Every NAD+ Trial Hits the Biomarker. Only Two of Them Hit the Endpoint.
NAD+ precursors have a rare quality among supplements, which is that they demonstrably do the thing they claim to do. Take a gram of nicotinamide riboside daily for six weeks and the NAD+ in your blood rises by roughly 60 percent. Take 300 to 900 milligrams of nicotinamide mononucleotide for 60 days and blood NAD+ climbs at every dose tested. There is no ambiguity about this and no serious dispute in the literature. The compounds are absorbed, they enter the salvage pathway, and the number on the assay goes up. What happened next is the part that gets left out of the marketing. When researchers in Dublin biopsied the thigh muscle of men with a median age of 75 who had been taking a gram of NR daily for three weeks, the NAD+ inside that muscle had gone from 197 to 210 picomoles per milligram, a difference that missed statistical significance at p equals 0.22, while the waste products of NAD+ breakdown in the same tissue had risen fivefold. The body had received the raw material and was clearing it, not stockpiling it. Since then a run of randomized trials has tested whether these compounds improve insulin sensitivity, muscle mass, grip strength, walking speed or arterial stiffness in the people most likely to buy them, and a 2025 meta-analysis pooled ten of those trials in adults over 60 and found nothing that reached significance. Two trials did hit their endpoints, and both are worth taking seriously. This article walks through what each trial measured, why the blood-versus-tissue distinction is the whole argument, what changed at the FDA in September 2025, and where the evidence genuinely runs out. It is educational background on published research, not medical advice, a protocol, or a recommendation to take or avoid anything.
Key Takeaways: What the NAD+ Precursor Trials Actually Show
Nicotinamide riboside and nicotinamide mononucleotide reliably raise NAD+ measured in blood, with 1,000 mg of NR daily for six weeks raising blood NAD+ by about 60 percent in 24 healthy adults aged 55 to 79. In aged human skeletal muscle, 1,000 mg of NR daily for 21 days did not significantly raise NAD+ itself, which moved from 197 to 210 pmol/mg at p equals 0.22, while the metabolite NAAD roughly doubled and methylnicotinamide rose around fivefold. A 2025 systematic review and meta-analysis of 10 randomized trials in adults with a mean age above 60 found no significant effect of NMN or NR on skeletal muscle index, handgrip strength on either side, gait speed, or five-time chair stand performance. A 12-week trial of 2,000 mg of NR daily in 40 middle-aged obese insulin-resistant men found no improvement in insulin sensitivity, mitochondrial function or body composition. A 2021 trial in Science reported that 250 mg of NMN daily for 10 weeks increased muscle insulin sensitivity in 25 prediabetic postmenopausal women, a result in women that has not been reproduced in men. A 2023 trial in GeroScience found that 600 and 900 mg of NMN daily for 60 days significantly increased six-minute walking distance and self-rated health, while 300 mg did not. Human trials of these compounds run three to twelve weeks and enroll 12 to 80 people, so nothing in the published record speaks to years of use. The FDA reversed its 2022 position excluding NMN from the dietary supplement definition in letters issued on 29 September 2025, concluding that NMN was marketed as a supplement before any investigational new drug authorization. No trial of any NAD+ precursor has used lifespan, disability or disease incidence as an endpoint.
What Is NAD+ and Why Does Anyone Take a Precursor for It?
NAD+ is a coenzyme that every cell in the body uses to move electrons around, and it is required for the chemical steps that turn food into usable energy. That much is textbook biochemistry from the 1930s. The reason it became a longevity target is a second job it picked up in the research literature: NAD+ is consumed, not just recycled, by three families of enzymes that matter for aging. Sirtuins use it to strip acetyl groups off proteins and are the enzymes the resveratrol story was built on. PARPs use it to repair damaged DNA, and they burn through it fast when damage accumulates. CD38, an enzyme on immune cells, degrades NAD+ directly and becomes more abundant as tissue inflammation rises with age. Every one of those enzymes is a drain. The body refills the tank mainly through the salvage pathway, recycling nicotinamide back into NAD+ using an enzyme called NAMPT, which is the rate-limiting step. Nicotinamide riboside and nicotinamide mononucleotide are both intermediates that enter this pathway downstream of the bottleneck, which is the entire pharmacological argument for taking them. NR is one phosphate group simpler than NMN. There is good evidence that orally administered NMN is dephosphorylated to NR before it crosses into cells, which means the two compounds may largely converge on the same molecule before anything interesting happens. That detail matters when people argue about which is superior.
Does NAD+ Actually Decline With Age?
The decline is real in specific tissues and specific measures, and it is less uniform than the marketing suggests. The strongest mechanistic case runs through CD38, the NAD+-consuming enzyme on immune cells. Work published in Cell Metabolism in 2016 found CD38 protein and messenger RNA rose two to threefold with chronological age across every tissue tested in mice, and that deleting CD38 protected tissue NAD+ levels and mitochondrial function. On the supply side, NAMPT abundance in human skeletal muscle correlates negatively with age, meaning older muscle has less of the enzyme that performs the rate-limiting salvage step. Human adipose tissue from donors averaging 61 years old shows higher CD38 and PARP1 expression than tissue from donors averaging 34. So the shape of the story holds up: more consumption, less recycling capacity, lower net NAD+. Two things complicate it. NAD+ is difficult to measure accurately because it degrades quickly after sampling, which means published decline figures vary widely depending on the assay and the tissue, and whole blood NAD+ does not track tissue NAD+ closely. And the decline is not fixed. Aerobic and resistance training reverses the age-related fall in NAD+ salvage capacity in human skeletal muscle, which is an inconvenient finding for anyone framing a capsule as the only available lever. A declining input does not by itself establish that topping it up produces a benefit, and that gap is where most of the disagreement in this field lives.
Do NMN and NR Raise NAD+ in People?
Yes, consistently, and this is the least disputed claim in the entire field. Christopher Martens and colleagues published a crossover trial in Nature Communications in March 2018 in which 24 healthy adults aged 55 to 79 took 500 mg of nicotinamide riboside twice daily for six weeks. Blood NAD+ rose about 60 percent against placebo, with no serious adverse effects. The 2023 GeroScience trial from Abinopharm randomized healthy middle-aged adults to placebo or 300, 600 or 900 mg of NMN daily for 60 days and found significant increases in blood NAD at all three doses, with the placebo group flat. A 2024 analysis of individual responses found the size of the NAD increase varies substantially between people at the same dose, particularly at lower doses, which is why some trials report large average gains alongside participants who barely moved. None of this is in question. The problem is what the number means. Blood NAD+ is a convenient sample, not a meaningful destination. Nobody takes these compounds hoping to improve the NAD+ concentration of their plasma. The target is muscle, liver, brain and vascular tissue, and blood is a proxy for those that has never been validated. The gap between the proxy and the target is where this literature splits in two.
The One Number That Did Not Move
Percent change from baseline in each compartment. Blood NAD+ rose about 60 percent on 1,000 mg of nicotinamide riboside daily for six weeks in 24 healthy adults aged 55 to 79. In aged men given the same daily dose for 21 days, the downstream muscle metabolites moved hard, with NAAD roughly doubling and methylnicotinamide rising around fivefold, while muscle NAD+ itself went from 197 to 210 pmol/mg and did not reach significance at p equals 0.22. Green marks a significant change against control, grey marks one that was not.
Does Blood NAD+ Tell You Anything About Muscle NAD+?
Not reliably, and the trial that demonstrated this is the most important single result in the field. Elhassan and colleagues published a placebo-controlled crossover study in Cell Reports in 2019 in which 12 men with a median age of 75 took 1,000 mg of nicotinamide riboside daily for 21 days, with muscle biopsies before and after. Muscle NAD+ itself went from 197 pmol/mg on placebo to 210 pmol/mg on NR, a difference that did not reach significance at p equals 0.22. What did rise sharply were the markers of NAD+ turnover: NAAD roughly doubled, and three methylated breakdown products including methylnicotinamide rose around fivefold. That pattern has a clear reading. The muscle was receiving the compound and metabolizing it, then methylating and excreting the excess rather than expanding its NAD+ pool. Mitochondrial respiration in the biopsied muscle did not change. Citrate synthase activity did not change. Mitochondrial DNA copy number did not change. Handgrip strength was 32.5 kg on NR against 34.7 kg on placebo at p equals 0.96. The one clearly positive finding was that circulating inflammatory cytokines including IL-6 and TNF-alpha fell from baseline. Later trials reproduced the same shape. Remie and colleagues gave 1,000 mg of NR daily for six weeks to healthy overweight and obese adults, raised muscle NAD+ metabolites, shifted muscle acetylcarnitine handling, and found no improvement in insulin sensitivity or mitochondrial respiratory capacity.
Do NAD+ Precursors Improve Insulin Sensitivity?
In men, the randomized evidence says no. Dollerup and colleagues published a 12-week trial in the American Journal of Clinical Nutrition in 2018 in which 40 middle-aged obese insulin-resistant men took 2,000 mg of nicotinamide riboside daily, which is double the dose used in most other trials. Insulin sensitivity did not improve. Endogenous glucose production did not change. Mitochondrial function in muscle did not change. Body composition did not change. The compound was safe and well tolerated, and it did nothing measurable to the metabolic problem the men had been enrolled for. Remie's 2020 trial reached the same conclusion at 1,000 mg over six weeks in overweight and obese adults. The one clear positive result belongs to a different compound and a different population. Yoshino and colleagues published a trial in Science in 2021 in which 25 prediabetic postmenopausal women with overweight or obesity took 250 mg of NMN daily for 10 weeks, and muscle insulin sensitivity improved by roughly 25 percent alongside changes in muscle insulin signaling. That result is real and it was published in a serious journal. It is also a small trial in women, at a quarter of the dose that failed in men, using a different molecule, with no change in body weight, blood pressure, liver fat or circulating metabolic markers. Reading it as evidence that NAD+ precursors improve metabolic health in middle-aged men requires stepping across three separate gaps at once.
Every Well-Known Human Trial, Plotted by Dose and Length
Daily dose on the horizontal axis, trial duration in weeks on the vertical. Green points are nicotinamide riboside trials, amber points are nicotinamide mononucleotide trials. The entire cluster sits between 250 and 2,000 mg daily and between three and twelve weeks, with enrollment running from 12 to 80 people. Nothing here runs a year, and nothing here enrolls thousands. That is the size of the evidence base a compound sold for decades of aging currently rests on.
- Nicotinamide riboside
- Nicotinamide mononucleotide
Do NAD+ Precursors Preserve Muscle Mass or Strength After 60?
The pooled answer is no. A 2025 systematic review and meta-analysis gathered 10 randomized controlled trials of NMN or NR in adults with mean ages between 60.9 and 83, covering doses from 250 to 2,000 mg daily and durations from three to 24 weeks. Six of the trials used NMN and four used NR. Skeletal muscle index showed a mean difference of minus 0.42 with a 95 percent confidence interval running from minus 0.99 to 0.14, at p equals 0.14. Handgrip strength on the left came in at a mean difference of 0.61 kg with an interval from minus 0.89 to 2.10, at p equals 0.42, and the right hand at 0.45 kg with an interval from minus 1.06 to 1.96, at p equals 0.56. Gait speed produced a mean difference of minus 0.01 m/s at p equals 0.79. The five-time chair stand test produced minus 0.21 seconds at p equals 0.41. The authors concluded that current evidence does not support NMN or NR supplementation for preserving muscle mass and function in adults over 60. Every confidence interval in that list crosses zero comfortably, which means the pooled data cannot even rule out small harm in either direction. This matters more than any single null trial, because sarcopenia prevention is the specific use case most often attached to these compounds for men in their fifties and sixties.
Pooled Muscle Outcomes, Sorted by How Far They Missed
P-values for every muscle outcome pooled in the 2025 systematic review and meta-analysis of 10 randomized trials of nicotinamide mononucleotide and nicotinamide riboside in adults with a mean age above 60. The dashed line marks the conventional significance threshold of 0.05. No outcome came close to it. Skeletal muscle index landed at a mean difference of minus 0.42 with a confidence interval from minus 0.99 to 0.14, and handgrip strength, gait speed and the five-time chair stand all sat further out still.
What Happened in the Blood Pressure and Arterial Stiffness Trials?
The vascular story is the most promising unresolved thread, and it is still unresolved. The 2018 Martens crossover trial included a preliminary observation that has driven a decade of follow-up work: in the subgroup of participants who started with above-normal systolic blood pressure, six weeks of NR was associated with a reduction of roughly 8 to 9 mmHg, alongside an improvement in aortic stiffness measured as carotid-femoral pulse wave velocity. That was a subgroup finding in 24 people and the authors treated it as hypothesis-generating rather than conclusive. A properly powered phase IIa trial followed, enrolling 94 adults aged 50 and over with systolic pressure between 120 and 159 mmHg for three months of 1,000 mg NR daily. Results were presented at the American Physiology Summit in 2025 reporting that NR lowered systolic blood pressure and improved vascular endothelial function, though a conference abstract is not the same thing as a peer-reviewed full report and the details needed to weigh it are not yet public. On the NMN side, Katayoshi and colleagues ran a 12-week trial of 250 mg daily in 36 healthy middle-aged adults published in Scientific Reports in 2023. Serum nicotinamide rose significantly. Pulse wave velocity trended downward in the NMN group but showed no significant difference against placebo. A trend is not a result, and the honest summary of the vascular question is that it is the one place where a real effect might still be waiting.
What Do NAD+ Precursors Reliably Do?
Three things hold up across trials, and none of them is what the category is sold on. The first is anti-inflammatory signaling. The Dublin trial in aged men found IL-6, IL-5, IL-2 and TNF-alpha all fell from baseline on NR, and reductions in circulating inflammatory markers have appeared across several independent trials. The second is a set of molecular changes in muscle that stop short of a functional result. A 2023 Science Advances study in BMI-discordant twins found NR improved systemic NAD+ metabolism, increased muscle mitochondrial number, improved myoblast differentiation, and shifted gut microbiota composition and DNA methylation, in both twins regardless of body mass index. The authors were explicit that the clinical potential of oral NR in humans remains to be understood, which is a careful way of saying the biology moved and the person did not. The third is a genuine performance signal at moderate NMN doses. The 2023 GeroScience trial found significantly longer six-minute walking distance and better self-rated health at 600 and 900 mg daily but not at 300 mg. A 2021 trial in amateur runners reported dose-dependent improvements in aerobic capacity measured as oxygen uptake at the ventilatory threshold, attributed to muscle oxygen utilization rather than to a higher VO2 max. Both are modest, both are in the NMN column, and neither has been replicated at scale.
The Biomarker Always Moves. The Endpoint Usually Does Not.
Each trial scored twice. The green bar records whether NAD+ or its metabolites rose significantly, which happened in all seven. The amber bar records whether the trial's own pre-specified clinical endpoint was met, which happened in two: the 2021 Science trial reporting improved muscle insulin sensitivity in 25 prediabetic postmenopausal women on 250 mg of NMN, and the 2023 GeroScience trial reporting longer six-minute walking distance at 600 and 900 mg. A one means the result was reached and a zero means it was not.
- NAD+ biomarker rose
- Primary clinical endpoint met
Why Did the FDA Reverse Its Position on NMN?
The FDA concluded in September 2025 that NMN had been marketed as a dietary supplement before any investigational new drug authorization existed, which removes it from the drug preclusion clause. The sequence is worth understanding because it shaped what was available to buy for three years. In November 2022 the agency told two ingredient suppliers that NMN could not be sold as a dietary ingredient, citing section 201(ff)(3)(B) of the Food, Drug and Cosmetic Act, the provision that bars an ingredient from the supplement market once it has been the subject of substantial clinical investigation as a drug. NMN was being developed as a pharmaceutical, so the clause applied on its face. Major retailers delisted NMN products. The Natural Products Association and the Alliance for Natural Health filed a citizen petition in March 2023 arguing the exclusion was wrong on the facts. In two letters dated 29 September 2025 the FDA agreed and reversed itself, and follow-up letters in December 2025 addressed new dietary ingredient status for specific suppliers. Nicotinamide riboside never faced this problem and has been continuously available. Two points are worth separating here. A regulatory reversal is a statement about market eligibility, not about efficacy, and nothing in the FDA's letters evaluates whether NMN does anything. And the three-year period of legal uncertainty pushed a lot of NMN sales through channels with weaker quality control, which is a real consideration for anyone evaluating product sourcing.
What Doses Were Studied and What Is Known About Safety?
Published trials have used 250 to 2,000 mg daily for periods between three and 24 weeks, and both compounds have been well tolerated across that range. NR has been given at 2,000 mg daily for 12 weeks without a safety signal in obese men, and at 1,000 mg daily for six weeks in adults aged 55 to 79 with no serious adverse effects. NMN has been given at 900 mg daily for 60 days and at 250 mg daily for 12 weeks without adverse events. That is a reassuring short-term record and a limited one, and three boundaries apply. The longest trial in the published literature runs 24 weeks, so continuous use over years is uncharacterized in humans. There is a mechanistic question about methyl groups, because the body disposes of excess nicotinamide by methylating it, and the fivefold rise in methylnicotinamide seen in muscle biopsies represents a real draw on methyl donor pools whose long-term significance nobody has established. And preclinical work has raised the question of whether raising NAD+ availability could support the metabolism of tumours that depend on it, a concern that comes from mouse models and has never been tested in a human trial in either direction. None of that constitutes evidence of harm. It constitutes an absence of the data that would settle the question, which is a different thing and should be described as such. Anyone with an existing cancer diagnosis, kidney impairment, or a medication list has specific reasons to raise this with a clinician rather than treating it as a neutral addition.
How Should the Whole NAD+ Evidence Base Be Read?
NAD+ precursors are a pharmacologically successful intervention in search of a clinical result. The mechanism is coherent, the biochemistry is well characterized, the compounds are absorbed, the target biomarker rises reliably, and the safety record over three months is clean. That is more than most supplements can claim and it should not be dismissed. Set against it: the tissue that matters most did not raise its NAD+ pool in the trial that biopsied it, mitochondrial respiration did not improve, insulin sensitivity did not improve in men at any dose tested including 2,000 mg, ten pooled randomized trials found no effect on muscle mass, grip, gait speed or chair-stand time in adults over 60, and the vascular signal that looked most promising in 2018 is still sitting in a conference abstract eight years later. Two positive endpoint results stand: muscle insulin sensitivity in 25 prediabetic postmenopausal women on NMN, and six-minute walking distance at 600 mg of NMN and above. Both deserve replication rather than extrapolation. The gap between the biomarker and the outcome is the entire story of this category, and it is a pattern with precedent, because antioxidant supplements also moved their biomarkers convincingly for years before large trials found no benefit. Anyone deciding what to do with this information is deciding whether a compelling mechanism plus a moving biomarker is enough to act on before the outcomes arrive. That is a legitimate personal judgement and it is not the same as an evidence-based one. This article is educational and describes published research. It is not a protocol and not a recommendation to start or stop anything, and questions about individual use belong with a clinician who can see the full picture.
The NAD+ Precursor Record on One Axis
Every strand of the human evidence arranged as a single schematic map. Green marks what randomized trials support, amber marks the claim the muscle biopsy data complicated, and grey marks claims the trials do not carry. Bar length is a rough visual weight rather than a shared unit, so read this as a guide to where the evidence is thick and where it thins out. Raising blood NAD+ sits at the strong end and is the least interesting thing on the chart, because it is the input rather than the outcome.
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