NAD+: The Coenzyme Researchers Are Racing to Understand

For Research Use Only (RUO). This content is written for scientific and research audiences. Nothing herein constitutes medical advice or a therapeutic claim.

What is NAD+ and NAD plus?

Nicotinamide adenine dinucleotide, abbreviated NAD+ (also written as "NAD plus") is a coenzyme found in every living cell. It exists in two primary forms: the oxidized form (NAD+) and the reduced form (NADH), and this redox cycling sits at the heart of cellular energy metabolism.

From a biochemistry standpoint, NAD+ is one of the most critical molecules in the body. It functions both as an electron carrier in metabolic reactions and as a substrate for a broad class of signaling enzymes, including sirtuins, PARPs (poly-ADP-ribose polymerases), and CD38 glycohydrolases. These enzymes mediate DNA repair, epigenetic gene regulation, inflammatory response, and mitochondrial function.

A 2019 study examining plasma samples from 67 healthy subjects aged 20–87 years confirmed a significant, age-associated decline in circulating NAD+, NADP+, and related metabolites establishing that reduced NAD+ bioavailability is a consistent feature of normal human aging, not simply disease states. (PMC6482912)

Why NAD+ declines with age

Researchers have identified three converging processes that deplete NAD+ as organisms age:

1. Reduced biosynthesis

The salvage pathway which recycles nicotinamide (NAM) back into NAD+ via the enzyme NAMPT is responsible for approximately 85% of NAD+ production in most tissues. With age, NAMPT activity decreases, limiting the throughput of this recycling loop. (PMC11205942)

2. Increased consumption by CD38

CD38, an NAD-consuming glycohydrolase expressed on immune cells, increases dramatically with age. Research published in Cell Metabolism (Camacho-Pereira et al., 2016) demonstrated that CD38 knockout mice maintain youthful NAD+ levels into old age. (PMID: 27304511)

A 2020 study in Nature Metabolism showed that senescent cells and their inflammatory secretions drive accumulation of CD38+ immune cells in adipose tissue, creating a feedback loop that progressively depletes NAD+ in aging organisms. (PMID: 33199925)

3. Chronic DNA damage activating PARPs

PARP enzymes, which detect and initiate repair of DNA strand breaks, are major consumers of NAD+. As DNA damage accumulates with age, PARP hyperactivation further depletes the available NAD+ pool. (PMC10776128)

About NAD peptides and precursor compounds

In the research context, the term "NAD+ peptide" typically refers to compounds — primarily NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside) — that serve as direct biosynthetic precursors to NAD+. These molecules are studied for their capacity to restore intracellular NAD+ levels.

Are NAD peptides true peptides in the chemical sense? Technically, NMN and NR are nucleotides, not peptides. However, within the research community, "NAD+ peptide" has become a colloquial term for these small-molecule NAD+ precursors.

A landmark 2025 human trial published in Nature Metabolism (Christen et al.) directly compared three oral NAD+ precursors NMN, NR, and nicotinamide (Nam) in 65 adults over 14 days. Both NMN and NR approximately doubled circulating blood NAD+ levels, while nicotinamide supplementation had no statistically significant effect.

NMN is considered to have higher chemical and biological stability than NAD+ itself, enabling more efficient cellular uptake. It can enter cells directly through the Slc12a8 transporter, or be synthesized from NR via NR kinase (NRK). (PMC12561839)

Key mechanisms under investigation

Sirtuin activation

Sirtuins (SIRT1–SIRT7) are a family of NAD+-dependent enzymes that regulate transcription, DNA repair, inflammation, energy metabolism, and circadian rhythm. Since sirtuins require NAD+ as a co-substrate, declining NAD+ levels directly reduce sirtuin activity across all seven family members. SIRT1 modulates p53, NF-κB, and PARP1, while mitochondrial sirtuins (SIRT3, 4, 5) govern oxidative stress and lipid metabolism. (PMC6206880)

PARP-mediated DNA repair

PARP1 is a primary sensor of DNA single-strand breaks, consuming NAD+ to generate poly-ADP-ribose chains that recruit repair machinery. Researchers study NAD+ availability as a limiting factor in this process, especially given the competition between PARP and sirtuins for a shared NAD+ pool.

Mitochondrial function and energy metabolism

In the mitochondria, NAD+ is central to the electron transport chain — the primary mechanism of ATP synthesis. Preclinical NMN studies in aged mice have shown improvements in mitochondrial oxygen consumption rate and skeletal muscle function after supplementation. (PMC9158788)

Skin biology and cellular senescence

A 2025 in vitro study using human skin fibroblasts found that NMN significantly elevated cellular NAD+ levels, activated sirtuin and autophagy pathways, enhanced mitochondrial function, suppressed cellular senescence markers, and accelerated wound healing. (PMC12561839)

NAD+ research findings: men

An emerging body of evidence suggests NAD+ metabolism has important implications for male health research:

• NMN supplementation improved muscle insulin sensitivity and glucose metabolism in a Tokyo University trial of older males over 12 weeks (Yamaguchi et al., 2022; PMC9158788)
• Chronic NMN elevated blood NAD+ concentrations and was associated with changes in muscle function and physical performance metrics in older male subjects
• Male mice showed significant metabolic health improvements — improved glucose tolerance and reduced fat accumulation — with long-term NMN treatment
• Sirtuin activity improvements linked to NAD+ restoration appear to influence pathways relevant to mitochondrial SIRT3 in preclinical male models

NAD+ research findings: women

Research increasingly highlights sex-specific patterns in NAD+ biology that are particularly relevant to women's health research:

• Female mice showed an 8.5% increase in median lifespan with long-term NMN treatment, a statistically significant benefit not replicated at the same magnitude in males (PMC10738409)
• Female animals show greater Preiss-Handler pathway activity — an alternative NAD+ biosynthesis route which may partly explain observed sex differences in response
• NMN oral supplementation improved insulin sensitivity in obese women with prediabetes (250 mg/day), with improved insulin signaling and muscle remodeling gene expression
• NAD+ levels have been independently associated with anaemia risk in community-based women's health studies
• NR supplementation studies suggest brain health benefits relevant to neurodegenerative disease, a class of conditions that disproportionately affects women

"A thorough characterization of NMN metabolism across age, sex and tissues shows context-specific sex differences, including greater Preiss-Handler pathway metabolism in females than males, which may contribute to observed sex differences in health and lifespan." Kane et al., Aging Cell, 2023 (PMC10738409)

The clinical trial landscape

Human clinical research on NAD+ precursors has accelerated significantly since 2020. A systematic review of randomized clinical trials published between 2020 and 2025 found that various doses and durations of NR and NMN supplementation are consistently well-tolerated and reliably increase whole-blood NAD+ levels in healthy middle-aged and older adults. (Geromedicine, 2025)

A placebo-controlled trial evaluating 1,250 mg/day of β-NMN in 31 healthy adults aged 20–65 over four weeks found no clinically significant adverse events and no concerning changes in hematological, biochemical, urinary, or body composition parameters. (PMC9400576)

Even when circulating NAD+ increases substantially, translating those increases to tissue-level outcomes particularly in the brain, heart, and adipose tissue requires further investigation. The question of whether peripheral blood NAD+ measurements accurately reflect intracellular NAD+ in target tissues remains unresolved.

Research goals and open questions

Established research accomplishments

• Confirmed that NAD+ decline is a consistent and measurable hallmark of human aging
• Identified CD38 as the primary driver of age-related NAD+ depletion in peripheral tissues
• Demonstrated safety and tolerability of NMN and NR supplementation in multiple human clinical trials
• Confirmed that oral NMN and NR reliably double circulating blood NAD+ in humans
• Documented sex-specific differences in NAD+ metabolism with implications for precision longevity research

Active research frontiers

• Developing strategies to improve NAD+ precursor delivery to brain, heart, and adipose tissue
• Investigating combination approaches: precursor supplementation + CD38 inhibition + senolytic strategies
• Conducting larger, longer-duration trials to determine clinical significance of NAD+ restoration on cognition, cardiovascular outcomes, and longevity markers
• Exploring NAD+ dysregulation in specific disease contexts: neurodegeneration, metabolic syndrome, cardiovascular disease, and cancer
• Understanding the role of the gut microbiome as both a modulator of and response indicator for NAD+ interventions

Peer-reviewed references

1. Fukamizu Y, et al. Safety evaluation of β-NMN oral administration. Scientific Reports. 2022. PMC9400576
2. Yamaguchi S, et al. Chronic NMN supplementation in healthy older men. npj Aging. 2022. PMC9158788
3. Kane AE, et al. Long-term NMN treatment in mice, sex-dependent lifespan effects. Aging Cell. 2023. PMC10738409
4. Chini CCS, et al. NAD metabolism: Role in senescence regulation. Aging Cell. 2024. PMC10776128
5. Camacho-Pereira J, et al. CD38 dictates age-related NAD decline. Cell Metabolism. 2016. PMID: 27304511
6. Covarrubias AJ, et al. CD38 ecto-enzyme in aging and NAD+ regulation. Nature Metabolism. 2020. PMID: 33199925
7. Massudi H, et al. The plasma NAD+ metabolome in aging. PLoS One. 2012. PMC6482912
8. Mei Z, et al. Sirtuins in metabolism, DNA repair and cancer. J Exp Clin Cancer Res. 2016. PMID: 27916001
9. Dantowitz M, et al. NMN supplementation: metabolic variability. PMC. 2024. PMC11205942
10. Christen C, et al. Human trial comparing three NAD+ precursors. Nature Metabolism. 2025.
11. Uchida A, et al. NMN gene expression profiles in skin fibroblasts. PMC. 2025. PMC12561839
12. Okabe K, et al. Clinical evidence for NAD+ precursors to slow aging. Geromedicine. 2025.

Disclaimer: This article is intended for research audiences only. All compounds referenced are for research use only (RUO) and are not approved for human therapeutic or diagnostic use. Nothing in this article constitutes medical advice. Researchers must comply with all applicable regulations governing the use of research compounds.