Hypothesis

Baseline heart‑rate variability (HRV) predicts the magnitude of anxiety reduction after supplementation with Lactobacillus paracasei PS23 in stressed adults.

Rationale

• Psychobiotics such as L. rhamnosus and L. paracasei PS23 exert anxiolytic effects through GABAergic signaling, BDNF elevation, and anti‑inflammatory pathways [L. rhamnosus GABA upregulation] [L. paracasei PS23 anxiety reduction].
• The enteric nervous system (ENS) communicates with the brain primarily via the vagus nerve; vagal efferent activity is indexed by HRV metrics (RMSSD, HF power) [HRV vagal link].
• No human trial has yet validated HRV as a mediator of psychobiotic efficacy, creating a methodological gap [HRV validation gap].

We propose that individuals with higher baseline vagal tone (greater RMSSD/HF) possess a more responsive ENS‑vagal axis, allowing probiotic‑induced GABA release to translate into stronger autonomic and anxiety improvements.

Novel Mechanistic Insight

Beyond GABA upregulation, we hypothesize that L. paracasei PS23 stimulates enterochromaffin cells to release 5‑HT, which activates 5‑HT₃ receptors on vagal afferents, enhancing nucleus tractus solitarius (NTS) firing. This amplifies central GABAergic inhibition in the amygdala, but only when vagal afferent signaling is already primed by tonic activity. Thus, baseline vagal tone sets the gain of the gut‑brain feedback loop.

It's well‑known that vagal afferents relay gut signals to the brain, and we don't expect the effect to occur without baseline tone.

Testable Predictions

1. In a double‑blind, placebo‑controlled RCT (n=120 stressed adults), participants will be stratified by baseline RMSSD (high vs low).
2. The high‑RMSSD group receiving L. paracasei PS23 will show a significantly greater increase in post‑intervention RMSSD and HF power (Cohen’s d ≥ 0.6) compared with placebo.
3. The low‑RMSSD group will show no significant HRV change despite similar increases in fecal GABA and plasma BDNF.
4. Mediation analysis will confirm that change in HRV mediates the relationship between probiotic intake and reduction in State‑Trait Anxiety Inventory (STAI) scores (indirect effect > 0, p < 0.05).

Methods (brief)

• Population: adults 18‑45 y with perceived stress scale ≥ 20.
• Intervention: 1 × 10⁹ CFU L. paracasei PS23 daily for 8 weeks.
• Outcomes: weekly HRV (5‑min resting, supine), STAI, fecal GABA, plasma BDNF, IL‑6.
• Analysis: mixed‑effects models with time × group × baseline HRV interaction; mediation via bootstrapped indirect effects.

Falsifiability

If the high‑RMSSD cohort does not exhibit greater HRV or anxiety improvement than placebo, or if HRV changes fail to mediate anxiety reduction, the hypothesis is refuted. Conversely, a significant interaction and mediation would support the claim that baseline vagal tone gates psychobiotic efficacy via ENS‑vagal‑GABA signaling.

Expected Impact

Validating HRV as a predictive biomarker would enable personalized psychobiotic prescriptions, reduce trial heterogeneity, and clarify the autonomic mechanism linking gut microbes to anxiety.