Hypothesis Individuals with depression who exhibit a high ratio of secondary to primary fecal bile acids (elevated deoxycholic acid, reduced cholic acid) will show greater improvement in depressive symptoms after an 8‑week high‑fiber synbiotic intervention, and this effect will be mediated by activation of the TGR5 receptor on vagal afferents leading to reduced neuroinflammation.

Rationale

• The gut‑brain axis communicates microbial metabolites to the brain through the vagus nerve, immune signaling, and neuroactive molecules 3.
• Bile acids are modified by gut bacteria; secondary bile acids such as deoxycholic acid (DCA) are potent agonists of the membrane receptor TGR5, which is expressed on vagal afferents and modulates anti‑inflammatory pathways 5.
• Depression is associated with gut dysbiosis, reduced SCFA producers, and increased intestinal permeability that drives systemic inflammation 4, 5.
• Synbiotic formulations (prebiotic fiber + probiotic strains) can reshape bile acid metabolism by enriching bacteria that deconjugate and transform primary bile acids 6.
• Heterogeneity in treatment response suggests that a baseline microbial metabolic profile could identify who benefits 2.

Predictions

1. At baseline, a higher fecal DCA/CA ratio will correlate with lower baseline depressive symptom scores (HAMD‑17) after accounting for covariates.
2. Participants with a high DCA/CA ratio receiving the synbiotic will show a ≥30 % reduction in HAMD‑17 scores at week 8, whereas those with a low ratio will show <10 % change.
3. Synbiotic treatment will increase fecal SCFA concentrations and decrease serum LPS‑binding protein, indicating reduced gut permeability.
4. Vagal tone, measured by heart‑rate variability, will increase preferentially in responders, and this change will be blocked by a selective TGR5 antagonist in a parallel animal validation study.

Experimental Design

• Population: 120 adults diagnosed with moderate depression (HAMD‑17 16‑22), antidepressant‑free for ≥4 weeks.
• Intervention: Double‑blind, randomized to either synbiotic (10 g galacto‑oligosaccharide + 1×10⁹ CFU Bifidobacterium longum + Lactobacillus helveticus) or matched placebo daily for 8 weeks.
• Baseline Measurements: Fecal bile acid profile (LC‑MS), SCFA quantification, gut permeability markers (zonulin, LPS‑BP), HAMD‑17, heart‑rate variability.
• Outcome Measures: Change in HAMD‑17 at weeks 4 and 8; secondary outcomes include anxiety scores (GAD‑7), inflammatory cytokines (IL‑6, TNF‑α), and microbiome sequencing.
• Analysis: Primary analysis uses ANCOVA with baseline DCA/CA ratio as a covariate; interaction term tests whether the ratio predicts differential treatment response. Mediation analysis will assess whether changes in vagal tone and systemic inflammation mediate the relationship between bile acid shift and symptom improvement.

Potential Outcomes and Implications If the hypothesis is confirmed, the fecal DCA/CA ratio could serve as a predictive biomarker to personalize microbiome‑based therapies, moving beyond trial‑and‑error prescribing. It would also highlight bile acid‑TGR5‑vagus signaling as a mechanistic bridge between gut microbial metabolism and mood regulation, offering a target for adjunctive pharmacological strategies (e.g., TGR5 agonists) in non‑responders. Conversely, a null result would refute the notion that bile acid metabolism alone drives synergistic effects of synbiotics on depression, prompting deeper exploration of other metabolite classes (e.g., tryptophan catabolites) or host genetic factors.