Editor, Science of NAD
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Does NMN mostly deliver NR to cells?
Yes. The science is very clear that NMN works by turning into NR outside cells...
Here are how scientists describe this problem:
NMN dephosphorylation to NR constitutes a critical step in order to act as an exogenous NAD+ precursor ...These data...provide compelling evidence for the extracellular conversion of NMN to NR...
--Nature Communications, October 11, 2016
We also showed that extracellular cleavage of NAD+ and NMN to NR is a prerequisite for using these nucleotides to maintain intracellular NAD contents….The degradation of NAD+ and NMN to NR or Nam is essential for these nucleotides to act as extracellular precursors of intracellular NAD.
-- Metabolites, November 29, 2019
These observations strongly support the conclusion that both NAD+ and NMN need to be degraded to NR outside the cell to serve as precursors of intracellular NAD+
-- Journal of Biological Chemistry, June 17, 2011
A study using stable isotope-labeled NR and NMN revealed that NMN is dephosphorylated into NR extracellularly. These results suggest that NMN is incorporated into cells after extracellular conversion to NR.
-- Journal of Biomedical Science, May 11, 2019
But all hope is not loss for NMN. It still works, because there are enzymes circulating in your body that break NMN into smaller pieces so that it can enter cells. Those smaller pieces are NR and Nicotinamide (NAM).
So that's mostly why and how NMN works -- it is a reliable way of delivering NR and/or NAM to your cells.