NAD (Nicotinamide Adenine Dinucleotide) is a crucial coenzyme found in every living cell that plays a central role in numerous biological processes, including cellular energy production, DNA repair, gene expression, and cellular defense mechanisms. NAD acts as a key molecule in redox reactions, facilitating the transfer of electrons and promoting the conversion of nutrients into energy (ATP) through metabolic pathways like glycolysis, the Krebs cycle, and oxidative phosphorylation.