NAD+

What is NAD+?#

NAD+ (nicotinamide adenine dinucleotide) is a coenzyme present in all living cells that serves as a central metabolic hub linking cellular energy production to signaling and repair processes. First discovered in 1906 by Arthur Harden and William John Young during fermentation studies, NAD+ has since been recognized as one of the most important molecules in biology.

NAD+ exists in two forms: the oxidized form (NAD+) and the reduced form (NADH). Together, this redox couple participates in over 500 enzymatic reactions, making it the most common cofactor in human metabolism. NAD+ accepts electrons during catabolic reactions (glycolysis, the TCA cycle, and fatty acid oxidation) and donates them as NADH to the mitochondrial electron transport chain for ATP synthesis.

Beyond its role as an electron carrier, NAD+ serves as a consumed substrate for three major classes of enzymes: sirtuins (SIRT1-7), poly-ADP-ribose polymerases (PARPs), and cyclic ADP-ribose synthases (CD38/CD157). These NAD+-consuming enzymes link cellular metabolic status to epigenetic regulation, DNA repair, calcium signaling, and immune function.

Mechanism of Action#

Overview NAD+ functions through two fundamentally different biochemical roles: (1) as a coenzyme in redox reactions, where it shuttles electrons without being consumed, and (2) as a substrate for NAD+-consuming enzymes, where it is cleaved and depleted. The balance between NAD+ synthesis, consumption, and recycling determines cellular NAD+ availability and, consequently, the activity of NAD+-dependent signaling pathways.

Redox chemistry and energy metabolism

• Glycolysis and TCA cycle: NAD+ accepts hydride ions (H-) from metabolic intermediates, generating NADH. Each glucose molecule produces NADH at multiple steps (glyceraldehyde-3-phosphate dehydrogenase, pyruvate dehydrogenase, isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, malate dehydrogenase).
• Electron transport chain: NADH donates electrons to Complex I (NADH dehydrogenase), initiating the proton gradient that drives ATP synthase. This process generates approximately 2.5 ATP per NADH molecule.
• Fatty acid oxidation: Beta-oxidation generates NADH at each cycle, linking lipid metabolism to NAD+ availability.
• NAD+/NADH ratio: The cytoplasmic NAD+/NADH ratio (~700:1 in well-oxygenated cells) serves as a metabolic sensor, influencing glycolytic flux, gluconeogenesis, and redox-sensitive signaling.

Sirtuin signaling (SIRT1-7)

• Sirtuins are NAD+-dependent protein deacetylases and ADP-ribosyltransferases that consume NAD+ to remove acetyl groups from target proteins, producing nicotinamide (NAM) and O-acetyl-ADP-ribose.
• SIRT1: Nuclear; deacetylates PGC-1alpha (mitochondrial biogenesis), p53 (apoptosis regulation), NF-kB (inflammation), FOXO transcription factors (stress resistance), and histones (epigenetic silencing).
• SIRT2: Cytoplasmic/nuclear; deacetylates alpha-tubulin and histones; involved in cell cycle regulation.
• SIRT3: Mitochondrial; deacetylates enzymes of the TCA cycle and electron transport chain, including SOD2 (superoxide dismutase), enhancing mitochondrial function and antioxidant defense.
• SIRT4-7: Additional mitochondrial and nuclear sirtuins with roles in fatty acid oxidation, urea cycle, rDNA silencing, and DNA repair.
• NAD+ dependence: Sirtuin activity is directly limited by NAD+ availability; declining NAD+ with age reduces sirtuin function, contributing to mitochondrial dysfunction, epigenetic dysregulation, and impaired stress responses.

PARP-mediated DNA repair

• PARP1 and PARP2 detect single-strand DNA breaks and consume NAD+ to synthesize poly-ADP-ribose (PAR) chains on histones and repair proteins, recruiting the base excision repair machinery.
• Under conditions of extensive DNA damage (aging, genotoxic stress), PARP hyperactivation can deplete the cellular NAD+ pool, starving sirtuins and other NAD+-dependent enzymes of substrate, creating a competition for limited NAD+.
• PARP inhibition or NAD+ replenishment can restore sirtuin activity in models of accelerated aging and DNA damage.

CD38 and NAD+ degradation

• CD38 is a transmembrane glycoprotein and the dominant NADase in mammalian tissues. It hydrolyzes NAD+ to produce cyclic ADP-ribose (cADRPR) and nicotinamide, regulating intracellular calcium signaling.
• CD38 expression increases with age and chronic inflammation, and CD38 has been identified as a major driver of age-related NAD+ decline. In mouse models, CD38 knockout prevents age-related NAD+ depletion and preserves mitochondrial function.
• SARM1 (sterile alpha and TIR motif-containing 1): An NADase activated during axonal injury that rapidly depletes NAD+ to trigger Wallerian degeneration; it represents a distinct NAD+ consumption pathway relevant to neurodegeneration.

NAD+ biosynthesis pathways

• De novo synthesis (kynurenine pathway): From dietary tryptophan via quinolinic acid to NAD+; a minor contributor to NAD+ pools in most tissues except the liver.
• Preiss-Handler pathway: Converts dietary nicotinic acid (niacin/vitamin B3) to NAD+ via nicotinic acid mononucleotide (NaMN) and nicotinic acid adenine dinucleotide (NaAD+).
• Salvage pathway: The predominant route for NAD+ maintenance; recycles nicotinamide (NAM, the byproduct of sirtuin and PARP activity) back to NMN via nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme. NMN is then converted to NAD+ by NMN adenylyltransferases (NMNATs).
• Precursor entry points: Nicotinamide riboside (NR) is phosphorylated by NR kinases (NRK1/2) to NMN, entering the salvage pathway. NMN can also be taken up directly by cells via the transporter Slc12a8.

Therapeutic Applications#

NAD+ replenishment is being investigated across multiple therapeutic areas, primarily through direct NAD+ administration (IV) and through oral precursors NMN and NR.

Aging and metabolic health

• Age-related NAD+ decline has been demonstrated in human tissues including skin, brain, liver, and blood. Studies show NAD+ levels decrease by approximately 50% between ages 40 and 60 in some tissues.
• In mouse models, NAD+ replenishment via NMN or NR improved mitochondrial function, insulin sensitivity, exercise capacity, and lifespan in some (but not all) genetic backgrounds.
• Human clinical trials with NR (NIAGEN) have demonstrated dose-dependent increases in blood NAD+ levels in healthy adults and older individuals. The Chromadex NIAGEN trials showed sustained elevation of NAD+ at 300-1000 mg/day doses.

Neurodegenerative diseases

• NAD+ depletion is observed in Alzheimer's disease brain tissue; NMN supplementation improved cognitive function and reduced amyloid pathology in APP/PS1 mouse models.
• In Parkinson's disease models, NAD+ supplementation protected dopaminergic neurons from degeneration.
• A Phase II clinical trial (NCT03568968) tested NR in Parkinson's disease patients and reported increases in cerebral NAD+ levels and some improvements in clinical scores.
• SARM1 inhibition is under investigation as a neuroprotective strategy for axonal degeneration in ALS and peripheral neuropathy.

Cardiovascular research

• NAD+ deficiency impairs cardiac function in heart failure models; NMN restored cardiac NAD+ and improved ejection fraction in mouse models of heart failure.
• NR supplementation reduced systemic inflammation and improved vascular endothelial function in a small crossover RCT in healthy middle-aged and older adults.

Substance use disorders

• IV NAD+ infusions have been used in addiction medicine clinics for decades, though controlled evidence is limited. A pilot study in alcohol use disorder patients found IV NAD+ was safe and showed preliminary improvements in craving and anxiety measures.

Research Evidence Quality#

The evidence base for NAD+ is growing rapidly but remains largely preclinical for most therapeutic claims.

• Strongest evidence: NR oral supplementation reliably raises blood NAD+ levels in humans (multiple RCTs); safety profile established up to 2000 mg/day.
• Moderate evidence: NMN oral supplementation raises NAD+ in humans (emerging RCTs); NR reduces inflammation markers in small trials.
• Preclinical only: Most disease-specific therapeutic claims (neurodegeneration, cardiovascular, lifespan extension) derive from animal models and have not been replicated in large human trials.
• Limited evidence: IV NAD+ therapy lacks controlled clinical trial data for most indications despite widespread clinical use.

Evidence Gaps and Limitations#

The current evidence base has important limitations:

• Large, long-term randomized controlled trials in disease populations are lacking
• Optimal dosing, route of administration, and treatment duration have not been established through dose-finding studies
• Whether raising blood NAD+ translates to meaningful tissue NAD+ increases in humans is incompletely characterized
• The relative efficacy of different NAD+ precursors (NMN vs NR vs niacin) has not been established in head-to-head trials
• Long-term safety data beyond 12 months is limited for all NAD+ augmentation strategies
• Publication bias may favor positive results, particularly in preclinical literature

Key Research Findings#

Chronic nicotinamide riboside supplementation is well-tolerated and elevates NAD+ in healthy middle-aged and older adults, published in Nature Communications (Martens CR et al., 2018; DOI: 10.1038/s41467-018-03421-7):

Crossover RCT in 24 healthy older adults showing NR 1000 mg/day for 6 weeks safely raised blood NAD+ by ~60% and tended to reduce systolic blood pressure and aortic stiffness

• NR 1000 mg/day increased whole blood NAD+ approximately 60% over baseline
• Trend toward reduced systolic blood pressure (-2 mmHg) and aortic stiffness
• Well tolerated with no serious adverse events

Nicotinamide riboside augments the aged human skeletal muscle NAD+ metabolome and induces transcriptomic and anti-inflammatory signatures, published in Cell Reports (Elhassan YS et al., 2019; DOI: 10.1016/j.celrep.2019.07.043):

Open-label study in 12 aged men showing oral NR 1000 mg/day for 21 days increased skeletal muscle NAD+ metabolome and induced anti-inflammatory gene expression signatures

• NR supplementation increased skeletal muscle NAD+ metabolites (NAAD, MeNAM)
• Anti-inflammatory transcriptomic signatures detected in muscle biopsies
• Downregulation of energy metabolism and mitochondrial pathways in transcriptome

Effect of oral nicotinamide mononucleotide on clinical parameters and nicotinamide metabolite levels in healthy Japanese men, published in Endocrine Journal (Irie J et al., 2020; DOI: 10.1507/endocrj.EJ19-0313):

First-in-human study of oral NMN 250 mg/day in 10 healthy men demonstrating safety and increases in NAD+ metabolites without significant adverse effects

• NMN 250 mg single dose was safe and well tolerated
• Increased plasma NMN and NAD+ metabolite levels
• No clinically significant adverse effects observed

NR-SAFE: a randomized, double-blind, placebo-controlled trial of high-dose nicotinamide riboside in healthy middle-aged and older adults, published in Nature Aging (Vreones M et al., 2023):

Safety-focused RCT testing NR at 1000 mg twice daily (2000 mg/day) for 12 weeks in healthy adults, establishing safety and tolerability at the highest studied dose

• NR 2000 mg/day was well tolerated over 12 weeks
• No clinically significant adverse events attributed to NR
• Confirmed dose-dependent NAD+ metabolite elevation in blood

Nicotinamide riboside supplementation to improve lower extremity function in peripheral artery disease (Kalil RS et al., 2023):

RCT investigating NR supplementation in peripheral artery disease patients, representing one of the first disease-specific human trials of NAD+ augmentation for vascular outcomes

• Investigated NR effects on walking performance in PAD
• Targeted disease-specific vascular outcomes rather than biomarkers alone

NAD+ augmentation with nicotinamide riboside in Parkinson's disease (NOPARK trial), published in Cell Metabolism (Brakedal B et al., 2022):

Phase II RCT testing NR 1000 mg/day in Parkinson's disease patients, showing increased cerebral NAD+ and some clinical improvements

• NR supplementation increased cerebral NAD+ levels measured by MRS
• Some improvements in MDS-UPDRS scores in the NR group
• Generally well tolerated in PD patients

Related Reading#

• NAD+ research studies and evidence
• NAD+ dosing protocols
• NAD+ side effects profile
• MOTS-c research guide
• SS-31 research guide