Core Idea

Intermittent fasting (IF) triggers a rapid, transient rise in circulating FGF21 that peaks within 12‑24 h of food withdrawal. We hypothesize that this FGF21 surge does not merely act as a biomarker but actively coordinates a tissue‑specific autophagy program by modulating the hepatic NAD+ salvage pathway in a manner that is tightly coupled to shifts in the respiratory exchange ratio (RER). When RER falls below 0.85 (indicating predominant fat oxidation), hepatic FGF21 stimulates SIRT1‑mediated deacetylation and activation of NAMPT, boosting NAD+ levels. Elevated NAD+ then activates SIRT1 and SIRT3 in peripheral tissues (muscle, adipose, liver), promoting deacetylation of autophagy regulators such as LC3, ATG5, and TFEB, thereby amplifying autophagic flux. This creates a feedback loop: enhanced autophagy improves mitochondrial fatty‑acid oxidation, further lowering RER and sustaining FGF21 secretion. Consequently, the metabolic flexibility index—defined as the magnitude and speed of RER decline during fasting—predicts the intensity of the FGF21‑NAD+‑autophagy axis and, ultimately, longevity outcomes.

Testable Predictions

1. Temporal coupling – In human volunteers undergoing a 24‑h fast, plasma FGF21 will peak at 12‑16 h, preceding a measurable increase in hepatic NAD+ (detectable via hepatic MR‑spectroscopy) and a concomitant drop in RER below 0.85.
2. Causal link – Pharmacological inhibition of hepatic NAMPT (e.g., with FK866) during the fasting window will blunt the FGF21‑induced rise in NAD+ and attenuate autophagy markers (LC3‑II, GFP‑LC3 puncta, p62 clearance) in peripheral blood mononuclear cells, despite normal FGF21 elevation.
3. Metabolic flexibility dependence – Subjects with a blunted RER shift (>0.90 at 12 h) will show a significantly smaller FGF21‑NAD+ response and lower autophagic flux compared with flexible responders, even when caloric intake is matched.
4. Rescue experiment – Exogenous NAD+ precursors (NR or NMN) administered to inflexible fasters will restore autophagy markers to levels seen in flexible individuals, confirming NAD+ as the downstream effector.
5. Longevity correlation – In a longitudinal cohort, baseline metabolic flexibility (quantified by the slope of RER decline during a standard 2‑h mixed‑meal tolerance test) will predict the magnitude of fasting‑induced FGF21 and NAD+ changes, which in turn will correlate with improvements in clinical biomarkers of aging (LDL‑C, CRP, insulin sensitivity) over 12 months.

Mechanistic Reasoning Beyond Cited Works

While prior studies established that fasting activates AMPK, inhibits mTORC1, and drives TFEB nuclear translocation (1), they did not address how systemic hormonal signals synchronize these cell‑autonomous pathways across tissues. FGF21 is known to rise during fasting (1), yet its role is largely viewed as a metabolic adjuster rather than a signaling orchestrator of autophagy. By integrating the concept of metabolic flexibility—highlighted as more predictive of lifespan than absolute metabolic rate (2)—we position RER as a real‑time read‑out of substrate utilization that directly influences hepatic redox state. The hepatic NAD+ salvage pathway is exquisitely sensitive to the NAD+/NADH ratio, which shifts when fatty‑acid oxidation dominates. FGF21‑driven SIRT1 activation of NAMPT provides a plausible mechanistic bridge: a drop in RER signals heightened fat burning, which hepatic FGF21 senses and amplifies via NAD+ production, thereby propagating a fasting‑autophagy signal to peripheral tissues. This loop explains why low‑protein, high‑carbohydrate diets that promote fat oxidation can mimic caloric restriction’s benefits (2)—they maintain a low RER, sustaining the FGF21‑NAD+ axis without caloric deficit.

Experimental Design (Outline)

• Participant groups: (a) metabolically flexible (RER <0.85 at 12 h fast), (b) inflexible (RER >0.90), each n=30.
• Intervention: 24‑h water fast with blood draws at 0, 4, 8, 12, 16, 20, 24 h for FGF21, NAD+, lactate, β‑hydroxybutyrate.
• Measurements: hepatic NAD+ via ^31P‑MRS, peripheral autophagy markers (LC3‑II/p62 in PBMCs, GFP‑LC3 imaging in a subset via transfected lymphocytes), RER via indirect calorimetry.
• Intervention arms: subset receives NAMPT inhibitor or NAD+ precursor during fast to test causality.
• Follow‑up: 12‑month monitoring of clinical aging biomarkers and incidence of age‑related morbidity.

Potential Impact

If validated, this hypothesis would reposition FGF21 from a passive fasting biomarker to an active regulator of inter‑tissue autophagy, with metabolic flexibility (RER dynamics) as the key determinant of longevity‑promoting fasting responses. It would suggest personalized fasting prescriptions based on an individual's RER shift capacity, and it opens therapeutic avenues—targeting the hepatic FGF21‑NAD+ axis—to mimic fasting’s autophagy benefits for those unable to adhere to prolonged caloric restriction.