NAD+

NAD+ (nicotinamide adenine dinucleotide, oxidized form) is a coenzyme central to redox reactions in energy metabolism and a required substrate for sirtuins, PARPs, and CD38. Cellular NAD+ levels decline substantially with age across tissues, impairing mitochondrial function, DNA repair, and sirtuin activity. NAD+ precursors such as NR (nicotinamide riboside) and NMN (nicotinamide mononucleotide) are studied as supplements aimed at restoring tissue NAD+, with mixed clinical evidence.