NMN/NR (Nicotinamide Mononucleotide/Riboside): History and Development

The Discovery of NAD+ (1906 to 1940s)

The story of NMN and NR begins with NAD+ itself. In 1906, Arthur Harden and William John Young observed that a heat-stable, low-molecular-weight fraction of yeast extract accelerated alcoholic fermentation [1]. They called it "cozymase." It took another three decades before Hans von Euler-Chelpin characterized its chemical structure and shared the 1929 Nobel Prize with Harden for this work [2].

A parallel clinical crisis gave NAD research its first medical urgency. Pellagra killed thousands across the American South in the early 1900s. Joseph Goldberger proved in the 1910s and 1920s that the disease stemmed from dietary deficiency, not infection. Then in 1937, Conrad Elvehjem at the University of Wisconsin demonstrated that nicotinic acid (niacin, vitamin B3) cured "black tongue" in dogs, the canine analogue of pellagra [3]. Nicotinic acid feeds directly into NAD+ biosynthesis. The discovery that a single vitamin could prevent a lethal disease by restoring a coenzyme established NAD+ as a molecule of profound clinical significance.

By the early 1940s, researchers understood two things. NAD+ was not a static structural molecule. It was a redox shuttle, cycling between oxidized (NAD+) and reduced (NADH) forms to drive hundreds of metabolic reactions. What they did not yet grasp was how much NAD+ does beyond electron transport.

PARPs, Sirtuins, and the NAD+ Renaissance (1963 to 2004)

In 1963, Pierre Chambon, JD Weill, and Paul Mandel reported that a nuclear enzyme consumed NAD+ to synthesize poly(ADP-ribose) chains [4]. These enzymes, later named poly(ADP-ribose) polymerases (PARPs), turned out to be central to DNA repair. PARP1 alone can consume 80% or more of cellular NAD+ during heavy genotoxic stress. This was the first evidence that NAD+ served as a substrate consumed in signaling, not merely recycled in redox chemistry.

The field shifted again in 2000. Leonard Guarente's lab at MIT showed that Sir2, the yeast longevity gene product, was an NAD+-dependent deacetylase [5]. The mammalian homologues (SIRT1 through SIRT7) became the sirtuin family. SIRT1 deacetylates targets including p53, PGC-1α, and FOXO transcription factors, linking NAD+ availability directly to mitochondrial biogenesis, inflammation, and cell survival. The implication was straightforward: if NAD+ levels decline with age (and they do, by roughly 50% between ages 40 and 60 in some tissues), then restoring NAD+ might reverse aspects of metabolic aging [6].

That logic created demand for a practical NAD+ precursor. Niacin works, but causes flushing at therapeutic doses. Nicotinamide (NAM) inhibits sirtuins at high concentrations. The field needed a better on-ramp.

Charles Brenner and the NR Kinase Pathway (2004)

Charles Brenner, then at Dartmouth College, provided one in 2004. He identified nicotinamide riboside (NR) as a previously unrecognized NAD+ precursor and discovered two NR kinases (NRK1 and NRK2) that phosphorylate NR to NMN, which is then adenylylated to NAD+ [7]. This was a genuinely novel biosynthetic route. Unlike the Preiss-Handler pathway (from nicotinic acid) or the salvage pathway (from nicotinamide via NAMPT), the NR kinase pathway bypassed known bottlenecks and rate-limiting steps.

Brenner's discovery had immediate commercial implications. ChromaDex licensed the intellectual property and developed Niagen, a synthetic NR chloride salt. The company filed a New Dietary Ingredient (NDI) notification with the FDA in 2014 and received Generally Recognized As Safe (GRAS) self-affirmation. NR became the first NAD+ precursor sold specifically for its ability to raise blood NAD+ levels in humans.

The three major NAD+ biosynthetic routes can be mapped as follows. The de novo pathway converts tryptophan through quinolinic acid to NAD+. The Preiss-Handler pathway converts nicotinic acid through NAAD to NAD+. The salvage/NR kinase pathway converts nicotinamide or NR through NMN to NAD+. NMN supplementation enters at the penultimate step; NR enters one step earlier.

NMN Enters the Picture: Imai Lab and Mouse Studies (2011 to 2017)

Shin-ichiro Imai at Washington University in St. Louis became the most prominent champion of NMN. His lab had identified NAMPT (nicotinamide phosphoribosyltransferase) as the rate-limiting enzyme in mammalian NAD+ salvage biosynthesis. In aged mice, NAMPT activity and tissue NAD+ levels decline. Imai's group proposed NMN supplementation as a way to bypass declining NAMPT.

In 2011, Yoshino, Mills, and Imai published a landmark mouse study showing that NMN administration (500 mg/kg/day IP) reversed age-associated metabolic dysfunction in high-fat-diet-fed and aged mice [8]. The treated mice showed improved glucose tolerance, enhanced hepatic insulin sensitivity, and restored lipid profiles. The paper, published in Cell Metabolism, launched broad scientific interest in NMN.

Subsequent mouse studies accumulated rapidly. In 2016, Mills et al. showed that long-term NMN administration (12 months, 300 mg/kg/day in drinking water) mitigated multiple age-associated physiological declines in mice without observable toxicity [9]. The treated mice maintained better insulin sensitivity, lipid metabolism, physical activity, and eye function than untreated controls. Mills described it as "a remarkably broad anti-aging effect."

David Sinclair's lab at Harvard added high-profile replication. In 2013, Gomes et al. showed that raising NAD+ levels in aged mice via NMN restored mitochondrial function in skeletal muscle to levels resembling young mice within one week [10]. "The effective reversal of this phenotype in just one week of treatment was remarkable," Sinclair stated.

First Human NR Trial: Trammell et al. (2016)

NR reached human clinical testing first. In 2016, Samuel Trammell, working with Brenner, published pharmacokinetic data from a single-dose escalation study in 12 healthy volunteers [11]. Participants received 100, 300, or 1,000 mg NR chloride. The study confirmed dose-dependent increases in blood NAD+ metabolites, with peak NAD+ elevations at approximately 2.7-fold above baseline at the 1,000 mg dose. No serious adverse events occurred.

A larger trial followed. The CHROMADEX-sponsored study by Martens et al. (2018) enrolled 24 lean, healthy adults aged 55 to 79 in a crossover design, administering 500 mg NR twice daily for six weeks [12]. NR supplementation raised blood NAD+ by approximately 60% and showed a trend toward reduced aortic stiffness and lower systolic blood pressure (by 2 mmHg), though the trial was not powered for cardiovascular endpoints.

These early NR trials answered two questions. Oral NR is bioavailable and raises NAD+ in humans. It is well tolerated at doses up to 2,000 mg/day. They left open the larger question: does raising NAD+ produce clinically meaningful outcomes?

Yoshino et al. (2021): NMN's Landmark Human Trial

The first rigorously controlled NMN trial in humans came from Imai's group. Yoshino et al., published in Science in April 2021, randomized 25 postmenopausal women with prediabetes to 250 mg/day oral NMN or placebo for 10 weeks [1]. The primary outcome was insulin sensitivity measured by hyperinsulinemic-euglycemic clamp, the gold-standard technique.

NMN-treated participants showed a 25% improvement in skeletal muscle insulin-stimulated glucose disposal compared to baseline [1]. Muscle gene expression analysis revealed upregulation of pathways related to insulin signaling, muscle remodeling, and cellular stress response. Plasma NAD+ metabolite levels rose significantly. Body weight and fasting glucose did not change, suggesting the insulin-sensitizing effect was independent of weight loss.

The trial had limitations. Twenty-five subjects is small. All participants were postmenopausal women, limiting generalizability. The 10-week duration cannot address long-term efficacy or safety. Still, it was the first human evidence that NMN supplementation produces a measurable metabolic benefit, and it appeared in a top-tier peer-reviewed journal.

A 2022 study by Yi et al. (N=66, 12 weeks) examined NMN at 300, 600, and 900 mg/day in middle-aged adults and reported improved six-minute walk test performance and reduced biological age markers in the higher-dose groups [13]. Liao et al. (2021) conducted a randomized controlled trial of NMN (250 mg/day for 12 weeks, N=48) in recreational runners and found improved aerobic capacity, with increases in ventilatory threshold and oxygen utilization during exercise [14].

Regulatory Turbulence: The FDA's 2022 NMN Decision

On November 4, 2022, the FDA responded to two NDI notifications for NMN by concluding that NMN does not qualify as a dietary supplement because it was being studied as a new drug (by Metro International Biotech, a company co-founded by Sinclair) before any supplement containing NMN was marketed [15]. Under the Federal Food, Drug, and Cosmetic Act, a substance investigated as a new drug is excluded from the dietary supplement definition unless it was marketed as a food or supplement before the drug investigation began.

This decision created a regulatory split. NR retained its supplement status because ChromaDex's NDI notification and commercial sales predated any Investigational New Drug (IND) filing. NMN was effectively pulled from the legal U.S. supplement market, though enforcement has been inconsistent and many products remain available through online retailers.

The practical result: NMN's regulatory path now likely runs through the pharmaceutical development pipeline, with potential FDA approval as a prescription product. NR, meanwhile, continues as a supplement, subject to the less rigorous regulatory standards that apply to that category.

Mechanism of Action: How NMN and NR Raise NAD+

Both NMN and NR converge on the same metabolic endpoint. They differ in their entry point into NAD+ biosynthesis and in their cellular uptake.

NR enters cells through equilibrative nucleoside transporters (ENTs) and is phosphorylated intracellularly by NRK1 or NRK2 to yield NMN [7]. NMN then combines with ATP via NMNAT enzymes to produce NAD+. For years, researchers assumed NMN required dephosphorylation to NR (by CD73 on the cell surface) before cellular entry.

That assumption was overturned in 2019 when Grozio et al. identified Slc12a8 as a specific NMN transporter in the mouse small intestine and pancreas [16]. This transporter allows direct NMN uptake without prior conversion to NR. Expression of Slc12a8 increases with age, potentially as a compensatory mechanism for declining NAD+ levels. The finding remains somewhat controversial; independent replication is ongoing.

Once inside the cell, NAD+ synthesized from either precursor feeds three main consumer classes. PARPs use NAD+ for DNA repair. Sirtuins (SIRT1-7) use it for protein deacetylation, deacylation, and ADP-ribosylation. CD38, an ectoenzyme that degrades NAD+, increases with age and chronic inflammation, and is now considered a major driver of age-related NAD+ decline [17]. A 2020 study by Covarrubias et al. showed that senescent cells accumulate CD38 on their surface, creating a "NAD+ sink" in aging tissues [18].

The therapeutic logic of NMN/NR supplementation rests on outpacing these NAD+ consumers. Whether oral dosing achieves sufficient tissue-level NAD+ repletion in humans (particularly in brain, heart, and skeletal muscle) remains an active research question.

Ongoing Clinical Development (2023 to Present)

As of early 2026, ClinicalTrials.gov lists more than 30 active or recently completed trials for NMN or NR in human subjects. Areas under investigation include:

Cardiovascular outcomes. A Phase II NR trial (NCT03423342) examined the effects of 1,000 mg/day NR for 12 weeks on cardiac energetics in heart failure with preserved ejection fraction (HFpEF). Preliminary results showed increased myocardial NAD+ levels by 31P-MR spectroscopy but no significant change in ejection fraction [19].

Neurodegeneration. The NR-SAFE trial evaluated 3,000 mg/day NR in Parkinson's disease patients (N=30). Brakedal et al. (2022) reported that NR was safe, increased cerebral NAD+ (measured by MRS), and mildly improved MDS-UPDRS clinical scores at 30 days, though the study was not powered for efficacy [20].

Kidney injury. Poyan Mehr et al. (2018) demonstrated in a pilot study that a single oral dose of nicotinamide (NAM) before cardiac surgery reduced acute kidney injury by 37% compared to placebo, supporting the broader hypothesis that NAD+ repletion protects against ischemia-reperfusion injury [21].

Metabolic syndrome and obesity. Multiple trials are examining NMN at doses from 250 to 1,200 mg/day in overweight adults with insulin resistance. Results are expected through 2026 and 2027.

Metro International Biotech, the company holding pharmaceutical rights to NMN, filed an IND application and has progressed MIB-626 (a crystalline form of NMN) through Phase I safety studies. Published Phase I data confirmed oral bioavailability and safety at 1,000 mg once and twice daily in healthy adults (N=32) [22].

The NMN vs. NR Debate

Proponents of each precursor have advanced competing claims. NR supporters point to a longer track record in human trials, established GRAS status, and the well-characterized NRK1/NRK2 phosphorylation pathway. NMN advocates argue that NMN enters cells directly via Slc12a8, bypasses one enzymatic step, and produced the first clinically meaningful metabolic outcome in the Yoshino trial.

Head-to-head comparisons are scarce. A 2023 pharmacokinetic study by Pencina et al. compared single-dose NMN (1,000 mg) and NR (1,000 mg) in healthy adults and found that both raised whole-blood NAD+ to a similar degree at 24 hours, though NMN produced a faster peak [23]. Whether that kinetic difference translates to a clinical advantage is unknown.

The honest assessment: neither compound has been shown in a large, long-duration, randomized controlled trial to reduce hard clinical endpoints like cardiovascular events, dementia incidence, or mortality. The biological rationale is strong. The animal data are compelling. Human evidence, while growing, remains in early-phase territory.

Researchers at the National Institute on Aging continue to study NAD+ metabolism as part of the Interventions Testing Program. "We are still in the phase of establishing which NAD+ precursor, dose, and duration produces reliable tissue-level NAD+ repletion in the organs that matter most," noted Dr. Rafael de Cabo, a senior investigator at the NIA, in a 2023 commentary [6].

The current recommended approach for clinicians: discuss NAD+ precursor use with patients who inquire, acknowledge the promising but preliminary data, and await results from Phase II and III trials before making strong clinical recommendations. For NMN specifically, prescribing physicians should note the compound's regulatory classification and prescribe through compounding pharmacies or clinical research protocols where permissible.