Your cells spend NAD+ to run DNA repair and sirtuin reactions, and they remake it from scratch every day. At Longevity Benchmark, we separate what a supplement measurably does from what it is sold for. With NAD+, those two have drifted a long way apart. The biology is well established and roughly a century old. The claim that swallowing a precursor changes how you age is neither.
Nicotinamide adenine dinucleotide (NAD+) has two jobs, and they are worth keeping separate. Only the second explains why the pool can shrink, and why anyone sells a precursor for it.
The Verdict
NAD+ Has Two Separate Jobs in a Cell
The first job recycles the molecule and the second consumes it, which is the whole reason cells need a constant supply. As a redox carrier, NAD+ picks up electrons to become NADH, its reduced form, and drops them off to become NAD+ again, over and over, in glycolysis and in the mitochondria. Nothing is used up in that loop. A cell running purely on redox chemistry would never need to make more.
The second job breaks the molecule. Three families of enzymes consume NAD+ as a substrate.
- Sirtuins, which sit at the center of most longevity claims.
- Poly-ADP ribose polymerase enzymes (PARPs), which activate in response to DNA damage.
- CD38, a cell-surface enzyme whose activity rises with age.
Every reaction they run costs the cell one NAD+ molecule. Synthesis has to keep pace with that consumption, and the phrase "NAD+ levels" means that balance.
Why the Pool Shrinks with Age
Both sides of the balance move in the wrong direction as people get older. CD38 activity rises, accumulated DNA damage keeps PARPs busier, and the salvage pathway that recycles nicotinamide back into NAD+ becomes less efficient. More is spent and less is remade.
One caveat belongs with that description. Most of the evidence for the decline comes from tissue samples and animal models rather than from following the same people for decades. The direction of the change is better established than its size in any particular human tissue. That gap matters when a product promises to restore a number, because the baseline it claims to restore has not been mapped as precisely as the marketing implies. Our guide to biological age covers the same measurement problem in a wider setting.
Five Routes Into the Same Pool
Cells build NAD+ through three pathways, and five different molecules feed them. The de novo pathway starts from the amino acid tryptophan in food. The Preiss-Handler pathway starts from nicotinic acid. The salvage pathway recycles nicotinamide and is where nicotinamide riboside and nicotinamide mononucleotide enter. All five arrive at the same destination.
| Precursor | Pathway | What it has going for it | The catch | Rough daily cost |
|---|---|---|---|---|
| Nicotinic acid (niacin, vitamin B3) | Preiss-Handler pathway | Decades of use, an established Recommended Dietary Allowance, and the best-documented safety record of the five | Causes flushing at the doses used for lipids | A few cents per day |
| Nicotinamide (niacinamide) | Salvage pathway | Long-established vitamin form, no flushing | Inhibits sirtuins at high concentrations, which cuts against the reason people take it | A few cents per day |
| Nicotinamide riboside (NR) | Salvage pathway via NRK enzymes | The most human pharmacokinetic data of the newer precursors | Raising blood NAD+ has not yet translated into a clinical outcome in healthy adults | Roughly $1 to $3 per day |
| Nicotinamide mononucleotide (NMN) | Salvage pathway | Widely sold again after the FDA reversed its 2022 position in 2025 | Less human outcome data than NR, and four years of regulatory uncertainty behind it | Roughly $1 to $3 per day |
| Tryptophan (dietary protein) | De novo synthesis | Comes with ordinary food, no supplement required | An inefficient route, roughly 60 mg of tryptophan per 1 mg of niacin equivalent | Included in the food you already eat |
The cost column changes the decision more than the pathway column does. Nicotinic acid and nicotinamide have been sold as vitamin B3 for decades at a few cents a day, and they reach the same pool. The commercial case for the newer precursors rests on the claim that they do something the older ones do not, and that claim has not been tested head to head on a health outcome. Our comparison of NMN and NR covers the choice between the two newer forms.
What the Human Trials Reported
Two trials in Nature Communications carry most of the weight in this field, and both studied nicotinamide riboside rather than NMN. Read together they show a precursor that reaches the blood NAD+ pool reliably and a set of health outcomes that remains open.
| Trial | What was tested | What it found | What it does not establish |
|---|---|---|---|
| Trammell et al. 2016, Nature Communications | Single oral doses of 100, 300 and 1,000 mg of NR | Dose-dependent rises in the blood NAD+ metabolome. A single dose raised blood NAD+ up to 2.7-fold in a one-person pilot | A pharmacokinetic study. It established that oral NR reaches the blood NAD+ pool, and was not designed to measure health outcomes |
| Martens et al. 2018, Nature Communications | 1,000 mg daily (500 mg twice daily) for 6 weeks, 24 healthy adults aged 55 to 79, crossover design | Whole-blood NAD+ rose about 60%. No serious adverse effects. Systolic blood pressure was about 10 points lower in the 13 participants who began with elevated pressure or stage 1 hypertension | The blood pressure result came from a subgroup of 13 and was not the primary endpoint, so it points at a hypothesis rather than settling one |
The blood pressure result from the 2018 trial is regularly quoted without its scope. It came from 13 people inside a 24-person study, and it was not the primary endpoint. A subgroup finding of that size generates a hypothesis for a larger trial. Nobody has run that larger trial. Supplement labels cite the subgroup anyway.
The NMN Regulatory Reversal
NMN is legal to sell as a dietary supplement in the United States again, after four years in which its status was unsettled. In 2022 the FDA determined that NMN fell outside the dietary supplement definition under the drug preclusion clause, because it had been authorized for investigation as a new drug before its supplement marketing was recognized. The agency reversed itself in September 2025. It concluded NMN had been marketed as a supplement first.
The practical consequence for a reader is narrow and worth stating plainly. The reversal restored a sales channel and produced no new evidence about whether NMN does anything. A product that was unavailable in 2023 and is on the shelf in 2026 has exactly the same clinical file behind it.
Who Should Skip NAD+ Supplements
Three groups get little from this category.
- Anyone whose diet already supplies adequate niacin, which is most people eating enough protein, is topping up a vitamin they are not short of.
- Anyone treating a diagnosed condition should be talking to the clinician managing it rather than substituting a supplement. That goes double for people on lipid-lowering medication, because the NIH Office of Dietary Supplements niacin fact sheet lists interactions at the higher nicotinic acid intakes.
- Anyone whose budget for their health runs to $1 to $3 a day would get more from that money spent on cardiorespiratory fitness or sleep, both of which have outcome data this category does not.
What Would Change Our Answer
A randomized trial in healthy adults reporting a health outcome rather than a blood marker would change this page. So would tissue-level NAD+ measurement in humans showing that oral precursors reach muscle or brain, because that would close the gap between the blood result and the claim. A head-to-head trial of nicotinic acid against nicotinamide riboside would settle whether the price difference buys anything, and it is the single most useful trial nobody has run.
If you want to follow this field, read the primary trial rather than the label, starting with the 2016 pharmacokinetic study that established oral NR reaches the blood NAD+ pool. Then check which of the two jobs the claim on the label depends on.
Frequently Asked Questions
What is NAD+?
NAD+ is nicotinamide adenine dinucleotide, a coenzyme present in every cell in your body. It does two separate jobs. As a redox carrier it shuttles electrons between NAD+ and its reduced form NADH in the reactions that turn food into usable energy, and in that role it is recycled rather than used up. As a substrate it is consumed by three families of enzymes: sirtuins, poly-ADP ribose polymerase enzymes (PARPs) which respond to DNA damage, and the cell-surface enzyme CD38. The second job is the reason cells must keep making more of it, and the reason the size of the pool can fall.
Why do NAD+ levels fall with age?
The pool reflects the balance between what a cell makes and what its enzymes consume, and both sides shift with age. Consumption by CD38 and by PARPs responding to accumulated DNA damage rises, while synthesis through the salvage pathway becomes less efficient. Most of the measurements behind the decline come from tissue and animal work rather than repeated whole-body measurement in the same people over decades. That limits how confidently anyone can describe the decline. The direction is consistent across the studies. The size of the decline in any given human tissue is less settled.
Does taking NAD+ itself work?
Swallowing NAD+ as a molecule is the least supported of the options, because it is large and unstable in the gut and is broken down into smaller pieces before absorption. That is why supplement companies sell precursors such as nicotinamide riboside and nicotinamide mononucleotide rather than NAD+ itself. Intravenous NAD+ bypasses the gut, but the infusions are expensive, the published human evidence is thin, and the reported infusion-site and flushing reactions are common enough to matter.
Is NMN legal to sell in the United States?
Yes, as of September 2025. The FDA had determined in 2022 that NMN was excluded from the dietary supplement definition under the drug preclusion clause, because it had been authorized for investigation as a new drug. In September 2025 the agency reversed that reading, concluding there was enough evidence NMN had been marketed as a supplement before the drug investigation began. Anything you read about NMN being banned dates from the 2022 to 2025 window. The FDA dietary supplement pages carry the agency's current position.
How much did NAD+ rise in the human trials?
In the 2018 crossover trial by Martens and colleagues, 1,000 mg of nicotinamide riboside daily for six weeks raised whole-blood NAD+ by about 60% in 24 healthy adults aged 55 to 79. In the 2016 pharmacokinetic study by Trammell and colleagues, a single oral dose raised blood NAD+ up to 2.7-fold in a one-person pilot. Both numbers describe blood. Neither tells you what happened in muscle, brain or liver, and every aging claim depends on the tissues rather than the blood.
Is niacin a cheaper way to raise NAD+?
Nicotinic acid feeds the same destination through the Preiss-Handler pathway and costs a few cents a day against roughly $1 to $3 for NR or NMN. It also has an established Recommended Dietary Allowance and decades of safety data, which no newer precursor can match. The trade-off is flushing, which is why the newer forms exist commercially. Whether the newer precursors do something niacin does not is the open question, and no trial has yet compared them head to head on a health outcome in healthy adults.
Do NAD+ precursors extend lifespan?
No human trial has tested that, and none is close to reporting. The lifespan results come from mice, and the human trials published so far measure blood NAD+ over six weeks to twelve weeks in groups of a few dozen people. A supplement that reliably raises a blood marker and has not been shown to change a health outcome is a reasonable thing to be curious about and a poor thing to be certain about. Anyone selling certainty here is ahead of the evidence.
Related
- NAD+ injections and IV infusions — what the human evidence actually shows
- NAD+ patches — whether transdermal delivery is plausible
- NMN vs NR: choosing between the two newer precursors
- NMN dosage: what the trials used
- NMN side effects: what has been reported
- Senolytics: another longevity category held to the same evidence bar
- Rapamycin for longevity: where the human data stands
- Biological age: the measurement problem behind these claims
- Methodology: how sources are verified