The NAMPT enzyme is downregulated with aging so the salvage pathway won´t be able to convert nicotinamide to NAD as efficiently. Bypassing NAMPT for NAD+ synthesis with precursors NMN or NR is an strategy to restore cellular NAD+.
"Since Nam is a substrate for the NAMPT enzyme its impact on the NAD + synthesis is limited by NAMPT activity. Therefore, administration of Nam under conditions where the NAMPT enzyme is inhibited or dysfunctional will not lead to increased NAD + synthesis by the salvage pathway.
To even more efficiently facilitate NAD + synthesis, by bypassing the rate-limiting step in the salvage pathway, one can administer NMN that will directly feed into the one step enzymatic generation of NAD + via NMNAT. There are several reports showing that intraperitoneal administration of NMN can significantly increase brain tissue NAD + levels within 15 min post-injection [31] [79].
This suggests that there is an active transport of NMN or its metabolites into the intracellular compartments, where it is converted to NAD + . However, the mechanisms of NMN transport across the BBB or cellular and mitochondrial membranes need to be determined."
AND
"NMN and NR demonstrated superior pharmacokinetics when compared to Nam, primarily due to involvement of active transport mechanisms that allow for better control of the translocation process of these compounds from plasma into intracellular compartments. As a result, there is more effective intracellular accumulation of NMN and its conversion to NAD + ."
Edited by Fredrik, 26 December 2019 - 11:35 AM.