## 1 Introduction  
Nicotinamide riboside (NR) is widely used as a dietary supplement. Structurally, it is a form of vitamin B\({}_{3}\) (nicotinic acid, niacin, NA), incorporating into its structure more elements of nicotinamide adenine dinucleotide (in its oxidized form, NAD\({}^{+}\)) [1]. NR influences, in particular, energy metabolism and neuroprotection [2; 3; 4]. From a non-medical perspective, as a part of the NAD\({}^{+}\) cofactor, NR is 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. This inference is supported by the ubiquity of NAD\({}^{+}\) in all branches of the tree of modern life [6]. NR is, however, a rather reactive molecule. Its glycosidic bond joins a positively charged pyridinium heterocyclic to a carbohydrate. This bond is therefore especially unstable to cleavage, making NR difficult to synthesize, store, and transport. This creates broad utility for any method to synthesize NR, as well as broad utility for derivatives of NR that are more stable. Stable forms of NR are especially important today. In July 2013, NR became accessible in dietary supplements in the form of NR chloride (NRCI). This is sold and widely consumed as a dietary supplement under various trademarks, including Tru Niagen\({}^{\text{TM}}\) and Niagen\({}^{\text{\textregistered}}\) (produced by ChromaDex, Los Angeles, CA, USA). Other products containing NRCI have been commercially available [7; 8]. Interestingly, analysis of some of these products at various times has shown that they do not contain pure NR but also products undoubtedly arising from the reactivity of NR. Notwithstanding, commercial NR products are suggested to elevate the level of NAD\({}^{+}\) in those who consume it [8]. Studies report that chronic NR supplementation (NRS) is well tolerated and elevates NAD\({}^{+}\) in healthy middle-aged and older adults [2; 3]. Other studies suggest NR oral bioavailability in mice and humans [9]. NR as a dietary supplement has also been the target of clinical studies [2; 3]. These have proven its effect of boosting NAD\({}^{+}\) in its consumers [8; 10]. These facts make it timely to review the current state of the art. NR, a pyridine-nucleoside form of vitamin B\({}_{3}\), consists of nicotinamide (NAM) and ribose as its fundamental components. It is found in various sources, such as milk, yeast, beer, bacteria, and mammals. NR-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 NR [9; 13]. In general, the amounts of NR in foodstuffs are quite low, most likely at micromolar levels. NR shares biological properties with other NAD\({}^{+}\) precursors, specifically NAM and NA. Indeed, these are old or \"classical\" vitamins B\({}_{3}\). Unlike NR, NAM and NA have disadvantages when consumed. Hepatotoxicity can be side effect associated with NAM, whereas a recent preclinical study indicates that NAM remains in the rat body for a shorter duration when compared to NR. NA 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 toward NAD\({}^{+}\) is nicotinamide mononucleotide (NMN). NMN has been recently discovered to be converted extracellularly to NR, which is transported into cells [11; 14; 15]. Since NMN is converted to NR in the body and the price of NMN is half of the price of NR, technology producing NR to replace NMN as a dietary supplement is especially needed [16]. Among the NAD\({}^{+}\) precursors, NR may be preferred, as it produces fewer reported unfavorable side effects [7]. In its classical metabolic roles, NAD\({}^{+}\) is a versatile receiver of hydride equivalents, becoming NADH.