The Rising Star of Anti-Aging: NAD+ A Comprehensive Introduction to its Synthesis Pathways
In recent years, increasing evidence suggests that a decline in NAD+ levels contributes to age-related pathophysiology [1,2,3]. The systemic reduction of NAD+ is caused by both a decrease in biosynthesis rate and an increase in NAD+ consumption.
NAD+ is crucial in cellular redox reactions, including most catabolic and anabolic reactions, such as glycolysis, fatty acid β-oxidation, tricarboxylic acid cycle, and the synthesis of fatty acids, cholesterol, and steroids [4,5,6].
There are bunch of enzymes, such as deacetylases, poly(ADP-ribose) polymerases (PARPs), cADP-ribose synthases (CD38/157 ectoenzymes) [7,8,9], and mono-ADP-ribosyltransferases (ARTs), contribute to the overall consumption of NAD+.
NAD+ can be synthesized from tryptophan (Trp), a process mediating NAD+ de novo synthesis, or replenished through salvage pathways using its four precursors: nicotinamide (NAM), nicotinic acid (NA), nicotinamide riboside (NR), and nicotinamide mononucleotide (NMN). The de novo synthesis from Trp requires eight steps. The salvage pathways (Preiss–Handler pathway) for NA and NAM require three or two steps (Figure 1).
Nicotinamide riboside (NR) is another salvageable NAD+ precursor that forms NAD+ through a two-step [10] or three-step pathway [11] (Figure 1). In mammals, the most common precursor is NAM, which can form NMN through the catalytic action of phosphoribosyltransferase (NAMPT), the rate-limiting enzyme in NAD+ synthesis [12]. Finally, NMN is converted to NAD+ by NMN/NaMN adenylyltransferases (NMNATs) [13,14].
The aging process or excessive nutrient intake can lead to a decrease in NAD+ content and NAMPT expression in various tissues [3,15,16,17], while the maintenance of NAD+ levels depend on different biosynthetic pathways and precursors in different tissues [18,19].
Nevertheless, the decreased expression of the NAMPT enzyme is one of the main reasons for the age-related decline in NAD+ [15,20]. NR can bypass the need for NAMPT by being directly converted to NMN through two nicotinamide riboside kinases, NMRK1 and NMRK2 [10].
This also bypasses the massive energy-consuming PRPP (phosphoribosyl pyrophosphate, Figure 2) and the feedback inhibition of NAD+ [21]. Alternatively, NR can be converted to NAM by purine nucleotide phosphorylase (NP) and then to NAD+ by NMNAT (Figure 1). Therefore, the utilization of NR depends on the expression of the Nrk pathway or the NP and NAMPT pathways.
Three biosynthetic pathways of NAD+
Activation of the NMRK2 pathway as a common coping mechanism for low NAD+ levels in failing hearts (less energy-consuming)
NAD+-consuming enzymes---regulators of aging and longevity
The importance of NAD+ is also reflected in the activity of NAD+-consuming enzymes, which are mediators of aging, mainly induced by stressors such as DNA damage, oxidative stress, and inflammation. The main downstream mediators, deacetylases/deacylases, are all NAD+-dependent.
Deacetylases are conserved regulators of aging and lifespan in various organisms and are considered as major metabolic switches [22] due to their numerous regulatory functions in metabolism, DNA repair, stress response, chromatin remodeling, and circadian rhythm (Table 1) [2,23].
Together with deacetylases, PARPs use NAD+ to produce chains of ADP-ribose (ADPR) molecules. PARP1 and PARP2 respond to DNA breaks in the nucleus and promote DNA repair processes [23]. Over time, DNA damage accumulation leads to the activation of PARPs, which in turn reduces the activity of SIRT1 for the competition for NAD+, plus PARP2 can directly inhibit the expression of SIRT1 [24].
Moreover, the content of CD38, the main NAD-consuming enzyme in mammals, increases with age. It can function as glycohydrolase or ADP-ribosyl cyclase, hydrolyzing NAD+ to NAM and ADP-ribose (ADPR), or NAM and cADPR [25,26,27].
CD38 can also degrade NR and NMN [28,29], further reducing NAD+ content [30]. In CD38-deficient mice, NAD+ is maintained at high levels, while mitochondrial and metabolic functions are protected during aging [31]. Furthermore, inhibiting CD38 can increase NAD+ levels and improve glucose and lipid metabolism [32].
The central roles of Sirtuins in DNA repair, cell cycle, and mitochondrial function
The enormous involvement of NAD+ in all these processes indicates that supplementation with NAD+ precursors have significant potential for treating various pathophysiological conditions. We have several supplementation options, however, the optimal precursor and dosage for specific conditions remain unclear.
Normally, dietary tryptophan or less than 20 mg of niacin (NA, NAM, but niacin can cause skin flushing, tingling, and burning reactions) per day is sufficient.
However, increasing evidence suggests that substantial increases in NAD+ synthesis can be achieved through precursor supplementation, leading to many beneficial and even therapeutic effects. These intermediates are present in various foods, including meat, eggs, dairy products, certain vegetables, and whole wheat products [35,36].
NR is the third natural NAD+ precursor in milk and has been marketed as a nutraceutical. Oral NR has been shown to increase NAD+ levels in multiple tissues while increasing SIRT activity [10,11], improving mitochondrial function [37], and enhancing the regenerative potential of stem cells [38].
Moreover, NR is currently considered a superior precursor compared to other NAD+ precursors because it has not been found to have serious side effects or flushing [14,39]. NR chloride has been granted GRAS status (generally recognized as safe), further supporting its rapid implementation as a drug-like therapy.
However, to adapt NR technology for therapeutic application, it is necessary to determine oral availability, therapeutic doses, and utilization in different tissues.
Effects of NR on Metabolism and Aging Physiopathology
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Insulin Sensitivity and Liver Function
NAD+ plays a crucial role in metabolism, and evidence suggests that increasing NAD+ levels can have beneficial effects on metabolic disorders such as type 2 diabetes (T2D), metabolic syndrome, and non-alcoholic fatty liver disease (NAFLD) [14,41,42].
Moreover, NR is not only one of the intermediates of NAD+, but also a precursor of NADH, as well as in hepatic tissue [14].
As the hepatic NAD+ metabolome is considered functional in prediabetic (PD) and T2D mouse models, NADP+ and NADPH can be used to assess disease progression. Specifically, both NADP+ and NADPH are important for combating oxidative stress, and NADPH is considered a major factor in insulin resistance [42]. In PD and T2D mice, hepatic NADP+ and NADPH levels are significantly reduced, and NR supplementation can restore these levels [41].
In mouse models, NR can increase NAD+ metabolism, thereby improving glucose tolerance, reducing weight gain, and providing neuroprotection against diabetic neuropathy and hepatic steatosis [41]. Similarly, in a high-fat-induced obese mouse model, a daily intake of 400 mg/kg of NR has been shown to improve insulin sensitivity and protect mice from weight gain [37].
However, these results have not yet been repeated in humans, as 12 weeks of NR supplementation at a dose of 2000 mg/day did not improve insulin sensitivity and other metabolic parameters in insulin-resistant obese men [43]. Further research is needed to determine the long-term effects of NR on insulin sensitivity [43].
On the other hand, evidence suggests that NR administration in mouse models increases the activity of SIRT1, which is an important factor in preventing T2D and maintaining insulin sensitivity [37,44].
Furthermore, SIRT1 inhibits the effects of oxidative stress in T2D mice [45], promotes glucose-stimulated insulin secretion in pancreatic β cells [46,47], and protects peripheral tissues from insulin resistance [48], while SIRT1 overexpression promotes fatty acid oxidation and inhibits lipogenesis, protecting the liver from steatosis.
By activating SIRT1 and other factors affecting cholesterol homeostasis, NR may lower cholesterol levels and improve liver health [51,52,53,54]. Fat accumulation may also be reduced through a mechanism that induces the mitochondrial unfolded protein response [44]. In regenerating livers, NR reduces fat accumulation, promotes hepatocyte replication, and increases hepatic ATP content, resulting in faster recovery of liver weight in mice [55].
Furthermore, dietary supplementation with NR can restore the decline in NAD+ levels caused by impaired biosynthesis in a mouse model of hepatocellular carcinoma, thereby preventing DNA damage and tumor occurrence [56].
Overall, there is sufficient evidence to suggest that increasing NAD+ content through NR supplementation can be considered a promising therapeutic strategy for treating metabolic dysfunction, including T2D and NAFLD.
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Cardiovascular Disease
Mitochondrial dysfunction leading to NAD+ homeostasis imbalance plays a central role in the development of cardiac hypertrophy and heart failure (HF) [57,58,59,60,61,62].
Moreover, during the development of HF, there is often a shift from fatty acid oxidation and oxidative phosphorylation to other forms of substrate metabolism (such as glycolysis and ketone oxidation) [63,64], while the NAD+/NADH ratio also decreases [64]. Changes in redox capacity further increase the heart's sensitivity to stress.
Protein hyperacetylation levels also increase, and NAD+-dependent deacetylation is reduced in mouse models of cardiac hypertrophy and in human patients with ischemic HF or dilated cardiomyopathy (DCM) [64,65].
NR supplementation can normalize the cardiac NAD+/NADH ratio and shows protective effects in adverse cardiac remodeling, while long-term supplementation can increase nucleocytoplasmic protein acetylation by stimulating citrate and acetyl-CoA metabolism as well as antioxidant gene expression [66].
In several models of cardiac injury, the NAMPT enzyme is inhibited [62], while Nmrk2 expression is dramatically upregulated [66]. Similar changes have also been observed in models of cardiomyopathy [66]. The activation of the NMRK2 pathway represents a common adaptive mechanism in heart failing, and the Nmrk2 gene can be activated in response to NAMPT inhibition.
Furthermore, the shift from NAMPT to NMRK2 for NAD+ synthesis is an energy-saving mechanism that may be favored because NMRK enzymes require only one ATP to synthesize NMN from NR, while synthesis from NAM by NAMPT requires more than three ATP (Figure 2).
Moreover, strong beneficial effects of NR have been found in mouse models of HF, which are associated with the maintenance of NAD+ levels in the heart. This further suggests that oral NR supplementation is a powerful approach to maintain cardiac function and limit remodeling in DCM（what is DCM?）.
NAD+ precursor supplementation may also protect against adverse cardiac remodeling through other mechanisms, such as activating SIRTs (Table 2) and maintaining Ca2+ homeostasis [67].
Specifically, in vivo activation of SIRT1 can protect against cardiac hypertrophy, metabolic disturbances, and cardiac inflammation in mouse models of cardiac hypertrophy and shows protective effects in other models of cardiac dysfunction [68,69,70,71].
Furthermore, the cardioprotective functions of SIRT2 and SIRT6 are quite prominent [72,73], as SIRT2 deficiency exacerbates cardiac hypertrophy in mice [73], and loss of SIRT6 in mice leads to the development of cardiac hypertrophy and HF [74].
The activity of mitochondrial SIRTs is associated with cardiac remodeling and the development of HF. For example, SIRT3 appears to be essential for maintaining cardiac function [75].
Further data suggest that NR is the premier NAD+ precursor in mitochondria [76], and the in vivo effects of NR are interpreted to be dependent on mitochondrial sirtuin activity [37,77].
However, the importance of nucleocytoplasmic targets should not be excluded [1,78]. The NR-induced reduction in systolic blood pressure (SBP) and aortic stiffness [39], both important risk indicators for cardiovascular function and health [79,80], may occur due to the activation of nuclear and cytoplasmic SIRT1 (Table 1).
The role of Sirtuins in the development of heart failure
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Neurodegenerative Diseases
Neurodegenerative diseases are associated with DNA damage and oxidative stress, which accumulate with age [85], leading to impaired mitochondrial function [86].
Moreover, NAD+ depletion has been observed in the aging process of various animals, including humans, and is considered a major risk factor for Alzheimer's disease (AD) [87]. When NR is administered in mouse models of AD, it shows beneficial effects in both oxidative stress and DNA repair by increasing NAD+ levels [88].
Furthermore, NR can also improve other aspects of AD neuropathology, including pTau, β-amyloid, neurogenesis, neuroinflammation, hippocampal synaptic plasticity, and cognition [89,90].
Specifically, NR treatment can reduce neuroinflammation and amyloid generation in the whole brain of high-fat diet (HFD)-fed mice by lowering β-amyloid levels and several inflammatory markers (NLRP3, CASP1, IL-1, TNF-α, and IL-6) [91].
As brain inflammation is closely associated with cognitive impairment [92,93,94], NR treatment can slightly recover cognitive function and recognition memory in as just 6 weeks [91].
Moreover, NR supplementation can reduce increased PARylation in AD mice [90]. Increased PARylation has also been detected in several other neurodegenerative diseases, such as Cockayne syndrome, xeroderma pigmentosum, and ataxia telangiectasia, all of which involve DNA repair defects.
Despite the presence of DNA repair defects, NR can significantly improve the phenotypes of these diseases in mouse models, and in mice with ataxia, NR can extend survival by more than three-fold [95,96].
Axonal degeneration is another early event in acute brain injury and chronic neurodegenerative diseases, including Alzheimer's and Parkinson's diseases [97,98,99,100]. In this context, axonal degeneration is caused by excitotoxicity, another feature involved in most neurodegenerative diseases affecting the central nervous system.
During excitotoxicity test, NAD+ is significantly reduced in neurons [101,102,103,104], while mice injected with NR are protected from excitotoxicity-induced axonal degeneration [105]. Among the three NAD+ precursors tested (NA, NAM, and NR), including NAD+, only NR can prevent axonal degeneration by altering local NR metabolism within axons [105,106].
Specifically, NR prevents nuclear condensation and axonal degeneration in neurons of Nmrk2-KO mice by inducing Nmrk1 [105], suggesting that Nmrk1 may be a key mediator of the neuroprotective activity of NR. This neuroprotection depends on mitochondrial and axonal NAD+ content [78].
Currently, there are two possible mechanisms of neuroprotection: increasing mitochondrial NAD+ to support SIRT3 [77], and maintaining axonal NAD+ to supply injury-induced SARM1 activation [107].
Through SIRT3 activation, NR-induced increases in NAD+ levels can produce additional beneficial and potentially therapeutic effects. Specifically, NR has been shown to prevent noise-induced hearing loss and neurite retraction of inner ear hair cells through a SIRT3-dependent mechanism [77].
Furthermore, both SIRT3 and SIRT5 are critical for the health of retinal photoreceptors [108]. The activity of SIRT3 is sensitive to the decrease in NAD+ content. This suggests that NR may be a potential treatment for a variety of diseases, including photoreceptor degeneration.
Similarly, NAD+ levels are generally reduced in neuromuscular diseases, which are often caused by inherited mutations that lead to progressive weakening and degeneration of skeletal muscles [38]. Increasing NAD+ content through NR can stimulate energy production and improve mitochondrial function. NR has been shown to have therapeutic effects in several muscle diseases in mouse models.
Although NR supplementation cannot correct the underlying genetic defects, it can improve mitochondrial abundance and function in two distinct mitochondrial myopathies [109,110]. Moreover, NR can reverse the Progressive emaciation syndrome in skeletal muscle of mice lacking Nampt, restoring endurance with just 1 week of treatment [111].
Furthermore, NR treatment has been shown to improve stem cell function, thereby improving the muscular emaciation phenotype in mdx mice, supporting the apply of NR for human diseases [38,112]. During NR treatment, the improvement in stem cell function appears to be a general phenomenon and may be the reason for the significant extension of lifespan in mice [38].
Overall, these findings support that NR can effectively manage the progression of muscle malnutrition and degeneration by improving muscle strength, rejuvenating aging muscle stem cells, and reducing levels of inflammation and fibrosis.
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Longevity
Calorie restriction (CR) is considered the most effective method for extending lifespan in eukaryotic organisms, since.
it was reported to extend lifespan in wild-type yeast cells by regulating Sir2 and NAD+ [114]. The lifespan-extending effects of CR may be partially achieved through increased sirtuin function, and the requirement of NAD+ for their activity suggests a possible link between aging and metabolism.
Although overexpression of genes through nutrition has been achieved in yeast, which can increase Sir2 activity and lifespan [115], NA did not extend lifespan and NAM shortened lifespan [116,117].
NR, on the other hand, can increase NAD+ levels and Sir2 function, and exogenous NR can promote Sir2-dependent recombination suppression, lead to gene silencing, and extend lifespan without calorie restriction [11].
Furthermore, the mechanism of action of NR is entirely dependent on increased net NAD+ synthesis via the Nrk1 and Urh1/Pnp1/4 pathways. The latter is independent of Nrk1 and represents a newly discovered NR salvage pathway [11].
Preclinical studies reported that NR reduces macrophage infiltration in damaged muscle [38,112] and plasma TNF-α in a model of fatty liver disease [44].
A recent clinical study confirmed the availability of NR in human muscle tissue in the elderly [120] and its anti-inflammatory effects.
21 days of NR supplementation reduced many circulating inflammatory cytokines [120], suggesting an additional mechanism by which NR may modulate the aging process and thus exhibit lifespan-extending effects.
NR for Infection Treatment and Immunomodulatory Effects
NAD+ intermediates have been recognized for their beneficial health effects in various pathogen infections. Studies have confirmed the anti-tuberculosis activity of NAM in patients infected with Mycobacterium tuberculosis [121,122], and immune-mediated clearance has been reported for Staphylococcus aureus, including MRSA and other major human pathogens such as Klebsiella pneumoniae and Pseudomonas aeruginosa [123].
Moreover, NAM and its analogues have shown antiviral effects in patients with HIV [121] and hepatitis B [124].
Recently, NAD+ intermediates have been considered as potential treatments to combat COVID-19 infection.
SARS-CoV-2 infection triggers a maladaptive immune response. In particular, it leads to an excessive pro-inflammatory response resulting in a "cytokine storm" in lung tissue, as well as lymphopenia with a sharp decline in CD4+ and CD8+ T cells [125]. At the molecular level, PARP activation is increased when the innate immune response is activated to combat infection, due to extensive DNA damage and IFN-induced MARylation (mono-ADP-ribosylation) of SARS-CoV-2 target proteins [126,127].
The PARP response is essential for inhibiting viral replication [128]; however, this antiviral effect is reversed by the ADP-ribosylhydrolase macrodomain of the viral nonstructural protein nsp3, whose activity is required for virulence [126,127,129].
Recent studies investigated the disruption of PARP expression and NAD+ metabolome caused by coronavirus infection. Examined SARS-CoV-2-infected ferret cell lines and lungs from deceased patients showed alterations in NAD+ metabolism and gene expression related to NAD+ synthesis and utilization [131].
Furthermore, the upregulation of the NMRK1 pathway, along with the upregulation of the expression of the concentrative nucleoside transporter CNT3, indicates an increased ability to convert NR to NAD+ and NADP+ during infection [10]. Upregulation of NMRK genes has previously been associated with the therapeutic effects of NR [66,105].
Moreover, the expression of NNMT (nicotinamide N-methyltransferase) is reduced due to decreased NAM methylation [131], suggesting enhanced NAM salvage pathways [132] and increased efficiency of NR treatment to supplement NAD+ [131].
These data suggest that elevating NAD+ content through the NAM and NR kinase pathways may restore the antiviral functions of PARPs to support innate immunity against SARS-CoV-2 [131].
After the activation of the adaptive immune response, the overexpression of CD38 in CD4+ and CD8+ lymphocytes further exacerbate NAD+ depletion [133,134], leading to increased production and release of pro-inflammatory cytokines, reactive oxygen species, and macrophage infiltration [135,136]. Additionally, severe NAD+ depletion impairs the function of sirtuins, which regulate cell death and survival [133].
Specifically, SIRT1 regulates the expression of genes including tumor suppressors, cytokines, and oncogenes, ultimately regulating inflammation, cell survival, and apoptosis mechanisms [137]. The loss of sirtuin function, combined with increased oxidative damage and overall energy decline, ultimately leads to cell death.
Supplementing intracellular NAD+ levels may restore energy levels and impaired sirtuin function and potentially rebalance the maladaptive immune response to SARS-CoV-2 infection.
New evidence suggests that NAD+ is released during the early stages of inflammation and has immunomodulatory effects in vivo [141,142]. Furthermore, niacin treatment has previously been suggested as an anti-inflammatory therapy, as an effective drug for reducing pro-inflammatory cytokines including IL-1, IL-6, and TNFα in a preclinical study [143].
NR similarly reduces IL-2, IL-5, IL-6, and TNFα [120]. Recently, anti-IL-6 therapy has been proposed as a promising treatment to stop the inflammatory storm, especially in severe COVID-19 patients [132]. This implies that NR should be considered as a potential treatment or supportive drug to reduce hyperinflammation and regenerate damaged lung tissue.
NR Bioavailability and Safety
The bioavailability of NR can be tested by measuring NAD+ levels or other related biomarkers (such as nicotinic acid adenine dinucleotide (NAAD)) in target tissue cells or blood. In various mammalian cell lines, including liver, skeletal muscle, and brown adipose tissue, there are records showing that NR can enhance NAD+ levels [37].
Studies conducted in healthy human volunteers and mice report that an NR dose of 1000 mg twice daily (total 2000 mg) can significantly increase steady-state whole blood NAD+ levels (2.7-fold increase after a single dose) [149] and effectively stimulate NAD+ metabolism [39,149,150].
These studies confirm that chronic oral NR supplementation has no serious adverse reactions [39,150], such as flushing, pruritus, hyperglycemia, hyperuricemia, or increased enzyme activity in the liver or muscle [149,150,151,152].
However, the blood NAD+ response appears to be unrelated to the absorption pattern of NR, with the peak increase in NAD+ reached after 9 days [150]. Furthermore, due to the relatively short elimination half-life of NR observed in several subjects, repeated dosing is suggested to prevent substantial fluctuations in NR levels in the body.
However, sustained blood NAD+ levels indicate that twice or even once daily NR dosing may be sufficient to achieve the desired clinical effects [150].
On the other hand, there is considerable interindividual variability in the oral bioavailability of a 1000 mg dose of NR [150]. The instability of NR in blood samples observed in several studies [150,153] may be a contributing factor, although it cannot fully explain the observed variability. Another proposed explanation is the hydrophilicity of NR [15], as NR is expected to have low passive permeability on human intestinal mucosa [150].
In addition, interindividual differences in NR transport mechanisms in the gut system may also affect the oral absorption of NR. Furthermore, NR may also be degraded to NAM in the gut, while another study showed that NR is metabolized to NAM in the liver [154]. Subsequently, NAM can be absorbed and converted to NMN, which is further metabolized to NAD+ or dephosphorylated to NR [155].
Moreover, in a study of male human subjects and C57Bl6/J mice, multiple pathways of NR conversion to NAD+ were identified [149], with NAAD significantly increased by 45-fold in response to NR. This suggests another possible conversion pathway from NR to NAD+. These studies indicate that further research into the metabolism and transport of NR may reveal the causes of variations in oral bioavailability.
NR: A Better NAD+ Precursor?
Currently, NR is emerging as a more promising precursor due to its bioavailability, safety, and potent ability to increase NAD+ levels compared to other precursors [149].
Among the various NAD+ precursors, NMN and NR exhibit better pharmacokinetic and pharmacological properties [156].
In preclinical studies, bioavailability among NAD+ precursors (NMN, NR, NAM, and NA) were evaluated by increasing intracellular NAD+. NR was able to increase NAD+ levels in mouse liver with greater oral bioavailability than NAM, which in turn had greater oral bioavailability than NA [149]. Similarly, NAD+ content in muscle could be significantly increased by NR and NA, but not by NMN [149].
These three precursors (NA, NMN, and NR) differ in the extent to which they promote ADPR accumulation, a measure of sirtuins activity and other NAD+-consuming activities [149]. Specifically, NR was found to increase ADPR by approximately 3-fold compared to NAM, supporting NR as the preferred precursor for increasing NAD+ in the liver [149].
Moreover, the activity of sirtuins was stimulated after NR-induced increases in NAD+ levels [37]. The activities of SIRT1 and SIRT3 were increased both in vitro and in vivo [37], supporting the hypothesis that NR can increase NAD+ levels at least in mitochondrial and nuclear compartments. The ability of NR to increase NAD+ in different subcellular compartments represents a key distinction compared to other methods of increasing intracellular NAD+ levels.
All three precursors, NA, NAM, and NR, can increase levels of NAD+ and NADP+ [14,149,157], but have different physiological responses.
For example, NA shows effects in lowering blood lipid levels and is used to treat dyslipidemia [151]. However, NA is associated with flushing at doses exceeding 50 mg per day [151].
In contrast, NAM does not affect blood lipid levels but may exhibit sirtuin inhibitory effects at higher doses [116,149].
Among the three precursors mentioned above, only NR was able to extend lifespan and induce hematopoietic stem cell regeneration [158]. Additionally, oral NR was found to increase resistance to and reversal of chemotherapy-induced neuropathy [159]. This implies potential usage advantages of NR precursor in cancer patients receiving chemotherapy or radiation therapy.
NAAD represents the most sensitive biomarker of effective NAD+ supplementation, as it is undetectable in blood prior to supplementation and increased levels in the liver have been observed after oral NAD+ precursor administration.
NR was found to increase NAAD by at least 45-fold compared to baseline [149]. While NA, as the only precursor expected to be converted to NAD+ via the NAAD intermediate, produced the least NAAD, both NAM and NR produced peak levels of NAAD in the liver [149].
NR has high availability in the normal human diet. NR enters cells without the need for conversion, partly explaining its high availability. In contrast, NAD+ and its precursors must be converted to NR or NAM before entering cells [153].
NAD+ and NMN are converted to NR extracellularly by CD73 [28] while their intracellular conversion relies on the NMRK pathway [153,160]. However, Grozio et al. recently identified an NMN-specific transporter encoded by the Slc12a8 gene in the gut [161]. Thus, the utilization of NR and extracellular NAD+ is limited by the activity of the NMRK pathway [153].
Considering side effects, NR may be a more suitable NAD+ precursor. Although NA and NAM can enter the NAD+ salvage pathway, preclinical studies have confirmed that either NA or NAM may cause painful flushing or other toxic effects at therapeutic doses [14,165,166].
Despite NMN showing significant beneficial pharmacological activities in preclinical studies, there is still a lack of sufficient clinical and toxicological data. To date, there have been no tests on the safety and human oral bioavailability of NMN, although a recent clinical study involving ten healthy men confirmed that single oral doses between 100-500 mg are safe and effective, with no apparent adverse effects [167].
On the other hand, multiple studies have confirmed that NR is well-tolerated, with daily doses up to 2 grams, and no association with flushing or any serious adverse events has been observed [39].
It is better, but not the best
In preclinical studies, the significant findings of numerous beneficial health effects of NR may ultimately lead to a breakthrough, making it possible to treat a large number of metabolic and neurodegenerative diseases.
The effects of NR are currently being investigated in a large number of clinical trials [181], including studies on various cardiovascular diseases, neurological and cognitive function, metabolic disorders, muscle and kidney injury, aging, and chemotherapy.
Furthermore, basic research on NR transport and metabolic pathways will further support its rapid translation into effective therapeutic applications. The advantages of using NR over other NAD+ precursors, including its safety and efficiency, suggest that it may replace niacin as a universal supplement in the future.
In-depth research on NR may also lead to its global availability in supplemental use and new therapeutic strategies, most importantly for pathophysiological conditions currently lacking effective treatments.
