Hypothesis

We hypothesize that selecting probiotic strains based on their genomic capacity to produce L‑lactate, and pairing them with a lactate‑rich prebiotic (e.g., lactulose), will enhance cognitive function in mild cognitive impairment (MCI) by increasing circulating L‑lactate that activates astrocytic HCAR1 receptors, thereby boosting BDNF expression and synaptic plasticity.

Mechanistic Rationale

Recent work shows that microbial metabolites such as SCFAs and tryptophan derivatives influence cognition via BDNF and neuroinflammation {1}. Computational gut‑brain modules (GBMs) now enable prediction of neuroactive potential {2}. However, lactate—a key neuronal‑astrocytic shuttle metabolite—has been overlooked in GBM screens. Certain Lactobacillus and Bifidobacterium strains possess lactate dehydrogenase genes that favor L‑lactate production, and L‑lactate can cross the blood‑brain barrier and bind HCAR1 on astrocytes, triggering cAMP‑PKA pathways that upregulate BDNF and reduce NF‑κB‑mediated inflammation {3}. Host factors such as HCAR1 polymorphisms and monocarboxylate transporter (MCT1) expression vary with age and cognitive status, explaining why identical strains yield heterogeneous outcomes.

Testable Predictions

1. MCI participants receiving a GBM‑selected high‑L‑lactate strain plus lactulose will show a greater increase in fasting plasma L‑lactate than those receiving a low‑lactate strain or placebo.
2. Elevations in plasma L‑lactate will correlate with increased serum BDNF and improved scores on the ADAS‑Cog subscale.
3. Participants with high HCAR1 expression (measured via peripheral blood monocyte mRNA) will experience larger cognitive gains than low‑expressors, irrespective of baseline microbiota composition.
4. In low‑HCAR1 expressors, the same intervention will not improve cognition despite elevated lactate, falsifying the lactate‑HCAR1 axis as necessary for benefit.

Experimental Design

A double‑blind, randomized, 24‑week trial with four arms: (A) high‑L‑lactate probiotic + lactulose, (B) low‑L‑lactate probiotic + lactulose, (C) high‑L‑lactate probiotic + maltodextrin placebo, (D) double placebo. Primary outcome: change in ADAS‑Cog score. Secondary outcomes: fasting plasma L‑lactate, serum BDNF, HCAR1 expression in monocytes, and fecal lactate concentrations. Sample size calculated to detect a 0.5 SD difference in ADAS‑Cog with 80% power (n≈50 per arm). Interim analysis at week 12 will assess lactate‑BDNF coupling.

Potential Outcomes and Falsifiability

If arm A shows significant cognitive improvement accompanied by raised lactate and BDNF, and this effect is moderated by HCAR1 expression, the hypothesis is supported. If arm A improves cognition without lactate increase, or if lactate rises equally across arms without cognitive benefit, the lactate‑HCAR1 mechanism is refuted. Conversely, if high‑HCAR1 participants benefit despite low lactate, alternative pathways (e.g., SCFA) must be considered. This design directly tests whether GBM‑guided strain selection combined with targeted prebiotic modulation of a specific metabolite yields reproducible cognitive gains, addressing the mechanistic opacity highlighted in prior work {[1][2][3]}