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 enters cells via the small intestines within 10 minutes converting to NAD+ through the Slc12a8 NMN transporter.
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. Doses of up to 500 mg was shown safe in men in a recent human study at Keio University School of Medicine, Shinjuku, Tokyo Japan.
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.
 The molecular structures of NMN and NR are roughly the same, except NMN has an added phosphate group, making it a larger molecule. Some scientists believe NMN is too large to cross cellular membranes and must convert to NR before entering cells, where NAD+ biosynthesis occurs. Otherwise, NMN would need to get transported into cells by a transporter specific for NMN, such as Slc12a8.
Both NR and NMN are vulnerable to extracellular degradation by CD38 enzyme, which can be inhibited by compounds such as CD38-IN-78c.