NAD+ Salvage Pathway
What it is
The NAD+ salvage pathway is the main mammalian recycling route that replenishes NAD+ from vitamin B3-related precursors and NAD+-consumption byproducts.
NAD+ is continuously consumed by:
- sirtuins
- PARPs involved in DNA repair
- CD38/CD157 and related NADases
The salvage pathway matters because NAD+ availability affects redox biology, mitochondrial signaling, DNA repair capacity, and cellular stress responses. In the Vitals vault, it is the reusable mechanism behind NMN NAD+, NAD Precursor Stacking, and NAD+/NADH-related interpretation.
Core pathway logic
Simplified routes:
| Input | Route | Practical implication |
|---|---|---|
| Nicotinamide (NAM) | NAM → NAMPT → NMN → NAD+ | NAMPT is the classic rate-limiting enzyme. |
| Nicotinic acid / niacin (NA) | NA → Preiss-Handler pathway → NAD+ | Can raise NAD+ but flushing limits tolerability. |
| Nicotinamide riboside (NR) | NR → NRK1/NRK2 → NMN → NAD+ | NR enters upstream of NMN via nucleoside transport. |
| Nicotinamide mononucleotide (NMN) | NMN may be converted extracellularly to NR, or transformed by gut microbiota, then contributes to NAD+ synthesis | Human direct NMN transport remains unresolved. |
Why it matters for NAD precursor stacking
NMN and NR are often marketed as if they use completely separate routes. The practical evidence is more cautious:
- They do not appear to compete at the same intestinal transporter level in a simple way.
- They converge downstream around NRK-dependent NMN generation and NMNAT-dependent NAD+ synthesis.
- Oral NMN and NR are substantially transformed by the gut microbiome before absorption.
- Blood NAD+ elevation does not prove tissue NAD+ delivery.
This is why NAD Precursor Stacking treats NMN+NR co-administration as unproven rather than automatically additive.
Evidence boundary
Source-backed:
- NMN and NR can each raise whole-blood NAD+ in humans.
- NRK1 deficiency studies support the importance of NRK-dependent conversion for oral NMN/NR effects.
- Gut microbiome transformation can account for a large share of oral precursor handling.
Still uncertain:
- Whether SLC12A8 meaningfully transports NMN in humans.
- Whether blood NAD+ elevation predicts brain, liver, or skeletal-muscle NAD+ elevation.
- Whether increasing precursor dose or combining precursors improves functional outcomes.
Vitals relevance
For Vitals, the salvage pathway is useful as a mechanistic explanation, not a wearable-detectable signal by itself.
- It explains why blood NAD+ labs may change before any subjective or wearable signal appears.
- It explains why HRV/RHR changes should not be over-attributed to NAD+ supplementation.
- It explains why exercise can be more relevant than oral precursors for skeletal muscle.
- It helps separate NAD+ precursor exposure from clinical outcomes.
Related notes
- NAD Precursor Stacking — practical NMN+NR stack evidence boundary
- NMN NAD+ — broader NMN/NR monograph
- NADH NAD+ disruption — alcohol-related redox imbalance
- Mitophagy — mitochondrial quality-control context
- PCC1 — senolytic context that may affect CD38-driven NAD+ consumption