A 2025 rodent study investigated dynamics of organ and circulating nicotinamide:
“Liver-derived circulating nicotinamide from nicotinamide adenine dinucleotide (NAD+) catabolism primarily feeds systemic organs for NAD+ synthesis. We surprisingly found that, despite blunted hepatic NAD+ and nicotinamide production in liver-specific nicotinamide nucleotide adenylyltransferase 1 (NMNAT1) deletion mice (liver-specific knockout [LKO]), circulating nicotinamide and extra-hepatic organs’ NAD+ are unaffected.
Metabolomics reveals a massive accumulation of a novel molecule in the LKO liver, which we identify as nicotinic acid riboside (NaR). The liver releases NaR to the bloodstream, and kidneys take up NaR to synthesize NAD+ through nicotinamide riboside kinase 1 (NRK1) and replenish circulating nicotinamide.
Serum NaR levels decline with aging, whereas oral NaR supplementation in aged mice boosts serum nicotinamide and multi-organ NAD+, including kidneys, and reduces kidney inflammation and albuminuria. The liver-kidney axis maintains systemic NAD+ homeostasis via circulating NaR, and NaR supplement ameliorates aging-associated NAD+ decline and kidney dysfunction.
While this study provides evidence of hepatic production and renal consumption of NaR for NAD+ homeostasis in mice, future human works are warranted to confirm these findings. In addition, genetic studies will be necessary to fully understand NaR metabolism at cellular and organismal levels.
While this study shows the oral availability of NaR and its effect on systemic NAD+ metabolism in mice, human studies testing NaR safety, oral availability, pharmacokinetics, and pharmacodynamics should be performed to test potential clinical usage of NaR supplements. Additionally, future studies are needed to investigate physiological significance of NT5C2-mediated hepatic production of NaR in healthy mice and identify NaR transporter(s).”
https://www.cell.com/cell-metabolism/abstract/S1550-4131(25)00217-7 “Nicotinic acid riboside maintains NAD+ homeostasis and ameliorates aging-associated NAD+ decline” (not freely available) Thanks to Dr. Dorota Skowronska-Krawczyk for providing a copy.
An elaborating commentary was published along with this study:
“Nicotinamide (NAM), nicotinamide riboside (NR), nicotinic acid (NA), and NAR are the salvageable precursors that feed into production of nicotinamide mononucleotide (NMN) and nicotinic acid mononucleotide (NAMN) to regenerate NAD coenzymes. NAMN is at an interesting juncture in NAD metabolism because it is formed in de novo synthesis and in salvage synthesis from both NA and NAR.
Song and coworkers did not specifically set out to determine endogenous sources of NR and/or NAR. Rather, they wanted to see what would happen when they deleted the major Nmnat isozyme, Nmnat1, in liver.
With depression of hepatic NAD+, they saw elevation of liver NMN and NAMN and discovered a huge increase in hepatic and circulating NAR. By viral knockdown, the step of conversion of accumulated NAMN to NAR was found to be catalyzed by a 5′ – nucleotidase encoded by the Nt5c2 gene, and the major tissue receiving the NAR was found to be the kidney.
Further, they showed that levels of NAR decline in aging while provision of supplementary NAR supports a newfound ability of the mouse kidney to circulate NAM. Of potential translational significance, supplementary NAR also supported mouse kidney function in aging.”
https://www.brennerlab.net/curriculumvitae/ “The NARly side of whole-body NAD homeostasis” (*pdf at page bottom)

























