Nicotinamide adenine dinucleotide (NAD+) is a critical coenzyme for redox reactions and a co-substrate for NAD+-dependent enzymes. Its metabolites act as regulatory centres and influence various energy metabolic processes in vivo, such as glycolysis, the tricarboxylic acid cycle (TCA) and alcohol metabolism. These processes are important for the maintenance of tissue and metabolic equilibrium. Notably, numerous diseases are accompanied by changes in NAD+ levels and physiological processes. Moreover, whether targeting NAD+ influences diseases through interference with the physiological processes of energy metabolism in vivo needs in-depth exploration. This chapter provides a summary and discussion of the mechanisms by which NAD+ participates in important physiological processes in vivo.