Nicotinamide adenine dinucleotide (NAD+) is a dinucleotide coenzyme present in all living cells, functioning as an essential electron carrier in oxidative phosphorylation and a critical substrate for a wide range of enzymes including sirtuins, PARPs, and CD38. NAD+ levels decline significantly with age, and this decline has been linked to impairments in mitochondrial function, DNA repair capacity, and cellular stress responses. Research into NAD+ repletion focuses on restoring these processes, with preclinical and early clinical studies suggesting benefits across metabolic health, neurological function, cardiovascular protection, and healthspan extension.