Quantitative Analysis of NAD Synthesis-Breakdown Fluxes
Ling Liu et al. · Cell Metabolism · 2018
Evidence type: Preclinical mechanistic studyInterpretive weight: Mechanistic contextResearch area: NAD synthesis and precursor flux
DesignStable-isotope tracer and metabolic-flux study in cell lines and mouse tissues
PopulationCultured mammalian cells and mice
SampleMultiple cell and animal experiments; see publication
InterventionIsotope-labeled NAD precursors including NMN and NR administered through different routes
EndpointsNAD synthesis and breakdown flux; Tissue-specific NAD turnover; Fate of oral versus intravenous NMN and NR; Nicotinamide production and tissue precursor use
What the publication reported
This study developed isotope-tracing methods to quantify how NAD is made and broken down across cells and mouse tissues.
NAD turnover differed substantially by tissue. In mice, intravenously administered NMN or NR could deliver intact precursor to multiple tissues, while oral precursor was extensively converted through hepatic metabolism, including to nicotinamide.
The study is important for understanding why route of administration matters, but its in-vivo oral data are from mice and cannot by themselves define human NMN bioavailability.
Key findings
NAD synthesis and breakdown rates varied widely across tissues.
Nicotinamide was a major NAD source in many tissues.
Intravenous precursor delivery differed from oral delivery.
Orally administered NMN and NR were substantially metabolized before reaching peripheral tissues intact in the mouse model.
What it cannot establish
In-vivo precursor-routing experiments were in mice.
Tracer conditions may not reproduce chronic supplement use.
Tissue-specific metabolism may differ between mice and humans.
The study addresses metabolic fate rather than clinical benefits.
HealthspanX claim boundary: Mouse tracer results do not establish the exact pharmacokinetics, tissue delivery, or clinical efficacy of orally administered NMN in humans.
