Hypothesis

Pulsed nutrient sensing that alternates short bouts of AMPK activation with brief mTORC1 refeeding windows maximizes autophagic flux while preserving metabolic flexibility, thereby outperforming chronic caloric restriction for human longevity.

Rationale

Chronic suppression of mTORC1 via prolonged fasting or caloric restriction (CR) drives autophagy but risks persistent AMPK activation that can impair anabolic signaling needed for tissue repair [4]. Conversely, intermittent mTORC1 reactivation during nutrient‑dense refeeding restores protein synthesis and replenishes NAD+ pools, which fuels sirtuin activity [2].

We propose that a 2-hour fasting pulse followed by a 4-hour nutrient‑dense refeed creates a rhythmic AMPK/mTORC1 oscillation. During the fasting window, falling ATP raises the AMP/ATP ratio, activating AMPK and ULK1‑dependent autophagy initiation. The subsequent refeed spikes intracellular leucine and insulin, transiently stimulating mTORC1 to phosphorylate S6K and promote lysosomal biogenesis via TFEB nuclear export, thereby expanding the autophagosome-lysosome capacity for the next cycle.

Novel Mechanistic Insight

The key innovation is that mTORC1‑driven lysosomal biogenesis acts as a "reset" that prevents autophagosome accumulation, a phenomenon observed in some lifelong CR rat models where autophagy markers plateau despite continued suppression [7]. By allowing periodic mTORC1 activity, the system avoids lysosomal exhaustion and maintains flux through the autophagy‑lysosome pathway. Additionally, the refeed‑induced rise in NAD+ activates SIRT1, which deacetylates autophagy genes (e.g., LC3, ATG5) and enhances their expression, synergizing with AMPK‑mediated ULK1 activation.

Testable Predictions

1. Flux measurement: Human participants undergoing the 2h fast/4h refeed pattern for 4 weeks will show higher LC3‑II turnover (measured by bafilomycin A1-adjusted immunoblotting in peripheral blood mononuclear cells) compared with matched subjects on continuous 30% CR.
2. Lysosomal capacity: Lysosomal-associated membrane protein 1 (LAMP1) fluorescence intensity will increase after each refeed pulse, indicating biogenesis, and will not decline over the intervention period.
3. Metabolic flexibility: Respiratory exchange ratio (RER) will shift more rapidly between fat and carbohydrate oxidation during the protocol, reflecting preserved mitochondrial plasticity.
4. Outcome biomarker: Serum IGF-1 will transiently rise during refeeds but remain lower than baseline over the study duration, capturing the anabolic‑catabolic balance.

Falsifiability

If the pulsed protocol fails to produce greater LC3‑II turnover or lysosomal marker elevation than continuous CR, or if lysosomal markers show progressive decline indicating exhaustion, the hypothesis is refuted. Likewise, absence of improved metabolic flexibility (no RER shift) would contradict the predicted preservation of mitochondrial function.