Hypothesis

Core idea: Oral administration of Lactiplantibacillus plantarum Lp815 increases vagal afferent signaling through a combined GABA‑short‑chain fatty acid (SCFA) mechanism, producing early detectable shifts in non‑linear heart‑rate‑variability (HRV) metrics before measurable changes in systemic GABA or anxiety scores.

Rationale

• Pre‑clinical work shows Lactobacillus strains reduce stress‑induced behaviors via vagal pathways and increase luminal GABA (Lactobacillus‑Based Probiotics Reduce the Adverse Effects of Stress in Rodents)[https://www.frontiersin.org/journals/behavioral-neuroscience/articles/10.3389/fnbeh.2021.642757/full].
• Human RCT of Lp815 raised urinary GABA and lowered GAD‑7 scores (Lactiplantibacillus plantarum Lp815 improves sleep and increases urinary GABA)[https://pmc.ncbi.nlm.nih.gov/articles/PMC12816741/].
• SCFA‑producing microbes stimulate colonic enterochromaffin cells to release 5‑HT and activate vagal afferents (Gut microbiota as a novel target for treating anxiety and depression)[https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2025.1664800/full].
• HRV non‑linear indices such as sample entropy (SampEnt) and detrended fluctuation analysis α1 (DFA α1) reflect vagal responsiveness and change earlier than time‑domain measures (Heart rate variability: a multidimensional perspective)[https://pmc.ncbi.nlm.nih.gov/articles/PMC12630126/].

Novel Mechanistic Insight

We propose that Lp815’s GABA production does not act alone; instead, GABA released in the gut lumen synergizes with locally generated SCFAs (acetate, propionate, butyrate) to potentiate GABA_B receptors on vagal afferent terminals. This co‑activation lowers the threshold for afferent firing, generating a burst‑like vagal signal that is reflected immediately in increased irregularity of heart‑rate dynamics—captured by rises in SampEnt and reductions in DFA α1—before hepatic metabolism elevates circulating GABA levels detectable in urine.

Testable Predictions

1. In a double‑blind, placebo‑controlled trial (n≈60) of anxious adults, daily Lp815 (1×10^9 CFU) will increase SampEnt and decrease DFA α1 within 48 h of first dose, while urinary GABA remains unchanged.
2. By week 2, urinary GABA will rise and GAD‑7 scores will fall, with the magnitude of early HRV non‑linear change predicting later anxiety improvement (moderation analysis).
3. Administration of a GABA_B antagonist (e.g., saclofen) peripherally will blunt the early HRV shifts without affecting luminal GABA concentrations, confirming the receptor‑mediated vagal pathway.
4. Metabolomic profiling of fecal samples will show a concurrent rise in SCFA concentrations correlating with the early HRV changes.

Falsifiability

If Lp815 fails to produce significant alterations in SampEnt or DFA α1 within the first 48 h, or if peripheral GABA_B blockade does not attenuate these HRV changes while urinary GABA still rises, the hypothesis would be refuted.

Implications

Demonstrating that specific probiotic strains can modulate vagal tone via a GABA‑SCFA co‑signaling axis would justify HRV‑based, mechanistic endpoints in future psychobiotic trials and accelerate translation from rodent models to human therapeutics.