1. Introduction
Nicotinamide riboside (NAR) is widely used as a dietary supplement, Structurally, it is an advanced form of vitamin B
3 (nicotinic acid), incorporating into its structure more elements of nicotinamide adenine dinucleotide (in its oxidized form, NAD
+) [
1]. NAR influences, in particular, energy metabolism and neuroprotection [
2,
3,
4].
From a non-medical perspective, as a part of the nicotinamide adenine dinucleotide (NAD
+) cofactor, it also inferred to be a vestige of the “ribonucleic acid (RNA) world” [
5], an episode of life on early Earth where RNA was the only encoded component of both metabolism and genetics, and this inference is supported by the ubiquity of NAD
+ in all branches of tree of modern life [
6].
NAR is, however, a rather reactive molecule. Its glycosidic bond joins a positively charged pyridinium heterocycle to a carbohydrate. This bond is therefore especially unstable to cleavage, making NAR difficult to synthesize, store, and transport. This creates broad utility for any method for the synthesis of NAR, as well as broad utility for derivatives of NAR that are more stable.
Stable forms of NAR are especially important today. Since July 2013, NAR became accessible in dietary supplement form of NAR chloride (NARC), NAR is sold and widely consumed as a dietary supplement, under various trademarks, including Tru Niagen™ and Niagen
® (produced by ChromaDex). Other products containing NARC have been commercially available since [
7,
8]. Interesting, analysis of some of these products at various times has shown that they do not contain pure NAR, but also products undoubtedly arising from the reactivity of NAR.
Notwithstanding, commercial NAR products are suggested to elevate the level of oxidized NAD (NAD
+) in those who consume it [
8]. Studies report that chronic NAR supplementation (NARS) is well tolerated and elevates NAD
+ in healthy middle-aged and older adults [
2,
3]. Other studies suggest NAR oral bioavailability in mice and humans [
9]. NAR as a dietary supplement has also been the target of clinical studies [
2,
3]. These have proven its effect to boost NAD
+ in its consumers [
8,
10]. These facts make it timely to review the current state of the art.
NAR, a pyridine-nucleoside form of vitamin B3, consists of nicotinamide (NA) and ribose as its fundamental components. It can be found in various sources such as milk, yeast, beer, as well as bacteria and mammals.
NAR-enriched foodstuffs
per se are not well established. Presumably, products that contain yeast are excellent natural sources for the compound [
11,
12]. However, dairy products have also been noted to contain NAR [
9,
13]. The amounts of NAR in foodstuffs are extremely low, most likely at micromolar levels.
NAR shares biological properties with other NAD+ precursors, specifically NA and nicotinic acid (NAc, niacin). Indeed, these are old or “classical” vitamins B3. Unlike NAR, the NA and niacin have disadvantages when consumed. Hepatotoxicity or flushing can be potential side effects associated with NA, whereas a recent preclinical study indicates that NA remains in the rat body for a shorter duration when compared to NAR.
Niacin taken in large amounts is associated with adverse effects, including cutaneous flushing when administered in an immediate release formulation. Sustained release formulations may cause hepatotoxicity.
A precursor that is structurally more advanced towards NAD
+ is nicotinamide mononucleotide (NMN). NMN has been recently discovered to be converted extracellularly to NAR, which is transported into cells [
11,
14,
15]. Since NMN is converted to NAR in the body and the price of NMN is half of the price of NAR, technology producing NAR rather than NMN is especially needed [
16].
Among the NAD
+ precursors, NAR is exceptional, as it produces fewer reported unfavorable side effects [
7].
In its classical metabolic roles, NAD
+ is a versatile receiver of hydride equivalents, becoming NAD (NADH). Analogous chemistry is seen with its phosphorylated derivative, nicotinamide adenine dinucleotide phosphate (NADP and NADPH). Traditionally, NAD
+ and its derivatives function as coenzymes for dehydrogenases and oxidoreductases, playing crucial roles in fundamental energy metabolism processes such as glycolysis, the citric acid cycle, and mitochondrial (MT) electron transport. In addition to its classical functions, NAD
+ also serves as a vital substrate for signaling enzymes,
e.g., poly [adenosine diphosphate (ADP)-ribosyl] polymerases, sirtuins (SIRTs), and ADP-ribosyl-transferases. These enzymes are referred to as “NAD
+ consumers” [
17,
18,
19,
20,
21].
Throughout classical understanding as well as recent discoveries, NAD
+ has been recognized as a crucial and abundant metabolite present in all mammalian cells. It participates in a wide array of cellular mechanisms, including essential processes such as metabolism and cell signaling that are vital for survival. However, the limited presence of NAR in food sources (with some quantitative studies available) and the challenges associated with obtaining significant quantities of pure NAR as a dietary supplement have hindered extensive research on the effects of NAR on cells and tissues [
22,
23]. Nevertheless, recently, there have been notable advancements in the synthesis techniques for producing NAR, resulting in new and refined approaches [
24]. However, these new technologies were developed for cell-based investigations and animal feeding tests, and not for human consumption [
25,
26].
Non-classical roles also exist for NAD
+. For example, NAD
+ also activates SIRTs and supports the mitochondrial response to unfolded proteins. Pursuing these non-classical roles, NAD
+ metabolism is seen to be involved in an extensive spectrum of pathologies, including cancer, neurodegenerative disorders, cardiomyopathy, obesity, diabetes, and even hearing loss [
4,
27,
28,
29,
30,
31].
In many tissues (
e.g., brain, muscle, skin, liver, pancreas, and adipose tissue), the level of NAD
+ decreases with age. A separate decrease in the NAD
+/NADH ratio affects the cellular redox state, highlighted by anaerobic glycolysis and oxidative phosphorylation (OXPHOS). This decreases, it is thought, the capacity of cells to generate adenosine triphosphate (ATP) [
32]. Consequently, healthy aging and longevity are thought to be related to NAD
+ metabolism, mainly through NAR and NMN, two essential and well-studied NAD
+ derivatives. One argument for NA-related dietary supplements is that these can have prophylactic and therapeutic impact on functional decline, improving age-associated neurodegenerative, cardiovascular (CV), and metabolic diseases and conditions, and promoting the beneficial effects of calorie restriction (CR) [
4,
33,
34,
35,
36].
As a precursor of NAD
+, NAR is also proposed to be important in regulating oxidative stress (OXS), inhibiting oxidative injury and inflammatory response, with beneficial effects in the treatment of sepsis [
37], systemic lupus erythematosus (SLE) [
38], and necrotizing enterocolitis (NEC) [
39]; furthermore, NAR showed significant protective effect on lung injury caused by paraquat (PQ) in mice [
40].
Separately, sepsis-caused multiple organ failure is the major risk of morbidity and mortality in intensive care units. In experimental models of mouse sepsis induced by lipopolysaccharide (LPS) injection, feces injection in peritoneum or by cecal ligation and puncture (CLP), NAR inhibited plasma high mobility group box 1 (HMGB1) release, OXS, and tissue infiltration, increased endogenous antioxidant ability, prevented lung and heart injury and improved survival [
37,
41]. This is mediated
via NAD
+/SIRT1 signaling.
NAR could be a potential adjuvant for SLE treatment:
in vitro testing in monocytes from patients with SLE showed that NAR restricted autophagy and attenuated interferon-beta (IFN-
β) release in an NAD
+-dependent manner but also through inosine signaling [
38]. Also, in the mouse experimental NEC model, NAR administration alleviated OXS, increased NAD
+ levels and intestinal microcirculatory perfusion, and relieved signs of endothelial dysfunction by modulating the SIRT1-associated endothelial nitric oxide synthase (eNOS) acetylation/deacetylation pathway [
39,
42].
Compared with control (CON) group, which received saline by one-time gavage, intraperitoneal (i.p.) injection of 300 mg/kg NAR solution led to the inhibition of the inflammatory response, peroxidation injury and apoptosis at the lung level, and to the survival time prolongation of the PQ intoxicated mice, mainly by upregulating SIRT1 and nuclear factor erythroid 2 (NF-E2)-related factor 2 (NRF2) protein expression [
40].
We recently reported an inexpensive synthesis of borate-stabilized NAR based on models for how nucleosides might have been formed on a prebiotic Earth. Unlike many commercially available NAR products, borate-stabilized NAR can be delivered in pure form, exhibiting high stability against thermal and chemical decomposition. Thus, NAR borate (NARB) is a promising nutraceutical to replace NARC, as currently sold on the US market. This paper reviews the current research supporting the value of NAR for healthy aging and longevity, the perspectives of senotherapeutic NAD+ supplementation, and thus forms of NAR that can be made, shipped, and stored without decomposition.
7. Conclusions and Perspectives
NAD+ is known classically as a metabolite that stands astride both catabolic and anabolic pathways throughout the metabolism that is taught in introductory biochemistry courses. However, non-classical studies starting over a decade ago found that it is also involved in higher order functions, in part because of its involvement in the activation of SIRTs and the support of the MT unfolded protein response. Many studies have suggested that NAD+ is involved in an extensive spectrum of pathologies, including neurodegenerative disorders, cardiomyopathy, obesity, and diabetes. Further, healthy aging and longevity appear to be closely related to NAD+ and its related metabolites, including through NAR and NMN. This system appears to have prophylactic and therapeutic value improving age-associated neurodegenerative, CV, and metabolic diseases and conditions.
Accordingly, many are now recommending the consumption of materials in this system as dietary supplements, hoping to achieve overall improvements in human health. Among NAD+ precursors, nicotinamide ribose (NAR) appears to have special values. These include better tolerance, better uptake, and overall greater potency.
Unfortunately, NAR is a reactive molecule, often unstable during its manufacturing, transport, and storage. Indeed, HPLC analyses of many commercial samples of NAR shows that they contain substantial amounts of material that are not, in fact, NAR. More stable derivatives of NAR that are easily converted upon consumption into NAR are therefore desired.
Recently work related to prebiotic chemistry provided the borate derivative of NAR. NARB is considerably more stable than NAR itself. However, immediately upon consumption, the borate dissociates from NARB, and is lost in the body through dilution and binding to other species, notably carbohydrates such as fructose and glucose. The NAR left behind is expected to behave pharmacologically in ways identical to NAR itself.
This review provides a comprehensive uncritical summary through Q1 of 2023 of literature that makes the case for the consumption of NAR. It then summarizes the challenges of delivering quality NAR to consumers using standard synthesis, manufacture, shipping, and storage approaches. It concludes by outlining the advantages of NARB in these processes.