Nicotinamide riboside (NR, SR647) is a pyridine-nucleoside and a form of vitamin B3. It functions as a precursor to nicotinamide adenine dinucleotide or NAD+ through a two-step and a three-step pathway. Chemistry While the molecular weight of nicotinamide riboside is 255.25 g/mol, that of its chloride salt is 290.70 g/mol. As such, 100 mg of nicotinamide riboside chloride provides 88 mg of nicotinamide riboside. History Nicotinamide riboside (NR) has been identified as an NAD precursor, involved in salvage NAD synthesis in both bacteria and eukaryotes. In bacteria, it was first described in 1944 as a necessary growth factor for the culture of Haemophilus influenza, H. influenza was identified as requiring both X factor (hemin) and V factor (NAD) to grow. V factor, purified from blood, was shown to exist in three forms: Nicotinamide adenine dinucleotide (NAD+), NMN and NR. NR was the compound that led to the most rapid growth of the H. influenza bacterium. H. influenza cannot grow on nicotinic acid (NA), nicotinamide (NAM), or amino acids such as tryptophan (Trp) or aspartic acid (Asp), which were the previously known precursors of NAD+. H. influenza depends entirely on salvage of NAD precursors from other cells in its environment. The identification of Nicotinamide riboside (NR) as an NAD precursor in eukaryotes developed out of the study of pellagra. Pellagra was the first disease to be associated with NAD+ deficiency. It was linked to nutritional deficiency by Joseph Goldberger in 1914, and to deficiency of niacin (vitamin B3) by Conrad Elvehjem in 1937. NAD+ (then called coenzyme I) was shown to be extremely low in cases of pellagra, and NA and NAM were identified as molecular precursors in rebuilding NAD+ levels. Pellagra is now understood as a severe, chronic depletion of NAD+, which can be treated through diet. Subsequent studies of NAD+ metabolism have identified regulatory pathways used by cells and tissues to maintain NAD+ availability. NAD+ and its precursors nicotinic acid (NA) and nicotinamide (NAM) have been shown to be vital cofactors in cellular oxidation/reduction reactions and ATP synthesis. Classic NAD+ synthesis pathways characterized in eukaryotes include an eight-step de novo pathway from Trp and two pathways using the NAD+ precursors NA and NAM: a three-step NA-based pathway known as the Preiss-Handler pathway; and an NAM-based pathway involving the enzyme Nicotinamide phosphoribosyltransferase (NAMPT) and the formation of nicotinamide mononucleotide (NMN). In 2004, a previously unknown pathway was reported when nicotinamide