What Is NAD+? Definition, Biology and What Research Reports

Definition: What Is NAD+?

NAD+ is the abbreviation for nicotinamide adenine dinucleotide in its oxidised form. Chemically, it is a dinucleotide: two nucleotides joined through their phosphate groups, one carrying an adenine base and the other carrying a nicotinamide group derived from vitamin B3. Biologically, it is a coenzyme — a small molecule that enzymes require in order to catalyse a reaction. It is present in bacteria, yeast, plants and animals, and it participates in hundreds of enzymatic reactions in human cells.

NAD+ is usually described in two distinct roles. In the first, it is a recyclable electron carrier: it accepts electrons, becomes NADH, delivers those electrons elsewhere, and returns to NAD+. In the second, it is destroyed and rebuilt: certain enzymes cleave NAD+ and use fragments of it as a chemical substrate, which means the cell must continuously resynthesise it. Both roles appear throughout the research literature discussed below.

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NAD+, NADH, NADP+ and NADPH: The Redox Family

Metabolism moves electrons. Breaking down glucose, fatty acids and amino acids releases electrons that must be carried to the mitochondrial electron transport chain, where their energy is used to generate ATP. NAD+ is the principal carrier for that traffic.

• NAD+ — the oxidised form; accepts a hydride (two electrons and a proton).
• NADH — the reduced form; donates those electrons, for example to complex I of the respiratory chain.
• NADP+ / NADPH — a phosphorylated version of the same pair, generated by NAD kinases. NADPH is used mainly for biosynthesis (fatty acids, cholesterol, nucleotides) and for antioxidant systems such as glutathione and thioredoxin recycling.

The ratio of NAD+ to NADH is one of the most cited descriptors of a cell's metabolic state, because it sets the direction of many dehydrogenase reactions. Research on how the NADP pool is generated continues: a 2021 review in Proceedings of the Japan Academy, Series B described a polyphosphate-dependent NAD kinase activity in human mitochondria, which the author framed as a previously missing link in mitochondrial NADP synthesis (PMID 34629356).

Beyond Redox: The NAD+-Consuming Enzymes

The second role of NAD+ is what has driven most of the recent interest in the molecule. Several enzyme families cleave the bond between nicotinamide and the rest of the dinucleotide and use the released ADP-ribose portion in signalling chemistry:

Sirtuins

Sirtuins (SIRT1–SIRT7 in humans) are NAD+-dependent deacylases. They strip acetyl and other acyl groups from lysine residues on histones, transcription factors and metabolic enzymes, and they cannot function without NAD+ as a co-substrate. Because their activity is tied to NAD+ availability, sirtuins are frequently described in the literature as sensors that couple the cell's metabolic state to gene expression and mitochondrial regulation.

PARPs

Poly(ADP-ribose) polymerases attach chains of ADP-ribose to proteins, most prominently as part of the DNA damage response. PARP activation consumes NAD+, which is why sustained genotoxic stress is often discussed as a drain on cellular NAD+ pools.

CD38, CD157 and other NAD+ glycohydrolases

These ectoenzymes degrade NAD+ and NAD+ precursors and generate calcium-mobilising messengers such as cyclic ADP-ribose. CD38 expression is commonly discussed in the context of immune activation and of age-related changes in NAD+ turnover.

How Cells Make NAD+: Three Pathways

Because NAD+ is continuously consumed, cells maintain several routes to rebuild it. Human biochemistry textbooks and reviews describe three:

In most mammalian tissues the salvage pathway dominates, because nicotinamide is released every time an NAD+-consuming enzyme fires. NAMPT is generally described as the rate-limiting step of that recycling loop. Riboside forms enter through a different door: nicotinamide riboside is phosphorylated by nicotinamide riboside kinases to NMN before joining the salvage route.

Compartments and Transport

NAD+ is not a single uniform pool. Distinct concentrations and turnover rates are described for the cytosol, nucleus and mitochondria, and how mitochondria obtain NAD+ has been actively debated. A 2018 study in eLife reported evidence that NAD+ itself is transported into mammalian mitochondria, rather than mitochondria depending exclusively on the import of precursors and local synthesis (PMID 29893687). The researchers framed the finding as relevant to how mitochondrial NAD+ levels are maintained during stress. Compartmentalisation matters for interpretation, because a measurement in whole blood or whole tissue does not necessarily describe the pool available to a particular enzyme.

Unexpected Roles Reported in the Literature

NAD+ research is not limited to energy metabolism. A 2017 report in PNAS identified NAD+-capped mRNAs in Saccharomyces cerevisiae, showing that NAD+ can be installed at the 5′ end of RNA molecules in place of the canonical cap (PMID 28031484). The researchers described this as evidence that the nucleotide participates in RNA biology as well as in enzymology. Separately, a 2019 paper in Science Advances characterised NAD+ acting as a photocatalyst, a chemistry-focused observation about the molecule's light-driven reactivity rather than a claim about human physiology (PMID 31334353). Both illustrate how broad the term "NAD+ biology" has become.

NAD+ and Ageing: What Research Reports

A recurring theme in the literature is that NAD+ levels measured in various tissues fall with age in model organisms, and that experimental restoration changes measurable outcomes in those models. Several recent papers illustrate the shape of that evidence.

A 2025 study in Nature Cancer reported that age-associated NAD+ decline drove failure of CAR-T cells, linking the availability of the coenzyme to the functional performance of engineered T cells in preclinical systems (PMID 40394194). The researchers positioned NAD+ metabolism as a variable in immune cell fitness rather than as a treatment.

In a 2025 ACS Nano report, investigators built lubricated hydrogel microspheres loaded with NAD+ and reported that this three-pronged delivery approach alleviated features of age-related osteoarthritis in the experimental model used (PMID 40315404). That work concerned a locally applied biomaterial, not an ingested compound, and the study's findings were limited to the animal and tissue systems tested.

A second 2025 ACS Nano paper described a strategy combining senescent-cell targeting with recycling of intracellular NAD+, and the researchers reported attenuation of senescence-associated phenotypes in their experimental models (PMID 40911860). Taken together, these reports describe mechanistic and preclinical findings; none of them establishes an outcome in humans.

Precursor Forms Studied in Humans

Because NAD+ itself is a large, charged molecule that is broken down extracellularly, most human research has used precursors. The forms most commonly appearing in the literature are:

Human trials to date have generally been small and short. A 2022 report in npj Aging examined chronic oral NMN in healthy older men and reported that it elevated blood NAD+ levels and altered measures of muscle function relative to the comparison condition (PMID 35927255). The researchers described the trial as exploratory and limited by its small sample.

A 2024 trial in Endocrine Journal evaluated long-term NMN supplementation in healthy, middle-aged Japanese men, assessing safety alongside metabolic parameters, sleep measures and markers of NAD+ biosynthesis (PMID 38191197). The study reported changes in NAD+-related metabolites and examined whether metabolic and sleep outcomes shifted over the supplementation period. Readers comparing such trials should note that outcome panels, participant characteristics and durations differ substantially between them, which is one reason the literature has not converged on a single interpretation.

Safety and Tolerability: What Studies Report

Safety information in this area comes mainly from small, short-duration precursor trials rather than from long-term studies. The 2024 Endocrine Journal report included safety and tolerability among its stated objectives for long-term NMN supplementation in healthy middle-aged men and reported on standard clinical laboratory monitoring alongside its metabolic and sleep endpoints (PMID 38191197). The 2022 npj Aging trial in healthy older men similarly monitored participants during chronic supplementation while measuring blood NAD+ and muscle function (PMID 35927255).

What these reports do not provide is data on long durations, on older or clinically complex populations, on interactions with medications, or on NAD+ administered by non-oral routes in people. Preclinical work such as the NAD+-loaded microsphere study concerned local delivery in animals and cannot be extrapolated to human systemic exposure (PMID 40315404). Questions about safety for any individual belong to a licensed clinician.

Limitations of the Current Evidence

1. Measurement is not standardised. Whole-blood, plasma and tissue NAD+ values are not interchangeable, and compartment-specific pools may move independently of the pool being sampled (PMID 29893687).
2. Raising a metabolite is not an outcome. Trials reporting elevated blood NAD+ after precursor supplementation reported functional measures separately, and the two do not automatically track together (PMID 35927255).
3. Most mechanistic findings are preclinical. Reports on CAR-T failure, osteoarthritis models and senescence phenotypes came from cell and animal systems (PMID 40394194, PMID 40911860).
4. Sample sizes are small. Published human precursor trials have typically involved dozens of participants over weeks to months.

Glossary of Related Terms

• Coenzyme — a non-protein molecule an enzyme needs to work.
• Redox reaction — a reaction transferring electrons between molecules.
• NAMPT — the enzyme converting nicotinamide to NMN; commonly described as rate-limiting in salvage.
• NMNAT1/2/3 — enzymes converting NMN (or NAMN) to NAD+ in different compartments.
• Sirtuin — an NAD+-dependent deacylase family.
• Senescence — a stable cell-cycle arrest state with an altered secretory profile, studied in relation to NAD+ handling (PMID 40911860).

References

• Age-associated nicotinamide adenine dinucleotide decline drives CAR-T cell failure (Nature Cancer, 2025)
• Nicotinamide Adenine Dinucleotide-Loaded Lubricated Hydrogel Microspheres with a Three-Pronged Approach Alleviate Age-Related Osteoarthritis (ACS Nano, 2025)
• A Targeting Senescence and Recycling Intracellular Nicotinamide Adenine Dinucleotide Strategy for Attenuation of Senescence-Associated Phenotypes (ACS Nano, 2025)
• Safety and efficacy of long-term nicotinamide mononucleotide supplementation on metabolism, sleep, and nicotinamide adenine dinucleotide biosynthesis in healthy, middle-aged Japanese men (Endocrine Journal, 2024)
• Chronic nicotinamide mononucleotide supplementation elevates blood nicotinamide adenine dinucleotide levels and alters muscle function in healthy older men (npj Aging, 2022)
• Nicotinamide adenine dinucleotide is transported into mammalian mitochondria (eLife, 2018)
• Polyphosphate-dependent nicotinamide adenine dinucleotide (NAD) kinase: A novel missing link in human mitochondria (Proceedings of the Japan Academy, Series B, 2021)
• Identification of NAD+ capped mRNAs in Saccharomyces cerevisiae (PNAS, 2017)
• Nicotinamide adenine dinucleotide as a photocatalyst (Science Advances, 2019)