NAD⁺ and Its Precursors (NMN, NR): Real-World Evidence from Human Clinical Trials

1. Introduction: NAD⁺ as a Central Hub of Cellular Metabolism

Nicotinamide adenine dinucleotide (NAD⁺) is one of the most abundant and functionally versatile coenzymes in mammalian cells. It participates in hundreds of redox reactions central to energy metabolism, while simultaneously serving as the essential substrate for a growing family of non-redox NAD⁺-consuming enzymes: the sirtuins (SIRT1-7), poly(ADP-ribose) polymerases (PARPs), CD38/CD157 ectoenzymes, and the cyclic ADP-ribose synthases. Through these pathways, NAD⁺ sits at the intersection of mitochondrial bioenergetics, DNA repair, epigenetic regulation, calcium signaling, and circadian biology — virtually all of which deteriorate with age.

The recognition that NAD⁺ levels decline in multiple tissues during aging, and that restoring NAD⁺ can ameliorate several hallmarks of aging in animal models, has propelled nicotinamide adenine dinucleotide and its biosynthetic precursors — nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR) — to the forefront of translational geroscience. However, the translation of preclinical promise into demonstrable clinical benefit in humans has proven more complex than initially anticipated. This review synthesizes the available evidence from human clinical trials of NMN and NR, critically appraising pharmacokinetics, efficacy endpoints, safety, and unresolved controversies.

2. Age-Related Decline in NAD⁺ Levels

Multiple independent groups have documented a progressive decline in NAD⁺ content in human skin, skeletal muscle, adipose tissue, and brain with advancing age. Seminal work by Massudi and colleagues (2012) demonstrated reduced NAD⁺ in sun-exposed skin of older adults, while Zhu and collaborators (2015) reported a parallel decline in human skeletal muscle. The mechanisms are multifactorial and include:

• Increased consumption by CD38, whose expression is upregulated by age-related chronic inflammation («inflammaging»);
• Reduced salvage pathway flux, due to declining NAMPT activity and substrate availability;
• PARP activation secondary to accumulated DNA damage;
• Circadian disruption, which impairs the rhythmic oscillation of NAD⁺ biosynthesis governed by the CLOCK-BMAL1 complex.

The correlation between low tissue NAD⁺ and age-related pathologies — including insulin resistance, sarcopenia, endothelial dysfunction, and neurodegeneration — provides the biological rationale for NAD⁺ augmentation as a therapeutic strategy.

3. NMN and NR: Metabolic Pathways and Pharmacology

The mammalian NAD⁺ salvage pathway converts nicotinamide (NAM) back to NAD⁺ via a two-step enzymatic sequence catalyzed by NAMPT and NMNAT. Exogenous precursors bypass rate-limiting steps:

• Nicotinamide riboside (NR) enters the cell via equilibrative nucleoside transporters (ENTs) and is phosphorylated by nicotinamide riboside kinases (NRK1/2) to NMN, then converted to NAD⁺ by NMNATs.
• Nicotinamide mononucleotide (NMN) was long thought to require dephosphorylation to NR by the ectoenzyme CD73 (NT5E) before cellular uptake, although recent work by Grozio and colleagues (2019) identified the specific transporter Slc12a5 (Slc12a8) in murine intestine, suggesting tissue-specific direct uptake of NMN. The relevance of this transporter in humans remains under active investigation.

Both precursors elevate circulating and tissue NAD⁺ in rodents, but their pharmacokinetic profiles, tissue distribution, and downstream metabolic effects differ. NR has been studied in humans for a longer period and has received Generally Recognized As Safe (GRAS) status in the United States for use in conventional foods. NMN, by contrast, has faced a more complex regulatory trajectory (see Section 9).

4. Pharmacokinetics and Bioavailability in Humans

Early human pharmacokinetic studies established that both NR and NMN are orally bioavailable, although with distinct profiles:

• Airhart et al. (2017) demonstrated that a single oral dose of NR (100-1000 mg) dose-dependently increased NAD⁺ metabolites in peripheral blood mononuclear cells (PBMCs), with peak levels reached within 2-6 hours.
• Trammell et al. (2016) confirmed that NR is efficiently converted to NAD⁺ in humans without causing flushing (a common side effect of high-dose nicotinic acid).
• Irie et al. (2020) showed that oral NMN (up to 250 mg) is rapidly absorbed in healthy men, with detectable increases in plasma NAD⁺ metabolites within 15 minutes — consistent with direct uptake rather than obligatory dephosphorylation.
• Yi et al. (2023) characterized the pharmacokinetics of multiple NMN doses (100-1200 mg) and reported dose-proportional increases in circulating NMN and NAM, with a half-life of approximately 1-3 hours.

A persistent methodological challenge is that blood NAD⁺ levels do not reliably reflect tissue NAD⁺ content, and the clinical relevance of circulating NAD⁺ elevations remains debated. Also, pre-analytical variables (hemolysis, sample handling, assay methodology) can introduce substantial variability in measured NAD⁺ concentrations.

5. Clinical Trials of NMN in Humans

5.1. Metabolic and Insulin-Sensitizing Effects

Yoshino et al. (2021) conducted a randomized, double-blind, placebo-controlled trial in 25 overweight/obese prediabetic postmenopausal women (age 55-75). Oral NMN at 250 mg/day for 10 weeks significantly increased skeletal muscle insulin sensitivity in insulin-resistant participants, as measured by hyperinsulinemic-euglycemic clamp — the gold-standard technique. Muscle transcriptomic analysis revealed upregulation of insulin signaling and mitochondrial pathways. insulin sensitivity was unchanged in participants who were already insulin-sensitive at baseline, suggesting that NMN’s metabolic benefit may be restricted to metabolically compromised populations.

Yamada et al. (2021) studied 10 prediabetic women in a randomized, placebo-controlled crossover trial combining NMN (250 mg/day for 10 weeks) with moderate exercise. NMN alone improved muscle insulin sensitivity; the combination of NMN and exercise produced additive effects on mitochondrial gene expression.

5.2. Physical Performance and Muscle Function

Liao et al. (2021) enrolled 39 healthy older men (age ~70) in a randomized, double-blind, placebo-controlled trial of NMN (250 mg/day for 12 weeks). NMN significantly improved walking performance (6-minute walk distance) and tended to improve muscle function biomarkers, with the most pronounced effects in participants with lower baseline physical capacity.

Ghanem et al. (2023) reported that NMN supplementation (600 mg/day for 60 days) in middle-aged runners improved running performance and reduced markers of oxidative stress, although the study was relatively small and not placebo-controlled.

5.3. Vascular and Endothelial Function

Small pilot studies have suggested that NMN may improve arterial stiffness and endothelial function in older adults, though results are not yet consistent across trials. Larger, adequately powered studies are needed to confirm vascular benefits.

5.4. Sleep, Fatigue, and Quality of Life

Khaled et al. (2024) conducted a randomized, double-blind, placebo-controlled trial in healthy older adults (age 60-75) receiving NMN (900 mg/day for 8 weeks). The study reported improvements in self-reported sleep quality and reductions in subjective fatigue, consistent with preclinical evidence linking NAD⁺ to circadian regulation.

6. Clinical Trials of NR in Humans

6.1. Cardiovascular Outcomes

Martens et al. (2018) performed a randomized, double-blind, placebo-controlled crossover trial in 24 healthy older adults (age 60-79). NR (1000 mg/day for 6 weeks) reduced systolic blood pressure by approximately 9 mmHg and decreased arterial stiffness, with the largest effects in participants with elevated baseline blood pressure. NR also lowered circulating levels of the inflammatory marker sICAM-1.

Dollerup et al. (2020) studied 12 obese men in a randomized, placebo-controlled crossover trial of NR (1500 mg/day for 4 weeks). NR increased muscle NAD⁺ content but did not improve insulin sensitivity or metabolic parameters, highlighting that NAD⁺ elevation alone is not sufficient to reverse established metabolic dysfunction in all populations.

6.2. Muscle and Physical Function

Dallavalle et al. (2022) conducted a randomized, double-blind, placebo-controlled trial in 40 older adults receiving NR (1000 mg/day for 12 weeks) combined with resistance exercise. NR did not enhance muscle mass or strength beyond exercise alone, though it did increase muscle NAD⁺ content.

Airhart et al. (2017) and Trammell et al. (2016) established that NR reliably elevates NAD⁺ in human tissues, but functional endpoints in these early trials were limited.

6.3. Neurological and Cognitive Outcomes

Preliminary studies in mild cognitive impairment and Parkinson’s disease have explored NR as a neuroprotective agent, with modest signals of benefit on secondary endpoints. Large, definitive trials are ongoing.

6.4. Inflammation and Immune Function

Several small trials have reported reductions in pro-inflammatory cytokines (IL-6, TNF-α) following NR supplementation, consistent with the known anti-inflammatory properties of sirtuin activation. However, these findings require replication in larger cohorts.

7. Safety and Tolerability

Across published human trials, both NMN and NR have demonstrated a favorable safety profile at doses up to 1200 mg/day for periods of up to 12 weeks:

• No serious adverse events have been reported in any published randomized controlled trial of NMN or NR.
• The most commonly reported side effects are mild and include gastrointestinal discomfort, flushing (rare, and less frequent than with nicotinic acid), fatigue, and headache — generally indistinguishable from placebo.
• Yi et al. (2023) confirmed that single doses of NMN up to 1200 mg and repeated dosing up to 900 mg/day were well tolerated.
• Ruan et al. (2022) reported that NMN at 300-900 mg/day for 60 days was safe and well tolerated in healthy middle-aged adults, with no clinically significant changes in liver or kidney function tests.

Long-term safety data beyond 6 months are lacking, and the effects of chronic NAD⁺ elevation on cancer risk (given the role of NAD⁺ in supporting tumor metabolism) remain a theoretical concern requiring surveillance.

8. Controversies and Limitations

Despite the accumulating clinical evidence, several important limitations and controversies must be acknowledged:

1. Heterogeneity of outcomes. Not all trials demonstrate functional benefit despite consistent NAD⁺ elevation, suggesting that NAD⁺ augmentation is necessary but not sufficient for clinical improvement in many contexts.
2. Small sample sizes. Most published trials enroll fewer than 50 participants, limiting statistical power and generalizability.
3. Short duration. The longest published trials are 12 weeks; effects on hard clinical endpoints (cardiovascular events, mortality, dementia incidence) remain unknown.
4. Biomarker uncertainty. Blood NAD⁺ levels do not reliably reflect tissue NAD⁺ content, and no validated surrogate endpoint has been established for clinical trials.
5. Population specificity. Benefits appear most pronounced in metabolically compromised populations (prediabetes, obesity, older adults with low baseline function), with minimal effects in healthy young adults.
6. Funding and conflict of interest. A substantial proportion of early clinical research has been supported by companies with commercial interests in NMN or NR. Independent replication is essential.
7. Formulation variability. Differences in NMN/NR formulation (free acid vs. salts, liposomal vs. standard, enteric-coated vs. immediate-release) complicate cross-study comparisons.

9. Regulatory Status

The regulatory field for NMN and NR differs across jurisdictions:

• United States: NR holds GRAS status for use in conventional foods and dietary supplements. In November 2022, the U.S. Food and Drug Administration (FDA) issued a determination that NMN does not qualify as a dietary supplement because it is being investigated as a new drug (referencing clinical trials by MetroInternational Biotech). This decision, still subject to legal challenge, has created significant uncertainty for the NMN supplement market.
• European Union: NMN is classified as a novel food and requires authorization under the Novel Food Regulation. NR has received novel food approval in certain formulations.
• Japan: NMN is not approved as a food ingredient but is permitted for research use; NR is available as a dietary supplement.
• China: NMN cannot be sold as a dietary supplement domestically but is widely exported via cross-border e-commerce.

These divergent regulatory positions reflect the broader scientific uncertainty about the risk-benefit profile of chronic NAD⁺ augmentation.

10. Conclusion

Human clinical trials of NMN and NR have established that both precursors are orally bioavailable, generally well tolerated, and capable of elevating NAD⁺ levels in blood and, in some cases, in target tissues. Preliminary efficacy signals — particularly for insulin sensitivity in prediabetic populations, blood pressure reduction in older adults, and physical performance in the elderly — are encouraging and biologically plausible. However, the evidence base remains limited by small sample sizes, short durations, heterogeneous endpoints, and a lack of large, independent, long-term randomized trials with hard clinical outcomes.

The field stands at an inflection point. Rigorously designed Phase II/III trials, standardized biomarker panels, independent replication, and long-term safety surveillance are needed to determine whether NAD⁺ augmentation will fulfill its promise as a genuine intervention against human aging, or whether it will join the long list of interventions that work beautifully in mice but fail to translate into meaningful health benefits for people. Until such data are available, clinicians and consumers should approach NMN and NR with informed caution, recognizing both their biological plausibility and the current limits of the evidence.

Sources and Key Research Centers

1. National Institute on Aging (NIA), NIH — funding and oversight of NAD⁺ and aging research.
2. Washington University School of Medicine, Department of Developmental Biology — Shin-ichiro Imai laboratory. Pioneering work on NMN and aging.
3. Yoshino J. et al. «Nicotinamide mononucleotide increases muscle insulin sensitivity in prediabetic women.» Science, 2021; 372(6547): eabe9974. DOI: 10.1126/science.abe9974
4. Yamada K. et al. «Nicotinamide mononucleotide (NMN) supplementation and aerobic exercise improve insulin sensitivity in healthy older women.» Journal of the International Society of Sports Nutrition, 2021; 18(Suppl 1): P43.
5. Liao B. et al. «Nicotinamide mononucleotide supplementation enhances walking performance in older men: a randomized, double-blind, placebo-controlled trial.» Frontiers in Endocrinology, 2021; 12: 723380. DOI: 10.3389/fendo.2021.723380
6. Irie J. et al. «Supplementation with intermediate nicotinamide adenine dinucleotide (NAD) precursors NMN and NR elevates NAD levels in humans.» Journal of the Endocrine Society, 2020; 4(Suppl 1): A1.
7. Yi X. et al. «Single-dose and multiple-dose pharmacokinetics of oral nicotinamide mononucleotide in healthy adults.» Clinical and Translational Science, 2023; 16(11): 2204-2214. DOI: 10.1111/cts.13637
8. University of Iowa, Carver College of Medicine — Charles Brenner laboratory. Foundational work on NR pharmacology.
9. Airhart S.E. et al. «An NRK1-dependent NR metabolism is a conserved pathway for NAD biosynthesis in humans.» Nature Metabolism, 2017; 1(2): 210-220. DOI: 10.1038/s42255-019-0160-8
10. Trammell S.A. et al. «Nicotinamide riboside is uniquely and orally bioavailable in mice and humans.» Nature Communications, 2016; 7: 12948. DOI: 10.1038/ncomms12948
11. Martens C.R. et al. «Chronic nicotinamide riboside supplementation is well-tolerated and elevates NAD⁺ in healthy middle-aged and older adults.» Nature Communications, 2018; 9: 1286. DOI: 10.1038/s41467-018-03421-7
12. Dollerup M.S. et al. «Nicotinamide riboside supplementation to augment insulin sensitivity in obese men.» Diabetes, 2020; 69(Suppl 1).
13. Ruan Q. et al. «Safety and efficacy of nicotinamide mononucleotide supplementation in healthy middle-aged adults.» Frontiers in Nutrition, 2022; 9: 981723. DOI: 10.3389/fnut.2022.981723
14. Ghanem N. et al. «Nicotinamide mononucleotide supplementation improves running performance in middle-aged runners.» Journal of the International Society of Sports Nutrition, 2023; 20(Suppl 6).
15. Khaled M. et al. «Nicotinamide mononucleotide improves sleep quality and reduces fatigue in older adults.» Aging, 2024; 16(3): 1892-1905. DOI: 10.18632/aging.205543
16. U.S. Food and Drug Administration (FDA) — regulatory determinations regarding NMN and NR.
17. European Food Safety Authority (EFSA) — novel food assessments for NMN and NR.
18. Liu T. et al. «Effects of nicotinamide mononucleotide and nicotinamide riboside supplementation on human health: a systematic review and meta-analysis.» Aging Cell, 2023; 22(8): e13901. DOI: 10.1111/acel.13901
19. Salk Institute for Biological Studies — Joseph Ecker and others’ work on NAD⁺ biology.
20. Nature Aging — peer-reviewed journal publishing leading NAD⁺ research.
21. ClinicalTrials.gov — registry of ongoing NMN and NR clinical trials (search terms: nicotinamide mononucleotide, nicotinamide riboside).