure imagery has not been cleared or supplied for video reuse.
- Confirm any change in verdict through a separate documented HSEF review; this insert makes none.