NAD+ has become an important area of research into longevity due to its links with cellular ageing, metabolism, and lifespan outcomes observed in animal models. Some of this research is of significant scientific importance, particularly at the mechanistic and preclinical level. However, evidence indicating that NAD+ affects ageing-related pathways in animals is not the same as evidence that NAD+ supplementation reverses ageing in humans. Throughout the article, we maintain this distinction, and also discuss how fasting fits into NAD+ biology, as fasting and caloric restriction are among the best-characterised metabolic states associated with changes in NAD+-dependent pathways.
Why do NAD+ levels decrease with age?
The age-related decline in NAD+ levels is one of the most consistently reported findings in this area of research. Lower NAD+ levels have been observed in the blood and numerous tissues with advancing age in animal models, and increasingly in human studies [1]. Several interrelated mechanisms may contribute to this decline.
One of the proposed mechanisms involves increased consumption of NAD+ by CD38. CD38 is an enzyme that degrades NAD+, and its activity appears to increase with age. This may be partly due to the accumulation of senescent cells, which have stopped dividing but remain metabolically active and can contribute to inflammatory signalling, as well as an increase in the number of immune cells expressing CD38.
Research on mice lacking the CD38 gene has shown that CD38 activity significantly contributes to the age-related loss of NAD+ and the accompanying mitochondrial dysfunction. These effects appear to involve, at least in part, pathways related to the NAD+-dependent mitochondrial enzyme SIRT3 [2].
The second mechanism is linked to accumulating DNA damage and increased NAD+ consumption by PARP enzymes. DNA damage builds up over time, and PARP proteins take part in DNA repair, using NAD+ as a substrate. Greater and more sustained repair activity can therefore increase NAD+ consumption [1].
Age-related changes in NAD+ synthesis may also contribute to this process. Some studies suggest that the activity of the NAD+ salvage pathway, including pathways regulated by the NAMPT enzyme, may decrease with age in certain tissues. If this is the case, NAD+ availability may be limited from two sides: through greater consumption and poorer regeneration [1].
Together, these mechanisms describe a biological situation in which ageing cells can produce or recover NAD+ less efficiently, while simultaneously consuming more of it. Such a combination may partly explain the gradual decline in NAD+ levels observed with age.
Does NAD+ supplementation really have the ability to „reverse” some markers of ageing?
The term „reversing ageing” must be separated from more specific and measurable research outcomes.
Broadly understood reversal of ageing, that is biological human rejuvenation across multiple organ systems and ageing-related processes, has not been demonstrated in controlled human clinical trials. No published human study has shown that NAD+ supplementation reverses ageing as a whole-body phenomenon.
Research into single biomarkers and biological processes related to ageing is more complex.
Animal studies have yielded significant results. In one influential mouse study, nicotinamide riboside increased NAD+ levels and activated the mitochondrial unfolded protein response, a cellular stress response pathway. The treatment was also associated with improved mitochondrial and muscle stem cell function, as well as an extension of the lifespan of the treated animals [3].
These results have significantly increased interest in NAD+ precursors in the biology of ageing. However, they stem from an animal model. This kind of life extension has not been demonstrated in humans, and studies using human lifespan as an endpoint would require very long periods of observation.
Early human studies analysed more specific biological and clinical parameters. In a phase I study involving people with Parkinson's disease, oral nicotinamide riboside increased brain NAD+ levels and was associated with minor clinical changes and changes in biomarkers related to inflammation and disease processes [4].
This represents an early signal from human research in a specific disease population. However, it should not be interpreted as evidence of a general anti-ageing effect in healthy individuals. This was an early-phase study, focusing primarily on safety, feasibility, biological activity and exploratory clinical outcomes, rather than proving that NR modifies the ageing process or reverses disease.
The most precise summary is therefore that NAD+ supplementation affected several molecular and cellular processes associated with ageing in animal models, whereas early human studies showed biological effects in selected situations. The widely understood reversal of ageing in humans has not been confirmed.
Fasting and NAD+: what is the connection really about?
The link between fasting and NAD+ has biologically plausible and relatively well-characterised mechanistic foundations.
One of the important pathways is AMPK, which is AMP-activated protein kinase. AMPK acts as a cellular energy sensor and becomes more active when cellular energy availability drops, such as during fasting and caloric restriction.
AMPK activation can affect NAMPT, an important enzyme in the NAD+ salvage pathway. Increased NAMPT activity may contribute to changes in NAD+ availability and the NAD+/NADH ratio, which in turn can affect NAD+-dependent enzymes such as SIRT1 [1].
This forms an interconnected metabolic pathway involving cellular energy status, AMPK signalling, NAD+ recycling, and sirtuin activity. Animal studies on caloric restriction have shown that these metabolic adaptations may help to preserve or modify pathways associated with NAD+, which otherwise change with age [1].
This relationship therefore has a stronger mechanistic basis than many generalised claims that a particular lifestyle element „increases NAD+”.
However, significant uncertainties remain when translating these findings to humans. Much of the detailed mechanistic research has been conducted in animals or cell models. Human studies directly measuring NAD+ changes in blood or tissues before and after strictly controlled fasting protocols remain limited.
As a result, current data do not allow us to determine the exact post length, its frequency, or the degree of calorie restriction that would reliably cause a specific increase in NAD+ in humans. The direction of the biological relationship is fairly well supported, but the magnitude, timing, and clinical significance of this effect in humans are less clearly defined.
Sirtuins, calorie restriction and NAD+ — how are they linked?
Sirtuins represent an important mechanistic link between caloric restriction, fasting, and NAD+ metabolism.
Sirtuins require NAD+ as a substrate. Because fasting and caloric restriction can alter cellular energy-sensing mechanisms and the NAD+/NADH ratio through pathways involving AMPK and NAMPT, these metabolic states may also influence sirtuin activity [1].
This relationship is one of the reasons why sirtuins have been extensively studied in the biology of caloric restriction. In many animal models, calorie restriction has been associated with a longer lifespan or delayed development of certain age-related changes. Sirtuins have been analysed as one of the possible groups of mediators linking altered nutrient availability with subsequent effects on mitochondrial function, stress resistance, metabolism and gene regulation.
Conceptually, the relationship can be summarised in this way: calorie restriction affects NAD+-related metabolism, which in turn can influence NAD+-dependent sirtuin activity. However, this pathway should not be interpreted as a simple linear sequence in which one intervention automatically reproduces all downstream effects of another.
Supplementation with NAD+ precursors can, under certain conditions, increase the availability of substrates associated with NAD+, but this does not mean it replicates the entire physiological state associated with fasting or caloric restriction.
Fasting induces broad metabolic and hormonal changes involving insulin, glucagon, fatty acid metabolism, ketone body production, AMPK, mTOR signalling, nutrient sensing and other pathways. Increasing NAD+ through supplementation only affects a part of this wider biological network.
For this reason, NAD+ precursor supplementation and fasting should not be treated as interchangeable strategies. Their mechanisms partially overlap, but they are not biologically equivalent.
How does NAD+ compare to other compounds promoted in the context of longevity?
NAD+ precursors belong to several groups of compounds often discussed in ageing and longevity research. Others include resveratrol, pterostilbene, metformin and rapamycin, although these substances act via different biological pathways and have very different evidentiary bases and regulatory contexts.
Resveratrol and pterostilbene are often discussed alongside NAD+ precursors because all of these compounds have been studied in the context of sirtuin biology. NAD+ is required as a substrate for sirtuins, whereas resveratrol and related compounds have been investigated for their potential effects on sirtuin-related signalling.
This mechanistic overlap has contributed to interest in combinations of NAD+ precursors with related polyphenols. One example is the combination of nicotinamide riboside with pterostilbene analysed in human studies [5]. However, the mere existence of a mechanistic rationale or a combination study does not prove that such combinations extend lifespan or provide broader anti-ageing effects in humans.
Other compounds commonly associated with longevity research affect other pathways in part. Metformin is often discussed in the context of AMPK and metabolic signalling, whereas rapamycin acts primarily by inhibiting mTOR. These pathways may intersect with the biology of NAD+ and sirtuins, but they should not be regarded as interchangeable mechanisms.
Compared to many compounds discussed in a commercial longevity context, oral NAD+ precursors such as NR and NMN have amassed a growing base of randomised, placebo-controlled human trials indicating that they can increase NAD+ levels or NAD+-related metabolites in the blood and are generally well tolerated over the short- and medium-term periods studied [6].
However, evidence that these compounds increase NAD+ biomarkers is not equivalent to evidence that they extend life, prevent ageing or provide broad anti-ageing effects in humans.
The same distinction applies to other compounds associated with longevity. None of the substances discussed here should be described as having definitively shown an extension of human life based on currently available clinical research.
A separate area of research is companion animals. In one randomised, controlled study, the combination of a novel NAD+ precursor and a senolytic compound was evaluated in older dogs with mild to moderate cognitive impairment. The study demonstrated a statistically significant association with an improvement in cognitive function as assessed by owners in the primary endpoint, as well as broader changes in parameters such as frailty, activity and reported satisfaction level, which were less statistically conclusive [7].
This result has translational significance, as relatively few controlled studies have evaluated NAD+-related interventions specifically in ageing companion animals. However, this was a single study, using a combination intervention rather than NAD+ alone, and improvements were observed across all treatment groups. The researchers therefore highlighted the possibility that some of the reported changes may have been influenced by placebo-like effects, owner expectations, increased attention or study participation itself.
What can realistically be expected, and what cannot?
Based on current evidence, several conclusions can be considered relatively well substantiated.
NAD+ levels appear to decline with age across many tissues and biological conditions, and several mechanisms have been proposed to explain this decline, including increased CD38 activity, greater NAD+ consumption during DNA repair, and possible alterations in NAD+ synthesis and salvage pathways [1,2].
Fasting and calorie restriction also have a biologically plausible and relatively well-supported link to NAD+ metabolism through pathways involving AMPK, NAMPT, the NAD+/NADH balance, and sirtuins [1].
Human studies have repeatedly shown that oral NAD+ precursors, such as NR and NMN, can increase circulating NAD+ levels or related metabolites under specific research conditions [6].
Animal studies have also demonstrated changes in mitochondrial function, stem cell activity, cellular stress responses and lifespan following the administration of NAD+ precursors in selected models [3]. Early human studies have identified changes in NAD+ biomarkers and exploratory clinical parameters in specific populations with diseases [4].
These results do not prove that NAD+ supplementation reverses ageing in humans.
Current data also do not show that NAD+ precursor supplementation extends human lifespan, nor that it reproduces all the physiological effects of fasting or caloric restriction.
Similarly, an increase in a biomarker associated with the biology of ageing does not in itself prove an improvement in healthspan, a reduction in biological age, a slowing of ageing, or an extension of lifespan.
This area remains an active and important direction of scientific research, but currently the strongest evidence concerns mechanisms, biomarker changes and selected experimental outcomes, rather than broad anti-ageing effects in humans.
Limitations of current evidence
- The strongest results linking NAD+ supplementation to lifespan extension, improved mitochondrial function and changes in the biology of ageing come mainly from animal studies, particularly mouse models. Human studies have shown biomarker changes and safety data in the short and medium term, but have not confirmed lifespan extension or comprehensive anti-ageing effects [3].
- Results concerning animal lifespans cannot be automatically translated to humans, as species differ in metabolism, lifespan, disease development, NAD+ biology, dosage, and experimental conditions.
- The link between fasting, calorie restriction, and NAD+ metabolism has mechanistic support, but the exact dose-response relationship in humans remains poorly defined. It is not yet known how long and how frequent a fast, nor what degree of calorie restriction would be required to induce a specific change in NAD+ levels in humans [1].
- Human studies directly measuring NAD+ in specific tissues before and after standardised fasting protocols remain limited, so mechanistic conclusions should not be translated into precise fasting recommendations.
- Human clinical trial evidence regarding NAD+ precursors in specific diseases, including Parkinson's disease, remains at an early stage and should not be interpreted as evidence of broad anti-aging or disease-modifying effects [4].
- Increasing the level of NAD+ or related biomarkers does not automatically mean an improvement in biological age, healthspan, lifespan, physical fitness, cognitive function, or disease risk.
- NAD+ precursor supplementation should not be considered biologically equivalent to fasting or caloric restriction, because fasting induces numerous metabolic, hormonal and signalling changes that go beyond NAD+ metabolism.
- Evidence for combinations, such as NR with pterostilbene, does not confirm that NAD+ precursors combined with resveratrol or other longevity-associated compounds provide additive or stronger anti-ageing effects [5].
- Comparisons with metformin, rapamycin, resveratrol or other longevity-associated compounds are mechanistic in nature and do not constitute proof that these interventions provide equivalent or interchangeable effects.
- The discussed study on cognitive function in dogs is a single controlled study using a combined intervention rather than NAD+ alone. Improvement was also noted across different treatment groups, which complicates the interpretation of the intervention-specific effect [7].
- Long-term human safety and clinical outcome data remain significantly more limited than short-term biomarker studies, particularly when NAD+ precursors are used for years rather than weeks or months.
Disclaimer
The article is exclusively educational and summarises scientific research. It does not constitute medical advice for humans or veterinary advice regarding animals.
NAD+, NAD+ precursors, fasting, calorie restriction, resveratrol, pterostilbene, metformin, rapamycin or any other compounds discussed in the context of longevity should not be interpreted as proven methods for reversing ageing, extending human lifespan, preventing diseases or treating medical conditions, unless there is a specific approved indication and appropriate clinical evidence.
NAD+ and its precursors should not be presented as FDA- or EMA-approved treatments for ageing, nor as therapies that have been shown to extend human life.
Fasting and calorie restriction are not suitable for everyone and may carry additional risks for certain individuals. Pregnant or breastfeeding women, people with diabetes, those taking glucose-lowering medications, individuals with a history of eating disorders, those who are underweight, people with certain chronic conditions, or those taking medications whose effects depend on food intake should discuss fasting or significant calorie restriction with a qualified healthcare professional.
Before starting a new supplement, fasting regimen, medication, or any other intervention intended to influence ageing processes, you should consult a qualified healthcare professional. Questions regarding pets should be consulted with a qualified veterinarian.
References
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