## 5 Safety and Bioavailability of Nicotinamide Riboside  
### Nicotinamide Riboside Safety  
The safety of NRCI was assessed by biological assays, including bacterial reverse mutagenesis tests (Ames test), in vitro chromosome aberration tests, in vivo micronucle tests, and toxicological studies in rats. These covered acute (24 h), sub-acute (14-day), and chronic (90-day) administration. Genotoxicity of NRCI was not observed, and lethality was absent even at a dose of 5000 mg/kg (p.o.). NRCI had the lowest-observed-adverse-effect level (LOAEL) and the no-observed-adverse-effect level (NOAEL) at the dose of 1000 mg/kg/day and 300 mg/kg/day, respectively. Here, the liver, kidney, ovary, and testicles were the target organs for oral toxicity assays [220]. A randomized, double-blind, PLA-controlled clinical trial for eight weeks was performed in overweight healthy women and men who received different doses of NRCI (orally 100 mg, 300 mg, 1000 mg). Between NRCI and PLA groups or between different NRCI groups, neither important differences nor adverse effects were observed. However, within two weeks of NRCI supplementation, significant increases in serum levels of NAD\({}^{+}\) and NAD\({}^{+}\) metabolites were recorded [3]. In a randomized, double-blind, PLA-controlled study, the safety and efficacy of NRPT (Basis\({}^{\text{TM}}\)) were assessed for eight weeks in 120 healthy adult volunteers (60-80 years old). In addition to the significant increase in NAD\({}^{+}\) blood levels, repeated doses of NRPT supplementation did not cause any adverse effects [221]. The European Food Safety Authority (EFSA) Panel on Nutrition, Novel Foods and Allergens (NDA) of the European Commission (EC) concluded that NR and NRCI are \"safe for the healthy adult population, excluding pregnant and lactating women, and that an intake of up to 230 mg/day is safe for pregnant and lactating women\". Moreover, for the two active compounds, there are no concerns about genotoxicity and their safety/tolerability in human studies [222; 223; 224]. Three more leading international regulatory authorities, including the U.S. Food and Drug Administration (FDA), Health Canada (HC), and the Therapeutic Goods Administration (TGA) of Australia, concluded that NR is safer for human use than other NAD\({}^{+}\) metabolites [225]. Similar studies were performed on NR-related compounds. A non-randomized study of single oral supplementation within the daily tolerable upper levels of NAM affects blood NAM and NAD\({}^{+}\) amounts in six healthy humans. NAM daily intake of up to 200 mg was found, at 0.5 h, a 30-fold increase in its maximum amount in the whole blood of male volunteers. At 12 h, NAD\({}^{+}\) serum concentrations have reached the maximum level. Metabolomic variations due to NAM dietary supplementation lasted one day, and then after two days, they returned to the baseline [226]. Moreover, in another study, oral supplementation of NAM (500 mg) significantly increased blood NAD\({}^{+}\) levels after 12 h in a cohort of five healthy adult subjects [227]. A toxicological study assessed the safety of Restorin\({}^{\text{\textregistered}}\) NMN, a direct NAD\({}^{+}\) precursor (\(\beta\)-NMN) in a high-purity formulation, by oral administration at 500, 1000, and 2000 mg/kg/day in Sprague-Dawley rats over three months followed by a recovery period of two weeks. No adverse effects were observed at the dose of 500 mg/kg/day. Restorin(r) NMN has a LOAEL of 2000 mg/kg/day and a NOAEL of 500 mg/kg/day in male rats and 1000 mg/kg/day in female rats [228]. In a double-blind, PLA-controlled, randomized clinical trial, oral administration of NMN (250 mg/day for 12 weeks) proved to be safe by increasing the level of blood NAD\({}^{+}\) in 30 healthy volunteers. A significant increase in the level of NA mononucleotide (NAMN), rather than in the level of NMN, was also seen [229].