NAD⁺ does two completely different jobs in your body, and almost every confusing thing about it comes from mixing them up.
In the first job it is a shuttle. It picks up electrons, drops them off, and is immediately reused — millions of times a second, never consumed. In the second job it is a raw material. Certain enzymes take a molecule of NAD⁺, break it apart, and throw the pieces away. That job destroys it.
Your cells have to keep rebuilding the supply because of the second job. Everything interesting about NAD⁺ and aging happens in the gap between how fast it is destroyed and how fast it is rebuilt.
Job one: moving electrons
Getting energy out of food means stripping electrons off it and running them through a chain of reactions that ends in your mitochondria. Those electrons need a carrier. NAD⁺ is the main one: it accepts a pair, which turns it into NADH, delivers them, and reverts to NAD⁺ ready for the next pass.
This is not a peripheral function. NAD⁺ and NADH are described as essential coupled redox metabolites that drive the oxidative reactions cells depend on, enabling energy generation through glycolysis and mitochondrial respiration [1]. Glycolysis stalls without free NAD⁺ to accept electrons. So does the citric acid cycle. The ratio of NAD⁺ to NADH is one of the numbers a cell watches to know whether it is in feast or famine.
Because this job recycles the molecule, it does not deplete anything. A cell running hard uses NAD⁺ constantly and ends up with the same amount it started with.
Job two: being eaten
A separate set of enzymes uses NAD⁺ as a co-substrate, meaning they consume it. They cleave the molecule, use one fragment to do chemistry on a protein or a strand of DNA, and release nicotinamide as waste. Four families do this: PARPs, sirtuins, CD38 and SARM1[2].
PARPs are DNA repair enzymes. When a strand breaks, PARP arrives and tags the site by attaching chains of ADP-ribose — built from NAD⁺ — which recruits the repair machinery. In genomic DNA, NAD⁺ is the sole substrate for this [3]. A lot of DNA damage means a lot of PARP activity, and a lot of PARP activity means a lot of NAD⁺ spent.
Sirtuins strip acetyl groups off proteins, including the histones that DNA is wound around, which changes which genes are accessible. They cannot work without NAD⁺, and each reaction costs one molecule. This is the connection people mean when they link NAD⁺ to gene regulation and to the claims about slowing aging.
CD38 and SARM1 are the other two consumers. The 2021 review of this system notes that the falling tissue NAD⁺ seen in aging is ascribed to an imbalance between making and spending it — less of the rate-limiting building enzyme NAMPT, alongside an increased activation state of the consumers PARP and CD38[2].
Between them these enzymes regulate DNA repair, immune cell function, senescence and chromatin remodeling — all processes whose maintenance matters for healthy aging [2]. And every one of them is paid for in NAD⁺.
The rebuild: the salvage pathway
The nicotinamide those enzymes throw away is not lost. An enzyme called NAMPT picks it up and converts it back toward NAD⁺ through what is called the salvage pathway — recycling the wreckage rather than waiting for fresh vitamin B3 to arrive from your diet.
NAMPT is the rate-limiting enzyme in that route [2], which makes it the bottleneck in the whole system. Oral precursors — nicotinamide riboside and nicotinamide mononucleotide — work by entering this pathway at a point downstream of that bottleneck. That is the mechanistic case for supplementing, and it is a good one as mechanisms go.
What happens to all this with age
Tissue NAD⁺ falls. That has been measured directly in people, in different organs and by completely unrelated methods.
Researchers analyzed pelvic skin samples from 49 people ranging from newborns to age 77 (n = 49) and found NAD⁺ falling steadily with age: in males p = 0.001, r = −0.706; in females p = 0.01, r = −0.537. DNA damage rose with age in both sexes. In males, PARP activity rose steeply (p < 0.0001, r = 0.768) and tracked inversely with tissue NAD⁺ (p = 0.0003, r = −0.639), while SIRT1 activity fell (p = 0.007) [3]. The authors read that as evidence for a specific mechanism: accumulated oxidative DNA damage drives PARP hyperactivation, which eats the NAD⁺ pool.
Separately, a magnetic resonance method was developed to measure NAD⁺ and NADH non-invasively inside the living human brain. It found an age-dependent increase in NADH alongside age-dependent reductions in NAD⁺, in total NAD, and in the NAD⁺/NADH redox potential of healthy brains [4]. Not just less of it — a shifted balance, which is what you would expect if mitochondrial capacity is declining.
So what is NAD⁺ used for, in one paragraph
Extracting energy from food; repairing broken DNA; switching genes on and off through the sirtuins; and supporting immune cell function and the clearance of senescent cells. The first of those recycles it. The rest consume it, and the supply has to be continuously rebuilt from the wreckage.
The part where this stops telling you anything useful
Everything above is settled biochemistry, and none of it establishes that taking more NAD⁺ will do anything for you. Those are two different questions and the category depends on you not noticing the seam.
Knowing that PARP consumes NAD⁺ to repair DNA does not tell you that raising blood NAD⁺ repairs more DNA. Knowing that sirtuins need NAD⁺ does not tell you that a bigger pool makes them work harder, or that working harder would be good. Mechanism tells you where to look; it does not tell you what you will find. In this category the mechanism is unusually strong and the human outcome data is unusually thin, which is exactly the gap the marketing lives in.
If you want the molecule described from the top — what NAD⁺ is, and what supplementing it has actually been shown to do — start there. If you have already decided to try it, the oral precursors are the format with the randomized trials and the lowest price, and our format finder lays out what each route does and does not have behind it.