Core Idea

Supplementing with nicotinamide (NAM) will selectively amplify the SIRT6‑dependent deacetylation and activation of NAMPT, creating a nuclear‑focused NAD+ boost that enhances SIRT6‑mediated DNA repair and metabolic regulation more effectively than NMN, leading to sex‑independent lifespan extension.

Mechanistic Rationale

• SIRT6 deacetylates NAMPT at lysine 53, increasing its catalytic efficiency for converting NAM to NAD+ via the salvage pathway (5).
• NAM serves as the direct substrate for this reaction, whereas NMN bypasses NAMPT and is phosphorylated to NAD+ by NMNAT enzymes, predominantly elevating cytosolic NAD+ pools that favor SIRT1 activation (6).
• Increased nuclear NAD+/NADH ratio drives SIRT6 deacetylation of histone H3K9ac and DNA repair factors (e.g., PARP1, Ku70), improving double‑strand break repair (2) and stabilizing telomeres (3).
• Hepatic SIRT6 activity maintains gluconeogenic flexibility and NAD+ homeostasis during aging (1).

Testable Predictions

1. In aged C57BL/6J mice, chronic NAM supplementation will raise nuclear NAD+ levels and SIRT6‑dependent deacetylation marks (H3K9ac, Ku70ac) to a greater extent than equimolar NMN.
2. NAM‑treated mice will show enhanced NAMPT activity (measured by NAM‑to‑NAD conversion rate in isolated nuclei) correlating with SIRT6 deacetylation of NAMPT at K53.
3. Median lifespan extension will be observed in both male and female NAM‑treated cohorts (≥15% increase), whereas NMN will extend lifespan primarily in males, replicating prior sex‑specific findings (7).
4. Pharmacological inhibition of NAMPT (e.g., with FK866) will abolish the longevity benefit of NAM, confirming dependence on the SIRT6‑NAMPT axis.

Experimental Design

• Groups (n=30 per sex): vehicle, NAM (400 mg/kg/day), NMN (400 mg/kg/day), NAM + FK866, NMN + FK866.
• Duration: 12 months treatment, followed by natural lifespan monitoring.
• Readouts: nuclear vs cytosolic NAD+ quantification (LC‑MS), SIRT6 activity (fluorometric deacetylase assay), NAMPT acetylation status (Western blot with acetyl‑lysine antibody), DNA damage markers (γH2AX foci), hepatic glucose tolerance, and survival curves.
• Statistical Analysis: Kaplan‑Meier with log‑rank test for lifespan; two‑way ANOVA for biochemical endpoints; sex as a factor.

Potential Impact

If validated, this hypothesis would reposition NAM as a superior NAD+ precursor for targeting the SIRT6‑NAMPT longevity circuit, clarify the mechanistic disconnect between generic NAD+ elevation and SIRT6‑specific signaling, and guide sex‑balanced intervention strategies for human aging trials.