Hypothesis

If autophagy operates as a siege‑rationing system, then chronic activation beyond the nutrient‑scarcity threshold will shift the cell from protective recycling to maladaptive self‑consumption, driving senescence rather than lifespan extension. Consequently, interventions that produce intermittent autophagy pulses aligned with natural feast‑famine cycles should extend healthspan, whereas continuous pharmacological or genetic upregulation will show a U‑shaped effect on longevity: beneficial at low‑to‑moderate flux, harmful at high flux.

Mechanistic Rationale

• Autophagy‑derived amino acids sustain TCA cycle flux during starvation, supporting ATP production and delaying AMPK‑mediated growth arrest [1]
• Persistent autophagic flux depletes essential organelles (e.g., mitochondria, ER) below a critical threshold, triggering ROS‑induced DNA damage and p53‑dependent senescence [2]
• The transition point is modulated by the cellular NAD⁺/NADH ratio; low NAD⁺ favors SIRT1 inhibition, reducing deacetylation of autophagy proteins and shifting the balance toward non‑selective bulk degradation [6]

Testable Predictions

1. In murine models, rapamycin dosing that yields autophagic flux measured by LC3‑II turnover of 0.5‑1.0 × baseline (intermittent dosing every 48 h) will increase median lifespan by ~15 % relative to controls, whereas constant dosing producing flux >2.0 × baseline will shorten lifespan by ~10 %.
2. Single‑cell RNA‑seq of liver hepatocytes after 2 weeks of intermittent fasting will show enrichment of selective autophagy signatures (e.g., NBR1, p62) and upregulated NAD⁺ biosynthesis enzymes, while continuous rapamycin treatment will elevate markers of organelle loss (e.g., TOM20 depletion, Calnexin loss) and senescence‑associated secretory phenotype (SASP) genes.
3. Using target‑trial emulation with time‑varying autophagy flux (measured by plasma sulfated glycosaminoglycans as a flux proxy) and nutrient state (circulating glucose/insulin), parametric g‑formula analysis will reveal a negative autocorrelation between flux and mortality only when flux remains below the empirically derived threshold (≈1.3 × baseline); above this threshold the association reverses.

Falsifiability

If continuous autophagy upregulation consistently extends lifespan across genetic backgrounds without inducing senescence markers, or if intermittent protocols fail to improve healthspan relative to ad libitum feeding, the siege‑rationing hypothesis would be refuted.

References (inline)

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