We demonstrate supplementing with nicotinamide (NAM) and related components of this pathway improve mitochondrial and overall hepatic metabolic dysfunction. We find that the effects of modulating the TRP-NAD+ pathway are mediated through SIRT1...NAM treatment can either reduce mitochondria degradation or increase its biogenesis in our model of severe malnutrition...NAM was shown to increase SIRT1 levels. The effect was not specific to NAM, as Nicotinamide Riboside (NR) and Tryptophan (TRP) demonstrated a similar effect...In our study, NAM treatment did not significantly restore NAD+ levels whereas NR did...

Interestingly, mice supplemented with NA revealed a significant delay in the onset of imbalance, whereas NR supplementation showed a trend towards delaying the median age of onset of tremors. Furthermore, NR significantly extended median lifespan by approximately 25% or 5 weeks, while animals receiving NA lived about as long as control-fed animals.

Ablation of the microbiome by antibiotic treatment increases the uptake and conversion of orally delivered NMN into the NAD metabolome, and that isotope labelled NMN overwhelmingly presents in intestinal tissue in the form of NR. Contrary to the assumption that exogenous NMN treatment raises NAD+ levels solely through its direct incorporation into the NAD metabolome, we show that treatment with isotope labelled NMN increases the levels of endogenous, unlabelled NAD metabolites....Given this evidence for the decomposition of NMN into free Nam prior to its uptake, a key question for the field is why downstream precursors in NAD+ synthesis such as NMN and NR lead to different outcomes compared to Nam alone...

Alterations in cellular nicotinamide adenine dinucleotide (NAD+) levels have been observed in multiple lifestyle and age-related medical conditions. This has led to the hypothesis that dietary supplementation with NAD+ precursors, or vitamin B3s, could exert health benefits. Among the different molecules that can act as NAD+ precursors, Nicotinamide Riboside (NR) has gained most attention due to its success in alleviating and treating disease conditions at the pre-clinical level. However, the clinical outcomes for NR supplementation strategies have not yet met the expectations generated in mouse models. In this review we aim to provide a comprehensive view on NAD+ biology, what causes NAD+ deficits and the journey of NR from its discovery to its clinical development...

The last decade has seen a strong proliferation of therapeutic strategies for the treatment of metabolic and age-related diseases based on increasing cellular NAD+ bioavailability. Among them, the dietary supplementation with NAD+ precursors—classically known as vitamin B3—has received most of the attention. Multiple molecules can act as NAD+ precursors through independent biosynthetic routes. Interestingly, eukaryote organisms have conserved a remarkable ability to utilize all of these different molecules, even if some of them are scarcely found in nature. Here, we discuss the possibility that the conservation of all of these biosynthetic pathways through evolution occurred because the different NAD+ precursors might serve specialized purposes.

This work demonstrates for the first time that NARH can act as a NAD+ precursor in mammalian cells and how different NAD+ precursors can interact and influence each other when co-administered...NARH achieved synergistic effects with NR in increasing NAD+ levels, but not with any of the other NAD+ precursors tested...The effect was remarkable, given the fact that NR or NARH had limited effects on NAD+ levels in these two cell lines when used as single treatments...The synergistic action of NR and NARH could be confirmed in vivo...NR degradation to NAM was not a critical step, as NAM and NARH did not lead to cooperative increases in NAD+ levels...The vigorous increase in NAD+ levels observed when combining NR and NARH was reminiscent of the effects of NRH...The simplest explanation for this would be that the combination of NR and NARH led to the production of NRH...The results showed that, when used together, NR and NARH increased the intracellular levels of NRH...Simply adding the two molecules to a vial of water at room temperature was enough to generate NRH

...The main route from oral nicotinamide riboside, a widely used nutraceutical, to host NAD is via conversion into nicotinic acid by the gut microbiome. Thus, NAD precursors cycle between the host and gut microbiome to maintain NAD homeostasis.

The present study demonstrated that orally administered NR increased NAD+ levels in a diphasic manner. First, in the early phase (within 1 h after administration), NR was directly taken up by the small intestine and utilized for NAD+ synthesis via the NR salvage pathway. In the late phase (~3 h after administration), NR increased NAD+ levels in a manner dependent on gut microbiota.

NAD+ is an essential coenzyme for all living cells. NAD+ concentrations decline with age, but whether this reflects impaired production or accelerated consumption remains unclear. We employed isotope tracing and mass spectrometry to probe age-related changes in NAD+ metabolism across tissues. In aged mice, we observed modest tissue NAD+ depletion (median decrease ∼30%). Circulating NAD+ precursors were not significantly changed, and isotope tracing showed the unimpaired synthesis of nicotinamide from tryptophan. In most tissues of aged mice, turnover of the smaller tissue NAD+ pool was modestly faster such that absolute NAD+ biosynthetic flux was maintained, consistent with more active NAD+-consuming enzymes. Calorie restriction partially mitigated age-associated NAD+ decline by decreasing consumption. Acute inflammatory stress induced by LPS decreased NAD+ by impairing synthesis in both young and aged mice. Thus, the decline in NAD+ with normal aging is relatively subtle and occurs despite maintained NAD+ production, likely due to increased consumption.

In summary, through chemical screen, global and targeted metabolic analyses, and functional genetic and biochemical assays, we discovered that bacteria contribute to the deamidated NAD biosynthesis in host cells through a microbial nicotinamidase (PncA).

NAD+ is a crucial metabolite and signaling molecule whose detailed functions and biology are incompletely characterized, at best. We are only beginning to elucidate how the temporal and spatial compartmentalization of NAD+ contributes to its numerous biological roles. The emerging view is that NAD+ concentrations are partitioned and dynamically modulated by NAD+ synthesis within the cell. Determining the extent of these mechanisms in vivo and their impact on signaling pathways will be a rich area for future research

...Accumulating evidence on Nicotinamide Riboside's health benefits has validated its efficiency across numerous animal and human studies for the treatment of a number of cardiovascular, neurodegenerative, and metabolic disorders. As the prevalence and morbidity of these conditions increases in modern society, the great necessity has arisen for a rapid translation of NR to therapeutic use and further establishment of its availability as a nutritional supplement. Here, we summarize currently available data on NR effects on metabolism, and several neurodegenerative and cardiovascular disorders, through to its application as a treatment for specific pathophysiological conditions. In addition, we have reviewed newly published research on the application of NR as a potential therapy against infections with several pathogens, including SARS-CoV-2. Additionally, to support rapid NR translation to therapeutics, the challenges related to its bioavailability and safety are addressed, together with the advantages of NR to other NAD+ precursors.

...FiNad sensors cover physiologically relevant NAD+ concentrations and sensitively respond to increases and decreases in NAD+. Utilizing FiNad, we performed a head-to-head comparison study of common NAD+ precursors in various organisms and mapped their biochemical roles in enhancing NAD+ levels. Moreover, we showed that increased NAD+ synthesis controls morphofunctional changes of activated macrophages, and directly imaged NAD+ declines during aging in situ...

Extracellular NAD is degraded by three main classes of specific ectoenzymes: CD38 and CD157, ARTs, ENPP1 and CD73. NADase, ENPP1 and CD73 can lead to the formation of adenosine (ADO), a potent immunosuppressant factor, independently of the activity of CD39. Beside generating ADO, eNAD can be degraded to nicotinamide mononucleotide (NMN) by CD38, generating Nam which can cross plasma membranes and be re-converted to NAD through NAMPT and NMN adenylyltransferase (NMNAT)...

A decay in intracellular NAD+ levels is one of the hallmarks of physiological decline in normal tissue functions. Accordingly, dietary supplementation with NAD+ precursors can prevent, alleviate, or even reverse multiple metabolic complications and age-related disorders in diverse model organisms. Within the constellation of NAD+ precursors, nicotinamide riboside (NR) has gained attention due to its potent NAD+ biosynthetic effects in vivo while lacking adverse clinical effects. Nevertheless, NR is not stable in circulation, and its utilization is rate-limited by the expression of nicotinamide riboside kinases (NRKs). Therefore, there is a strong interest in identifying new effective NAD+ precursors that can overcome these limitations. Through a combination of metabolomics and pharmacological approaches, we describe how NRH, a reduced form of NR, serves as a potent NAD+ precursor in mammalian cells and mice. NRH acts as a more potent and faster NAD+ precursor than NR in mammalian cells and tissues. Despite the minor structural difference, we found that NRH uses different steps and enzymes to synthesize NAD+, thus revealing a new NRK1-independent pathway for NAD+ synthesis. Finally, we provide evidence that NRH is orally bioavailable in mice and prevents cisplatin-induced acute kidney injury. Our data identify a new pathway for NAD+ synthesis and classify NRH as a promising new therapeutic strategy to enhance NAD+ levels.

...It is still debated whether nucleotides enter cells directly or whether they are converted to nucleosides and bases prior to uptake into cells. Here, we studied the metabolism of extracellular NAD+ and its derivatives in human HEK293 cells...Using pharmacological inhibitors of plasma membrane transporters, we also showed that extracellular cleavage of NAD+ and NMN to NR is a prerequisite for using these nucleotides to maintain intracellular NAD contents. We also present evidence that, besides spontaneous hydrolysis, NR is intensively metabolized in cell culture by intracellular conversion to Nam...

...Here we report physicochemical properties of nicotinamide riboside including pKa, log P, solubility, melting point, degradation mechanics, and kinetics, with a special focus on its stability under thermal and physiologically relevant conditions. A simple and rapid HPLC method confirms a base-catalysed hydrolysis degradation of NRCl to nicotinamide and sugar in simulated gastrointestinal (GI) fluids. Given the antagonising effect of nicotinamide against NR, the presented data have a profound impact on how NRCl should be handled both during formulation and storage to prevent formation and to limit accumulation of nicotinamide...

This work provides evidence that (1) endogenous NR metabolism is required to sustain hepatic NAD+ levels in situations of metabolic damage and lipotoxicity and, (2) the inability to use NR as a NAD+ precursor leads to mitochondrial dysfunction and amplifies the detrimental effects of HFD...There is no compensation through NAM to generate NMN and maintain hepatic NAD+ in this situation...The administration of NAM to NRK1 LKO mice neither increased NMN and NAD+ levels nor recovered their metabolic defects. Instead, the supplemented NAM seemed to be largely diverted towards clearance methylation/oxidation paths...

NRK1 deficiency, inhibition of CD73 (the cell surface protein thought to convert extracellular NMN to NR), and inhibition of ENTs all prevented NAD+ generation from exogenous NMN, providing evidence for conversion of NMN to NR before cellular uptake.

...While studies have documented the efficacy of some NAD+ precursors such as nicotinamide riboside (NR) in raising plasma NAD+, no data are currently available on the fate of directly infused NAD+ in a human cohort. This study, therefore, documented changes in plasma and urine levels of NAD+ and its metabolites during and after a 6 h 3 μmol/min NAD+ intravenous (IV) infusion. Surprisingly, no change in plasma (NAD+) or metabolites [nicotinamide, methylnicotinamide, adenosine phosphoribose ribose (ADPR) and nicotinamide mononucleotide (NMN)] were observed until after 2 h. Increased urinary excretion of methylnicotinamide and NAD+ were detected at 6 h, however, no significant rise in urinary nicotinamide was observed. This study revealed for the first time that: (i) at an infusion rate of 3 μmol/min NAD+ is rapidly and completely removed from the plasma for at least the first 2 h; (ii) the profile of metabolites is consistent with NAD+ glycohydrolase and NAD+ pyrophosphatase activity; and (iii) urinary excretion products arising from an NAD+ infusion include NAD+ itself and methyl nicotinamide (meNAM) but not NAM.

Despite genetic, pharmacological and kinetic evidence validated by a quantitative assay showing that nicotinamide mononucleotide (NMN) is dephosphorylated to nicotinamide riboside before cellular internalization1 , solute carrier family 12 member 8 (Slc12a8), which is widely expressed and annotated as a Na+/K+ Cl− transporter, has been nominated to be an NMN transporter. The analytical methods, transport data and interpretation underlying this assignment are not sound and do not support transport of NMN by Slc12a8.

...NAD concentration falls during aging, which has triggered intense interest in strategies to boost NAD levels. A limitation in understanding NAD metabolism has been reliance on concentration measurements. Here, we present isotope-tracer methods for NAD flux quantitation...NAD fluxes varied widely across tissues, with high flux in the small intestine and spleen and low flux in the skeletal muscle. Intravenous administration of nicotinamide riboside or mononucleotide delivered intact molecules to multiple tissues, but the same agents given orally were metabolized to nicotinamide in the liver. Thus, flux analysis can reveal tissue-specific NAD metabolism.

...Restoration of NAD+ levels in old or diseased animals can promote health and extend lifespan, prompting a search for safe and efficacious NAD-boosting molecules that hold the promise of increasing the body’s resilience, not just to one disease, but to many, thereby extending healthy human lifespan.

The apparent oral bioavailability of a 1000 mg dose of NR was highly variable among individuals. While half of the participants showed a significant increase (≥ 100%) in peak blood NR concentration during the pharmacokinetic portion of the study, the remaining subjects showed no or only modest (≤ +50%) changes in blood NR levels...Because NR increases circulating NAD+ in humans, NR may have potential as a therapy in patients with mitochondrial dysfunction due to genetic and/or acquired diseases.

...Here, we show that intracortical administration of nicotinamide riboside (NR) but not NAD+ reduces brain damage induced by NMDA injection. Using cortical neurons, we found that provision of extracellular NR delays NMDA-induced axonal degeneration (AxD) much more strongly than extracellular NAD+. Moreover, the stronger effect of NR compared to NAD+ depends of axonal stress since in AxD induced by pharmacological inhibition of nicotinamide salvage, both NAD+ and NR prevent neuronal death and AxD in a manner that depends on internalization of NR. Taken together, our findings demonstrate that NR is a better neuroprotective agent than NAD+ in excitotoxicity-induced AxD and that axonal protection involves defending intracellular NAD+ homeostasis.

NAD+ is a vital redox cofactor and a substrate required for activity of various enzyme families, including sirtuins and poly(ADP-ribose) polymerases. Supplementation with NAD+ precursors, such as nicotinamide mononucleotide (NMN) or nicotinamide riboside (NR), protects against metabolic disease, neurodegenerative disorders and age-related physiological decline in mammals. Here we show that nicotinamide riboside kinase 1 (NRK1) is necessary and rate-limiting for the use of exogenous NR and NMN for NAD+ synthesis. Using genetic gain- and loss-of-function models, we further demonstrate that the role of NRK1 in driving NAD+ synthesis from other NAD+ precursors, such as nicotinamide or nicotinic acid, is dispensable. Using stable isotope-labelled compounds, we confirm NMN is metabolized extracellularly to NR that is then taken up by the cell and converted into NAD+. Our results indicate that mammalian cells require conversion of extracellular NMN to NR for cellular uptake and NAD+ synthesis, explaining the overlapping metabolic effects observed with the two compounds.

Nicotinamide riboside (NR) is in wide use as an NAD+ precursor vitamin. Here we determine the time and dose-dependent effects of NR on blood NAD+ metabolism in humans. We report that human blood NAD+ can rise as much as 2.7-fold with a single oral dose of NR in a pilot study of one individual, and that oral NR elevates mouse hepatic NAD+ with distinct and superior pharmacokinetics to those of nicotinic acid and nicotinamide. We further show that single doses of 100, 300 and 1,000 mg of NR produce dose-dependent increases in the blood NAD+ metabolome in the first clinical trial of NR pharmacokinetics in humans. We also report that nicotinic acid adenine dinucleotide (NAAD), which was not thought to be en route for the conversion of NR to NAD+, is formed from NR and discover that the rise in NAAD is a highly sensitive biomarker of effective NAD+ repletion.