Hypothesis

Hormetic lifespan extension depends on repeated, transient activation of threat‑sensing pathways (NRF2, HSF1, FOXO) that act as a molecular readout of imminent danger, rather than on sustained pathway activity.

Mechanistic Basis

We propose that the key signal is a brief surge in mitochondrial ROS that shifts the NAD+/NADH ratio, transiently inhibiting TOR and activating AMPK, which then phosphorylates FOXO and NRF2. This ROS‑NAD+ pulse is sufficient to trigger autophagy and proteostasis without triggering apoptosis. Constitutive activation of FOXO or NRF2 bypasses the ROS‑NAD+ trigger, leading to chronic transcriptional programs that induce feedback inhibitors (e.g., KEAP1 upregulation, Sestrin2‑mediated TOR reactivation) and thus blunt the protective output. In contrast, senolytic removal of damaged cells directly reduces the source of chronic ROS, allowing the NAD+/NADH ratio to normalize without requiring a threat pulse.

Predictions & Experimental Design

1. Genetic test – Mice with liver‑specific, inducible FOXO3 overexpression will show no lifespan increase when FOXO3 is kept constantly active, but will recapitulate the ~30 % extension seen with intermittent fasting if FOXO3 is activated only during 24‑h fasting windows every week.
2. Pharmacological test – Chronic NAC (an antioxidant) will abolish the lifespan benefits of resveratrol and sulforaphane in C. elegans, whereas pulsing NAC (short exposure before each hormetic dose) will preserve the benefit.
3. Metabolite biomarker – Fecal IPA spikes will precede each hormetic intervention in wild‑type mice; maintaining IPA at a constant high level via continuous infusion will not extend lifespan and will suppress NRF2 target gene induction after a subsequent sulforaphane challenge.
4. Comparative arm – In a single cohort, intermittent fasting, senolytic (dasatinib + quercetin), and constant NAD+ booster (NR) will be administered; only the intermittent fasting and senolytic groups will show reduced senescence‑associated secretory phenotype (SASP) and improved frailty scores, while the constant NR group will resemble controls.

Potential Confounds & Controls

• Ensure that inducible FOXO3 lines have leak‑free expression; include a vehicle‑only inducible control.
• Verify NAC pharmacokinetics to distinguish chronic vs pulsing regimens.
• Measure fecal IPA by LC‑MS/MS to confirm temporal dynamics.
• Use blinded assessment of survival and frailty.

If constitutive pathway activation fails to extend lifespan while pulsed activation succeeds, the hypothesis that hormesis measures threat‑response capacity rather than directly implementing repair will be supported. Conversely, if constant FOXO/NRF2 activity yields comparable longevity gains, the threat‑detector model would be falsified.