Nicotinamide mononucleotide
Nicotinamide mononucleotide ("NMN" and "β-NMN") is a nucleotide derived from ribose and nicotinamide. Like nicotinamide riboside, NMN is a derivative of niacin, and humans have enzymes that can use NMN to generate nicotinamide adenine dinucleotide (NADH). In mice, NMN has been proposed to enter cells via the small intestines within 10 minutes converting to NAD+ through the Slc12a8 transporter. However this observation has been challenged, and remains unsettled. Because NADH is a cofactor for processes inside mitochondria, for sirtuins, and for PARP, NMN has been studied in animal models as a potential neuroprotective and anti-aging agent . Dietary supplement companies have aggressively marketed NMN products claiming those benefits. Single-dose administration of up to 500 mg was shown safe in men in a study at Keio University School of Medicine, Shinjuku, Tokyo, Japan. A 2021 clinical trial found that NMN improved muscular insulin sensitivity in prediabetic women. Nicotinamide riboside (NR) kinase enzymes are essential for exogenously administered utilization of NR and NMN. Some research suggests when administered exogenously, NMN must be converted to NR in order to enter a cell and be re-phosphorylated back to NMN. Both NR and NMN are vulnerable to extracellular degradation by  CD38 enzyme, which can be inhibited by compounds such as CD38-IN-78c. Dietary sources. NMN is found in fruits and vegetables such as edamame, broccoli, cabbage, cucumber and avocado.