Hypothesis

We propose that the longevity benefit of intermittent fasting (IF) over prolonged fasting (PF) stems from IF‑induced lysosomal biogenesis that enables efficient removal of senescent cells, whereas PF triggers lysosomal stress that blocks this clearance and activates pro‑inflammatory signaling.

Mechanistic Basis

Autophagy functions as a rationing system that recycles intracellular components to sustain essential metabolism during nutrient siege. In aged cells, elevated basal autophagy recycles amino acids that reactivate mTORC1, blunting the autophagy response—a feedback loop noted in ’s analysis【https://doi.org/10.1083/jcb.201610113】. We add that the fate of recycled material depends on lysosomal capacity. IF cycles produce short, repeated bursts of AMPK activation and TFEB nuclear translocation, driving lysosomal gene expression (e.g., LAMP1, Cathepsins) without allowing amino acid accumulation to reach mTORC1‑activating thresholds. This creates a "lysosomal window" where autophagosomes fuse efficiently with healthy lysosomes, degrading damaged organelles and, importantly, senescent cell‑associated macromolecules.

In contrast, PF maintains prolonged AMPK activation but also sustains high levels of autophagic flux. The continuous supply of recycled amino acids eventually reactivates mTORC1 despite low extracellular nutrients, inhibiting TFEB and halting lysosomal biogenesis. Lysosomes become overloaded with undigested material, leading to lysosomal membrane permeabilization (LMP), cathepsin release into the cytosol, and NLRP3 inflammasome activation. Consequently, senescent cells accumulate and secrete SASP factors, counteracting any systemic benefit of autophagy.

Predictions & Experimental Design

1. Lysosomal markers – In liver and muscle of aged mice, IF (e.g., 24 h fast every other day) will increase LAMP1‑positive lysosomes and TFEB nuclear localization compared with ad libitum fed controls, while PF (e.g., 48 h fast) will show no increase or a decrease relative to IF【https://pmc.ncbi.nlm.nih.gov/articles/PMC4500936】.
2. Autophagosome‑lysosome fusion – Using mCherry‑GFP‑LC3 reporters, IF will show a higher ratio of red‑only (autolysosomes) to yellow puncta (autophagosomes) than PF, indicating improved flux.
3. Senescent cell burden – SA‑β‑gal staining and p16^INK4a^ immunoblot will be reduced after 4 weeks of IF but unchanged or increased after PF.
4. Inflammasome read‑outs – Cytosolic cathepsin B activity and cleaved caspase‑1 will be elevated in PF‑treated tissues but not in IF‑treated tissues.
5. Functional outcome – Mice undergoing IF will exhibit improved glucose tolerance and lower circulating IL‑6 and TNF‑α compared with PF counterparts.

These predictions are falsifiable: if IF does not enhance lysosomal biogenesis or senescent cell clearance, or if PF reduces senescence without inflammasome activation, the hypothesis must be revised.

Potential Implications

If validated, this mechanism reframes fasting regimens not merely as autophagy inducers but as modulators of lysosomal health. It suggests that longevity strategies should prioritize regimens that promote lysosomal renewal—such as time‑restricted eating or periodic refeeding—rather than attempting to extend the siege of continuous nutrient deprivation.