Hypothesis

Specific psychobiotic strains that produce D‑lactate enhance hippocampal mTORC1 signaling via the HCAR1 receptor on vagal afferents, and baseline fecal D‑lactate levels combined with host HCAR1 expression predict which older adults will benefit from these strains.

Mechanistic Rationale

• Lactococcus lactis WHH2078 and related strains encode lactate dehydrogenase genes that favor D‑lactate secretion (2).
• D‑lactate acts as an endogenous agonist of the hydroxycarboxylic acid receptor 1 (HCAR1) expressed on enteroendocrine and sympathetic neurons, triggering vagal signaling to the nucleus tractus solitarius (3).
• Vagal afferent activation increases hippocampal BDNF release, which activates the PI3K‑AKT‑mTORC1 pathway, promoting neuronal survival and synaptic plasticity (1).
• Host variability in HCAR1 polymorphism or expression, and in microbiome‑encoded lactate pathways, creates a bimodal responder landscape; individuals with low baseline D‑lactate and high HCAR1 responsiveness are predicted to show the greatest cognitive gain.

Novel Insight Beyond Current Data

While prior work links strain‑specific tryptophan modulation to cognition (2), it overlooks lactate as a rapid, metabolite‑based signal that can gate neurotransmitter effects. We propose that lactate‑HCAR1 vagal signaling is a necessary upstream permissive factor for serotonin‑dependent improvements; without it, tryptophan shifts alone fail to engage mTORC1.

Testable Prediction

In a prospective, double‑blind trial of adults aged 65‑80 with mild cognitive impairment, stratification by (1) fecal D‑lactate concentration (> median = high) and (2) peripheral blood mononuclear cell HCAR1 mRNA expression (> median = high) will identify a responder subgroup. Participants receiving L. lactis WHH2078 (10⁹ CFU daily) will show, after 24 weeks, a significant increase in hippocampal volume (≥ 2% change) and a ≥ 1.5‑point improvement on the ADAS‑Cog compared with placebo, whereas non‑stratified or low‑marker groups will show no difference.

Falsifiability

If the stratified high‑marker group receiving the psychobiotic fails to demonstrate superior hippocampal or cognitive change relative to placebo, or if low‑marker participants exhibit equal or greater benefit, the hypothesis is falsified. Likewise, absence of increased vagal nerve activity (measured via heart‑rate variability) or hippocampal p‑S6K (a downstream mTORC1 read‑out) in responders would refute the proposed mechanistic chain.

Implementation Steps

1. Screen 300 participants for baseline fecal metabolomics (LC‑MS for D‑lactate) and PBMC HCAR1 expression (qRT‑PCR).
2. Randomize high‑marker stratum to psychobiotic or placebo (1:1), low‑marker stratum similarly.
3. Collect serial fecal samples, plasma kynurenine/serotonin, and episodic MRI at baseline, 12, and 24 weeks.
4. Primary analysis: interaction between treatment, marker status, and cognitive outcome using mixed‑effects models.
5. Secondary analysis: mediation via vagal tone (HRVF) and hippocampal p‑S6K from PET‑derived proxy or CSF biomarkers.

Expected Impact

Success would validate a precision‑microbiome framework that moves beyond taxonomic labels to functional metabolite capacity, enabling clinicians to match psychobiotics to individuals most likely to experience measurable brain health benefits.