Hypothesis

Individuals with lower baseline heart‑rate variability (HRV) will show greater improvement in anxiety scores after supplementation with Lactobacillus strains that produce high levels of GABA, and this effect will be mediated by increased activation of intestinal hPAT1 transporters and subsequent vagal afferent signaling.

Mechanistic rationale

• Lactobacillus spp. such as L. rhamnosus JB‑1 and L. plantarum LP‑28 synthesize GABA that can cross the epithelial barrier via the hPAT1 transporter (4).
• GABA binding to GABA_B receptors on enterochromaffin cells triggers release of serotonin, which stimulates 5‑HT3 receptors on vagal afferents, enhancing vagal tone (5).
• Enhanced vagal afferent firing raises HRV and inhibits HPA‑axis activity, reducing corticosterone and anxiety‑like behavior (1).
• Baseline HRV reflects resting vagal tone; those with lower vagal tone have more capacity for increase, predicting larger HRV gains and anxiety reductions when the gut‑brain GABA‑serotonin‑vagal pathway is engaged.

Testable predictions

1. In a randomized, double‑blind, crossover trial, participants receiving a high‑GABA Lactobacillus formulation (e.g., L. rhamnosus JB‑1 ≥10⁹ CFU/day) will exhibit a significant rise in HRV (RMSSD) after 8 weeks compared with placebo, but only if their baseline HRV falls below the cohort median.
2. The HRV increase will correlate positively with fecal GABA concentration and inversely with plasma corticosterone levels.
3. Administration of a vagal blocker (e.g., temporary transcutaneous vagal nerve inhibition) will abolish the HRV and anxiety benefits, confirming vagal mediation.
4. Microbiota sequencing will show that baseline abundance of GABA‑producing taxa modulates the magnitude of response, linking pre‑existing community composition to individualized efficacy.

Falsifiability

If high‑GABA Lactobacillus supplementation fails to raise HRV in low‑baseline HRV participants, or if HRV changes occur irrespective of baseline vagal tone, the hypothesis is refuted. Likewise, if vagal blockade does not diminish the anxiety‑reducing effect, the proposed vagal‑mediated mechanism is invalid.

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