The NAD+ Salvage Pathway
What the salvage pathway is
The NAD+ salvage pathway is the route mammalian cells use to rebuild NAD+ from nicotinamide (NAM), the byproduct left over every time an NAD+-consuming enzyme — a sirtuin, a PARP, CD38 — uses up a molecule of NAD+.[1] In simplified form, the pathway runs: NAD+ is consumed, releasing nicotinamide → the enzyme NAMPT converts nicotinamide into NMN → the enzyme NMNAT converts NMN into NAD+ again.[1] This is textbook, well-established cell biology, not a hypothesis under debate — the two-step NAM→NMN→NAD+ conversion sequence has been characterized in detail at the enzyme level.[1] What is genuinely a separate, open question is what happens when a person takes NMN as an oral supplement — see How NMN Is Absorbed and Metabolized and NMN Transport and Cellular Uptake for that distinct topic.
Why salvage, specifically, matters
Mammalian cells can technically build NAD+ two other ways — from scratch, or from a related B3 vitamin — but the salvage pathway is the dominant route in most human tissues, because it recycles nicotinamide the body already has rather than requiring new raw material.[1] Because NAD+-consuming enzymes are constantly active, cells depend on continuously salvaging nicotinamide back into NAD+ just to keep pace — a cell that could not salvage NAD+ would deplete its supply relatively quickly given how much is used in normal metabolism and signaling.[1] This is why researchers have focused on salvage-pathway enzymes, particularly NAMPT, as a possible point of intervention — not because supplementing at that point has been shown to produce a specific human health outcome, but because the pathway's central role in NAD+ maintenance is well established biochemistry.
How salvage differs from the other two NAD+ routes
Mammalian cells have three distinct ways to build NAD+, and it's easy to blur them together:[1]
- Salvage pathway: recycles nicotinamide (NAM) — the pathway this page covers.
- De novo synthesis: builds NAD+ from the amino acid tryptophan, through a longer multi-step route ending in quinolinic acid.
- Preiss–Handler pathway: builds NAD+ from nicotinic acid (a different form of vitamin B3, also called niacin), via a separate enzyme (NAPRT) and intermediate (NAMN).
NR (nicotinamide riboside) feeds into NAD+ production by a related but distinct route: it is converted into NMN by a different enzyme (NRK), which then joins the same final NMNAT step as salvage-pathway NMN.[1] See What Are NAD+ Precursors? for how NMN, NR, and nicotinic acid compare as supplement ingredients specifically.
What this page is not claiming
None of the above establishes that taking NMN as a supplement measurably changes how efficiently a person's cells run this pathway, or that doing so produces any specific health benefit. The salvage pathway's importance to basic cell survival is settled biochemistry; whether boosting one of its intermediates from the outside changes human physiology in a beneficial way is a separate question, covered with actual human trial data on NMN Dosage and NMN Safety and Side Effects.
Related reading
For the two enzymes that carry out this pathway's two steps, see NAMPT and NAD+ Biosynthesis and NMNAT Enzymes Explained. For what actually consumes the NAD+ this pathway rebuilds, see NAD+ and Sirtuins, NAD+ and PARPs, and NAD+ and CD38.
- The salvage pathway rebuilds NAD+ from nicotinamide in two enzyme-catalyzed steps: NAMPT (nicotinamide → NMN), then NMNAT (NMN → NAD+).
- It is the dominant NAD+-producing route in most mammalian tissues, distinct from de novo synthesis (from tryptophan) and the Preiss–Handler pathway (from nicotinic acid).
- NR joins the same pathway one step later than NMN, via a different entry enzyme (NRK).
- This is established cell biology, not a claim that NMN supplementation has been shown to produce a specific human health benefit.
- Xie N, Zhang L, Gao W, Huang C, Huber PE, Zhou X, Li C, Shen G, Zou B. NAD+ metabolism: pathophysiologic mechanisms and therapeutic potential. Signal Transduction and Targeted Therapy. 2020. doi:10.1038/s41392-020-00311-7. PMID: 33028824.Comprehensive peer-reviewed review of NAD+ biosynthesis pathways, consuming enzymes, and disease relevance; used as the primary synthesizing source for this page's foundational biochemistry.