Tissue‑Specific Autophagy Timing Driven by Fasting‑Refeeding Cycles and Metabolic Flexibility Predicts Longevity Benefits More Than Basal Metabolic Rate

Hypothesis

Intermittent 24‑hour fasting periods followed by a low‑protein, high‑carbohydrate refeed generate tissue‑specific autophagy waves that peak earlier in liver (≈12 h post‑fast) and later in skeletal muscle (≈24 h post‑fast), and the magnitude of these waves correlates with metabolic flexibility measured by the respiratory exchange ratio (RER) shift during refeeding, rather than with basal metabolic rate.

Mechanistic rationale

• Fasting activates AMPK and inhibits mTORC1, triggering ULK1‑dependent autophagy. The rate of AMPK activation depends on cellular AMP:ATP ratios, which fall faster in glycogen‑rich hepatocytes than in myofibers, producing an earlier autophagic burst in liver ([2]).
• A refeed low in leucine (the primary mTORC1 activator) sustains mTORC1 suppression while replenishing glycogen, allowing autophagy to continue in muscle where protein‑sensing is dominant ([5],[6]).
• Metabolic flexibility—the ability to switch from fatty‑acid oxidation to carbohydrate utilization—reflects efficient NADH/FADH2 feeding into the electron transport chain and is a readout of mitochondrial readiness to support autophagosome formation; individuals with greater RER flexibility show higher LC3‑II turnover in peripheral blood mononuclear cells ([3],[4]).

Testable predictions

1. In a randomized crossover trial, participants undergoing 24 h fast → low‑protein/high‑carb refeed will show:
   • Higher hepatic autophagy flux (measured by liver‑derived extracellular vesicle LC3‑II/I ratio) at 12 h post‑fast compared to a continuous 48 h fast.
   • Higher skeletal‑muscle autophagy flux (vastus lateralis biopsy LC3‑II/I) at 24 h post‑fast under the same condition.
2. The change in RER from fasting (≈0.7) to refeeding (≈0.9) will positively correlate with autophagy flux in both tissues (r > 0.5, p < 0.01).
3. Basal metabolic rate (measured by resting VO₂) will not predict autophagy flux after controlling for RER flexibility.
4. Adding resistance exercise during the refeed window will prevent any decline in lean mass despite repeated low‑protein periods, indicating that the autophagy‑induced proteostatic benefit does not translate to sarcopenia when mechanical loading is present.

Falsifiability

If the trial finds no difference in tissue‑specific autophagy timing between the intermittent protocol and continuous fasting, or if autophagy flux does not track RER flexibility but does correlate with basal metabolic rate, the hypothesis is refuted. Similarly, if low‑protein refeeds consistently reduce muscle protein synthesis markers (e.g., phosphorylated S6K) without rescue by exercise, the claim that autophagy benefits can be decoupled from sarcopenia risk fails.

Broader impact

Demonstrating that metabolic flexibility, not mere caloric restriction, drives tissue‑specific autophagy would refine fasting prescriptions, allowing personalized timing of low‑protein refeeds to maximize longevity pathways while preserving muscle mass across age groups.