NAD+ falls with age. The useful question is why — because the answer determines whether adding more precursor is a sensible response or a leaky-bucket strategy.
The short version: it is mostly not a manufacturing problem. Your cells do not largely lose the ability to make NAD+. What changes is that more of it is being destroyed, by enzymes that become more active with age, while the recycling system that rebuilds it gets somewhat less efficient at the same time.
Production versus consumption
If declining NAD+ were purely a supply problem, supplying more precursor would be an obvious fix. Because it is substantially a demand problem, precursors help by keeping up with a faster drain — which is a real benefit, but a different one from the "refilling the tank" image most marketing uses.
Where NAD+ comes from
Cells have more than one route to NAD+, but in most tissues most of the time the one that matters is the salvage pathway — recycling nicotinamide, the fragment left behind when an enzyme consumes NAD+, back into fresh NAD+.
The rate-limiting step in that recycling is an enzyme called NAMPT. How much NAD+ your cells can rebuild per hour is largely a question of how much NAMPT activity they have. And NAMPT activity tends to fall with age, which is part of the picture.
Where it goes
Three families of enzymes consume NAD+ rather than merely borrowing it.
PARPs — DNA repair
When DNA is damaged, PARP enzymes activate and consume NAD+ as they work. Damage accumulates over a lifetime, so PARP activity trends upward. This is NAD+ being spent on something genuinely necessary.
Sirtuins — stress and metabolic regulation
Sirtuins also consume NAD+. They are the enzymes most often invoked in longevity marketing, and importantly their activity depends on NAD+ availability — which is the mechanistic basis for the whole precursor field. Low NAD+ means constrained sirtuin function.
CD38 — the one that changes most
CD38 is the drain that matters most for this story. It is an NAD+-consuming enzyme associated with immune cells, and its expression rises substantially with age. The increase appears to be driven by chronic low-grade inflammation — the kind that accumulates as senescent cells build up in tissue and release inflammatory signals.
So the picture assembles into a loop: senescent cells accumulate → inflammation rises → CD38 expression increases → NAD+ is consumed faster → the NAD+-dependent machinery that helps manage cellular stress has less to work with.
What that means for a supplement
| If decline were... | Then the fix would be... | Reality |
|---|---|---|
| Purely reduced production | Add precursor; levels restore | Only part of the story |
| Purely increased consumption | Reduce the drain | Also only part |
| Both at once | Precursors help keep pace; they do not stop the drain | This one |
That framing explains something people find confusing: why precursor supplementation reliably raises measured NAD+ markers, yet the downstream benefits are harder to demonstrate. Raising the input into a system with an elevated drain is not the same as restoring the system to its younger state.
It is still a reasonable thing to do. Keeping the substrate available for repair and signalling enzymes is a coherent goal with a well-described mechanism. It is just not the same claim as reversing the underlying process, and you should notice when a product page quietly swaps one for the other.
What is genuinely unsettled
- How much of the measured tissue decline translates into functional consequences in otherwise healthy adults.
- Whether raising NAD+ in blood reflects what happens in the tissues you care about.
- Whether targeting the drain — CD38 in particular — would outperform adding substrate. This is an active research direction rather than a shelf product.
Related: NAD vs NMN · NMN vs NR · When to take your dose