Intermittent pulses of gut‑derived microbial metabolites activate the enteric nervous system (ENS) to trigger a systemic hormetic response that upregulates repair pathways without inducing chronic inflammation.

Microbial metabolites such as short‑chain fatty acids (SCFAs) and tryptamine derivatives fluctuate rhythmically with feeding cycles. These molecules stimulate afferent fibers of the vagus and spinal ENS, leading to coordinated release of acetylcholine and serotonin in the gut wall. The resulting neurochemical signal travels to the hypothalamus and peripheral tissues, where it transiently activates AMPK, SIRT1, and HSF1 in a pattern that mimics classic hormetic stressors (e.g., exercise, fasting). Because the signal is brief and followed by a recovery window, downstream effectors—FOXO transcription factors, Nrf2, and mitophagy machinery—are phosphorylated and then allowed to reset, preventing the pathway exhaustion seen with constant agonist exposure.

We hypothesize that this ENS‑mediated metabolite rhythm provides the constitutive low‑amplitude threat signal described in the seed idea, keeping repair networks primed. In the absence of such pulses, repair pathways drift toward basal activity, accelerating accumulation of lipofuscin and mitochondrial damage; conversely, excessive or continuous metabolite exposure drives maladaptive stress, mimicking the insulin resistance observed with continuous rapamycin dosing.

Testable predictions:

1. Germ‑free mice colonized with a strain that produces a rhythmic SCFA pulse (e.g., engineered Lactobacillus releasing butyrate in a circadian‑dependent manner) will show extended median lifespan and reduced lipofuscin compared with germ‑free controls or mice colonized with a constitutive‑producer strain.
2. We'll silence ENS afferent neurons chemogenetically during the metabolite pulse and expect the lifespan extension and transient AMPK/SIRT1/HSF1 activation in liver and muscle to disappear.
3. Administering a non‑metabolite ENS agonist (e.g., low‑dose capsaicin) at the same frequency as the microbial pulse should recapitulate the hormetic signature, whereas continuous delivery will blunt the response and induce insulin resistance.

Experimental approach:

• Construct a plasmid‑based butyrate synthase under a promoter responsive to the bacterial circadian clock (e.g., KaiC‑based oscillator) in Lactobacillus plantarum.
• Colonize age‑matched C57BL/6 mice (n=20 per group) and monitor feeding‑linked fecal butyrate levels via LC‑MS.
• We'll assess lifespan, frailty index, lipofuscin autofluorescence in heart and brain, and tissue‑specific phosphorylation of AMPK, acetylation of p53, and nuclear translocation of HSF1 at 4‑hour intervals over 24 h cycles.
• Use DREADDs to inhibit vagal afferents during predicted pulse windows; compare outcomes with vehicle‑treated controls.
• Include groups receiving continuous butyrate via osmotic pump to test the exhaustion prediction.

Falsifiable outcome: If rhythmic microbial metabolite exposure fails to produce transient pathway activation or lifespan benefit, or if ENS silencing does not attenuate the effect, the hypothesis is refuted. Conversely, observing the predicted intermittent signaling pattern and its dependence on ENS activity would support the model that the gut‑brain axis supplies the low‑amplitude threat cues required for hormetic longevity.

Key references: intermittent rapamycin dosing improves healthspan by avoiding pathway exhaustion 2; hormetic interventions extend C. elegans lifespan via FOXO/DAF‑16 upregulation 1; mitochondrial UPR activation prolongs life 3; HSP overexpression alone is sufficient for lifespan extension 4.