Hypothesis\nAging-related decline in vagal efferent signaling remodels the intestinal bile acid milieu, which selects for microbial taxa that produce senescence‑promoting metabolites; this microbial shift, in turn, reinforces vagal hypoactivity, creating a self‑amplifying gut‑brain loop that drives biological age.\n\n## Mechanistic Basis\nVagal efferent fibers release acetylcholine that regulates hepatic bile acid synthesis and intestinal transporters (e.g., FXR‑FGF19 axis). When vagal tone falls with age, hepatic CYP7A1 activity drops, shifting the bile acid pool toward more hydrophobic, microbiota‑unfriendly species. These altered bile acids favor expansion of bile‑acid‑transforming bacteria such as certain Clostridia and Enterobacteriaceae that generate metabolites like δ‑valerobetaine and secondary bile acids known to activate microglial inflammation [1][2]. Elevated δ‑valerobetaine impairs hippocampal plasticity via CCKAR‑dependent vagal afferent silencing [3], while secondary bile acids increase gut permeability, allowing LPS to trigger systemic inflammation that further depresses vagal efferent output via central nitrosative stress [4] [5]. Thus, the microbiome age is not a passive readout but an active effector shaped by vagal efferent output.\n\n## Testable Predictions\n1. Pharmacological or optogenetic enhancement of vagal efferent firing in aged mice will normalize the fecal bile acid profile, reduce δ‑valerobetaine‑producing taxa, and restore youthful performance on memory tasks.\n2. Selective vagotomy or chemogenetic inhibition of vagal efferents in young adult mice will rapidly reproduce an aged‑like microbiome signature (increase in Clostridium scindens‑like bile‑acid 7α‑dehydroxylators, rise in fecal δ‑valerobetaine) and impair cognition within two weeks.\n3. Supplementation with an FXR agonist that mimics vagal efferent‑driven bile acid signaling will rescue the microbiome‑cognitive deficit even when vagal efferent remains suppressed, placing bile acid signaling downstream of the nerve.\n\n## Experimental Approach\n- Subjects: Young (3 mo) and aged (20 mo) C57BL/6 mice.\n- Interventions:\n * Chemogenetic activation of dorsal motor nucleus of the vagus (DMV) cholinergic neurons using hM3Dq + CLO.\n * Vagal efferent transection or hM4Di inhibition in young mice.\n * FXR agonist (GW4064) treatment arms.\n- Readouts:\n * Fecal metabolomics (targeted δ‑valerobetaine, bile acid spectrum) via LC‑MS.\n * 16S rRNA + shotgun metagenomics to quantify taxa linked to bile‑acid transformation.\n * Vagal efferent activity recorded by electroneurogram.\n * Cognitive testing (novel object recognition, Morris water maze).\n * Hippocampal microglial activation (Iba1 immunostaining) and cytokine profiling.\n- Analysis: Compute geodesic distance between gut and brain multi‑omics latent spaces as a proxy for gut‑brain biological age disparity; test whether interventions reduce this distance in parallel with cognitive improvement.\n\n## Potential Implications\nIf vagal efferent tone sets the pace of microbial aging, then therapies that boost brain‑to‑gut communication (e.g., biofeedback, vagus nerve stimulation, bile‑acid mimetics) could decouple microbiota‑driven inflammaging from chronological age, offering a leverage point where a single intervention simultaneously rejuvenates both organ systems.