Nicotinamide adenine dinucleotide (NAD) is a vital coenzyme found in all living cells, playing essential roles in cellular metabolism, energy production, and cellular homeostasis maintenance. It exists in two forms, namely, NAD+ (oxidized) and NADH (reduced), which participate in numerous biochemical reactions, including glycolysis, the citric acid cycle, and oxidative phosphorylation. NAD+ also serves as a substrate for enzymes involved in DNA repair, gene expression, and cell signaling pathways. Dysregulation of NAD metabolism has been implicated in various age-related diseases, including metabolic disorders, neurodegenerative diseases, and cancer. Understanding the functions and regulation of NAD metabolism is crucial for elucidating the underlying mechanisms of these diseases and developing potential therapeutic interventions.