Hypothesis: Extended fasting boosts lysosomal acidification and cathepsin activity, completing autophagic flux and clearing lipofuscin aggregates

Core idea We propose that fasting beyond 20 hours triggers a sustained rise in AMP‑activated protein kinase (AMPK) activity that, besides inhibiting mTORC1, drives the nuclear translocation of transcription factor EB (TFEB). TFEB up‑regulates genes for vacuolar‑ATPase subunits, lysosomal hydrolases, and membrane lipids, thereby increasing lysosomal acidity and proteolytic capacity. This shift moves autophagic flux from mere autophagosome buildup to efficient degradation of resistant cargo such as lipofuscin and protein aggregates.

Mechanistic steps

1. Energy sensing – Prolonged fasting lowers the ATP/AMP ratio, activating AMPK.
2. TFEB activation – AMPK phosphorylates TFEB on serine residues, weakening its interaction with 14‑3‑3 proteins and promoting nuclear import.
3. Lysosomal biogenesis – Nuclear TFEB induces expression of V‑ATPase subunits (ATP6V0A1, ATP6V1A) and cathepsins (B, L, D), raising luminal proton concentration and enzyme activity.
4. Flux completion – Higher lysosomal acidity speeds autophagosome‑lysosome fusion and cargo breakdown, detectable as reduced lipofuscin fluorescence and increased lysosomal protease activity in peripheral blood mononuclear cells.
5. Feedback loop – Successful degradation lowers cytosolic amino acid levels, further sustaining AMPK activation and maintaining autophagic flux throughout the fasting window.

Testable predictions

• In humans following a 20:4 intermittent fasting schedule for four weeks, TFEB nuclear localization in circulating monocytes will be significantly greater than in a 16:8 control group (p < 0.01).
• Lysosomal pH measured with LysoSensor probes will drop by ~0.2 units in the 20:4 group, correlating with heightened cathepsin D activity (r > 0.6).
• Lipofuscin content, quantified by autofluorescence imaging of skin biopsies, will decline by ≥15 % after the 20:4 intervention, whereas the 16:8 group shows no significant change.
• Pharmacological inhibition of V‑ATPase with bafilomycin A1 will erase the lipofuscin‑clearance advantage of 20:4 fasting, confirming lysosomal acidification as the key mediator.

Falsifiability If the 20:4 regimen fails to produce higher TFEB nuclear translocation, lysosomal acidification, or lipofuscin reduction compared with 16:8 despite comparable AMPK activation, the hypothesis is refuted. Likewise, if lysosomal alkalinization does not track with flux‑completion readouts (LC3‑II turnover, p62 degradation), the proposed mechanism is insufficient.

Relevance Linking fasting duration to lysosomal function bridges the gap between metabolic improvements and genuine cellular cleanup, offering a biomarker‑driven framework to optimize intermittent fasting for age‑related conditions where lipofuscin and protein aggregates drive pathology.

It's unlikely that mTOR inhibition alone explains the observed clearance. This won't happen if lysosomal acidity remains unchanged. We don't expect changes in autophagosome number to directly reflect flux completion.