NAD+ (also shown as NADH) is one of the most critical coenzymes that exists in  every living cell within your body. Generally, a coenzyme is a small, organic  molecule (typically a vitamin) that binds to an enzyme (a protein) and provides a  catalyst to create a reaction. In the case of NAD+ specifically, this Vitamin B3  coenzyme is essential to energy metabolism and to maintaining proper cell  function. Every cell in the human body relies on NAD+ for the proper functioning  of our mitochondria (the power plants in our cells). In short, NAD+ enables the  body to take molecules from food and donate them to the mitochondria to be  utilized as energy. 

Recent research suggests that NAD+ also appears to help sirtuins (a class of  proteins that regulate cellular health) do their job. Among their many important  functions, sirtuins are responsible for regulating biological pathways, turning  genes on and off and protecting cells from age-related decline. NAD+ increases  the activity of specific sirtuins that encourage the formation of new mitochondria  and helps maintain the length of telomeres (the end caps on DNA strands) which  are associated with physical longevity.