Hypothesis

Combining nicotinamide riboside (NR) supplementation with intermittent rapamycin dosing will produce a greater-than-additive improvement in DunedinPACE (pace‑of‑aging) compared to either intervention alone, mediated by gut microbiota‑derived nicotinic acid (NA) that simultaneously raises NAD+ via the Preiss‑Handler pathway and activates SIRT1‑dependent AMPK signaling to suppress mTORC1 more effectively than rapamycin alone.

Rationale

• NR and NMN raise circulating NAD+ (~2‑fold after 2 weeks) but their efficacy depends on gut bacterial conversion to NA, which also boosts short‑chain fatty acids (SCFAs) [1, 2].
• Intermittent rapamycin (e.g., weekly 1 mg) extends mouse lifespan with fewer metabolic side effects by allowing mTORC1 rebound between doses [3, 4].
• NAD+ activates SIRT1, which deacetylates and activates LKB1‑AMPK, a known upstream inhibitor of mTORC1; thus, higher NAD+ can potentiate rapamycin’s mTOR suppression [5]
• DunedinPACE is sensitive to short‑term changes in metabolism and inflammation, making it ideal for detecting rapid effects of NAD+ and mTOR modulation [6]

Novel Mechanistic Insight

We propose that gut‑derived NA does more than simply raise NAD+: NA binds the GPR109A receptor on intestinal epithelial cells, triggering a paracrine release of prostaglandin‑E2 that activates hepatic AMPK independently of NAD+ levels. This dual‑hit (NAD+‑SIRT1‑AMPK + GPR109A‑AMPK) yields a stronger and more sustained inhibition of mTORC1 during the rapamycin “off” window, reducing the dose needed to achieve the same signaling suppression and thereby lowering side‑effects such as glucose intolerance.

Testable Predictions

1. Biochemical – Participants receiving NR + intermittent rapamycin will show:
   • Higher fecal NA and SCFA concentrations than NR alone or rapamycin alone.
   • A >2‑fold increase in circulating NAD+ (vs baseline) that exceeds the sum of the individual effects.
   • Greater AMPK phosphorylation (p‑AMPK Thr172) in peripheral blood mononuclear cells.
2. Epigenetic – Quarterly DunedinPACE will decline significantly more in the combination arm than predicted by additive effects of NR and rapamycin (interaction term p < 0.05). GrimAge will show comparable long‑term risk reduction across all active arms.
3. Physiological – Fasting insulin and HOMA‑IR will improve most in the combination arm, with fewer episodes of transient hyperglycemia than rapamycin‑only.

Experimental Design

• Population: 120 healthy adults aged 45‑65, stratified by sex and baseline BMI.
• Design: 2‑×‑2 factorial, randomized, double‑blind, placebo‑controlled crossover with 8‑week treatment periods and 4‑week washout.
• Arms: (1) Placebo, (2) NR 1000 mg/day, (3) Rapamycin 1 mg/week (intermittent: 3 days on/4 days off), (4) NR + Rapamycin (same doses).
• Outcomes: Serum NAD+, fecal NA/SCFA (LC‑MS), p‑AMPK (Western blot), DunedinPACE and GrimAge (Illumina EPIC array + online calculators), fasting glucose/insulin, lipid panel, adverse‑event logs.
• Analysis: Mixed‑effects models with interaction term (NR*rapamycin) for DunedinPACE change; mediation analysis testing whether fecal NA mediates the interaction effect on AMPK activation and DunedinPACE.

Falsifiability

If the combination arm does not produce a statistically significant greater reduction in DunedinPACE than the sum of the single‑agent effects, or if fecal NA and AMPK activation do not mediate this interaction, the hypothesis is falsified. Similarly, if NAD+ increases are merely additive without synergistic AMPK/mTORC1 suppression, the proposed mechanistic link is refuted.

Implications

Confirmation would suggest that modulating gut microbiota to enhance NA production (e.g., via targeted prebiotics) could optimize NAD+‑based regimens when combined with intermittent mTOR inhibition, offering a precision‑nutrition strategy for accelerating pace‑of‑aging improvement while minimizing drug burden.