Hypothesis

Baseline efferent vagal tone determines whether gut‑derived signals or brain‑driven autonomic output dominate the gut‑brain axis in aged individuals. Low vagal tone shifts causality upward, making gut metabolites and enteroendocrine signals the primary drivers of brain aging; high vagal tone enables downward control, allowing brain‑originated cholinergic anti‑inflammatory pathways to shape gut microbiota and barrier function. Consequently, longevity interventions that first restore vagal efferent activity will create a permissive state for subsequent microbiome‑targeted therapies, whereas reversing this order yields diminished or null effects.

Mechanistic basis

Vagal efferent signaling releases acetylcholine onto the enteric nervous system and immune cells, activating the cholinergic anti‑inflammatory pathway that suppresses NF‑κB–mediated cytokine production in the gut lamina propria [2]. This tone also regulates intestinal permeability via tight‑junction protein expression and modulates enteroendocrine cell secretion of serotonin and peptide YY, which feed back to the brain via vagal afferents [3]. When vagal efferent activity declines with age, reduced acetylcholine signaling permits low‑grade intestinal inflammation, increased translocation of microbial products (e.g., LPS), and altered metabolite profiles that then signal to the brain through afferent vagal and humoral routes, exacerbating neuroinflammation and cognitive decline [1]. Conversely, preserved vagal outflow can maintain gut homeostasis, limiting harmful upstream signals and allowing central circadian and stress‑response systems to entrain peripheral metabolism.

Testable predictions

1. Individuals aged >65 yr with baseline heart‑rate variability (HRV) below the age‑adjusted 25th percentile will show stronger correlations between fecal microbial diversity (or specific metabolite levels) and subsequent changes in hippocampal‑dependent memory performance over 6 months than those with HRV above the 75th percentile.
2. Administering transcutaneous vagus nerve stimulation (tVNS) for 4 weeks prior to a prebiotic/fiber intervention will produce a significantly greater increase in butyrate‑producing taxa and a larger improvement in episodic memory scores compared to the same microbiome intervention given without prior tVNS.
3. In mice, chemogenetic inhibition of the dorsal motor nucleus of the vagus (reducing efferent tone) will invert the direction of causality: gut‑targeted antibiotic treatment will no longer rescue age‑related hippocampal LTP deficits, whereas direct vagal activation will restore memory even when the microbiome remains dysbiotic.

Experimental approach

• Human arm: Recruit 120 older adults, stratify by HRV (low, medium, high). Collect baseline stool metatranscriptomics, plasma metabolomics, and cognitive testing. Randomize low‑HRV participants to either (a) tVNS → prebiotic (12 weeks each) or (b) prebiotic → tVNS. Primary outcome: change in Rey Auditory Verbal Learning Test score; secondary: shifts in microbial short‑chain‑fatty‑acid producers and plasma IL‑6/TNF‑α.
• Mouse arm: Use aged (20‑month) C57BL/6J mice with DREADDs to inhibit or excite cholinergic vagal efferents. Apply broad‑spectrum antibiotics or vagal stimulation, then assess gut barrier integrity (FITC‑dextran permeability), hippocampal cytokine levels, and spatial memory (Morris water maze).

Falsifiability

If low‑HRV individuals derive equal or greater cognitive benefit from microbiome‑first interventions compared to vagal‑first sequencing, or if tVNS fails to alter microbial composition or inflammatory markers despite confirmed efferent activation, the hypothesis would be falsified. Similarly, in mice, if vagal efferent inhibition does not abolish the protective effect of gut‑targeted antibiotics on hippocampal plasticity, the proposed directional switch lacks mechanistic support.

Potential impact

Demonstrating that efferent vagal tone gates the direction of gut‑brain communication would reorient personalized aging protocols: autonomic profiling becomes a prerequisite decision node for sequencing neuromodulation and microbiome therapies. This approach could reduce trial‑and‑error, improve cost‑effectiveness of longevity programs, and provide a clear biomarker‑driven framework for digital‑twin models that integrate both afferent and efferent axis data.