Hypothesis: Chronic efferent vagal signaling suppresses the production of gut‑derived phenylacetic acid (PAA) and trimethylamine‑N‑oxide (TMAO) by modulating enteroendocrine cholinergic pathways, thereby preserving tropoelastin synthesis and preventing vascular smooth muscle cell (VSMC) osteochondrogenic shift. This neural‑metabolic brake explains why vascular aging is plastic and can be reset by young systemic environments.

Mechanistic rationale:

1. Vagal afferents relay gut metabolite levels to the nucleus tractus solitarius; efferent output releases acetylcholine (ACh) onto intestinal epithelial cells expressing the muscarinic M3 receptor (Chrm3).
2. ACh‑Chrm3 signaling triggers enteroendocrine L‑cells to secrete peptide YY (PYY) and glucagon‑like peptide‑1 (GLP‑1). Both peptides have been shown to inhibit bacterial tyrosine decarboxylase and choline‑trimethylamine lyase, the enzymes that generate PAA and TMAO from dietary precursors.
3. Reduced luminal PAA and TMAO lower their systemic concentrations, decreasing endothelial senescence (via PAA‑induced ROS) and VSMC calcification (via TMAO‑driven Runx2 activation).
4. With less oxidative stress and osteogenic signaling, aortic tropoelastin transcription (via TGF‑β/Smad2/3) remains intact, preserving elastic fiber recoil and arterial compliance.
5. This creates a feedback loop: healthier vasculature improves baroreceptor sensitivity, augmenting vagal tone and reinforcing the metabolite‑lowering effect.

Testable predictions:

• In aged mice, chronic vagal nerve stimulation (VNS) will reduce plasma PAA and TMAO by ≥30% compared with sham‑stimulated controls.
• VNS‑treated aged mice will show increased aortic tropoelastin mRNA and protein levels, decreased VSMC α‑SMA+/Runx2+ cells, and lower pulse‑wave velocity (PWV).
• The vascular benefits of VNS will be abolished in mice with gut‑specific Chrm3 knockout or after broad‑spectrum antibiotic depletion, confirming the gut‑mediated route.
• Administering exogenous PYY or GLP‑1 to aged, vagotomized mice will mimic VNS effects on PAA/TMAO and arterial stiffness, while receptor antagonists will block them.
• Conversely, elevating luminal PAA (via phenylalanine‑rich diet) or TMAO (via choline supplementation) will override VNS‑induced protection, restoring stiffness despite intact vagal firing.

Falsifiability: If VNS fails to lower PAA/TMAO, does not improve tropoelastin expression, or improves stiffness independent of gut microbiota status (e.g., in germ‑free mice), the hypothesis is refuted. This framework shifts focus from passive wear‑and‑tear to an active neuro‑metabolic circuit that can be therapeutically tuned, offering a concrete target for interventions aimed at vascular rejuvenation.