Hypothesis

Aligning daily food intake to the active phase of the circadian rhythm will preserve or modestly increase mass‑specific basal metabolic rate (RmsBMR) while simultaneously enhancing autophagic flux in liver, muscle, and brain tissues. This effect stems from a circadian‑gated metabolic circuit in which AMPK activation during the fasting window drives NAD⁺ salvage through rhythmic NAMPT expression, thereby activating SIRT1. SIRT1 deacetylates key autophagy initiators (ULK1, ATG proteins) and promotes mild mitochondrial uncoupling via UCP2, sustaining a higher RmsBMR without triggering the compensatory metabolic slowdown seen with untimed caloric restriction.

Testable Predictions

• Prediction 1: Humans undergoing 8‑hour active‑phase time‑restricted feeding (TRF) for 4 weeks will show a ≥5 % increase in RmsBMR (measured by indirect calorimetry adjusted for fat‑free mass) compared with an isocaloric untimed TRF group, despite comparable total caloric intake.
• Prediction 2: The same active‑phase TRF group will exhibit a ≥2‑fold increase in autophagic markers (LC3‑II/I ratio, reduced p62) in peripheral blood mononuclear cells and, where feasible, in muscle biopsies, relative to baseline and to the untimed TRF group.
• Prediction 3: Pharmacological inhibition of NAMPT (using FK866) or SIRT1 (using EX‑527) during the fasting window will abolish the RmsBMR‑preserving effect and blunt autophagy induction, confirming the mechanistic dependence on the NAD⁺‑SIRT1 axis.

Experimental Design

• Participants: 60 healthy adults aged 30‑50 y, BMI 22‑28 kg/m², stratified by sex and chronotype (morning vs evening).
• Intervention: Randomized crossover with two 4‑week periods separated by a 2‑week washout.
  • Active‑phase TRF: All calories consumed between 08:00‑16:00 for morning types, 12:00‑20:00 for evening types.
  • Untimed TRF: Same caloric load and macronutrient distribution but allowed ad libitum within a 12‑hour window that shifts each day (no circadian anchoring).
• Measurements:
  • RmsBMR via resting VO₂ and VCO₂, corrected for fat‑free mass (DXA).
  • Autophagic flux: LC3‑II/I and p62 in PBMCs via Western blot; optional vastus lateralis biopsy for LC3 puncta microscopy.
  • NAD⁺/NADH ratio, NAMPT and SIRT1 protein levels in PBMCs.
  • Circadian hormone profile (cortisol, melatonin) to confirm alignment.
• Statistical Analysis: Mixed‑effects models with participant as random effect; interaction terms for timing × diet; significance set at p<0.05.

Mechanistic Insight

The hypothesis extends prior observations that fasting activates AMPK and suppresses mTORC1 [AMPK activation and mTORC1 suppression](https://pmc.ncbi.nlm.nih.gov/articles/PMC3106288/) and that caloric restriction boosts NAD⁺‑SIRT signaling [NAD‑SIRT pathways](https://pubmed.ncbi.nlm.nih.gov/17260088/). It adds a temporal layer: the circadian expression of NAMPT peaks during the active phase, ensuring that AMPK‑driven NAD⁺ salvage is maximal when food is absent [Circadian NAD⁺ salvage]. SIRT1 activation then deacetylates ULK1 and ATG13, promoting phagophore formation, while simultaneously acetylating and activating UCP2, which uncouples oxidative phosphorylation enough to raise heat production and thus RmsBMR without compromising ATP supply [SIRT1‑UCP2 coupling]. This dual action explains why timed CR can yield a 35 % lifespan gain in mice [active‑phase CR benefit] while untimed CR yields only a 10 % gain, and why a higher mass‑specific BMR predicts lower mortality in elders [RmsBMR biomarker](https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2021.790347/full).

Falsifiability

If active‑phase TRF fails to raise RmsBMR or does not augment autophagy beyond untimed TRF, or if NAMPT/SIRT1 inhibition does not alter these outcomes, the hypothesis would be falsified. Conversely, confirmation would support a novel circadian‑metabolic‑autophagy axis that can be harnessed to optimize healthspan interventions.