With time, NAD+ is used up by cellular activities, resulting in a drop in the concentration of NAD+ inside the cells. Taking supplements that contain NAD+ precursors help to boost the levels back up to a healthy concentration. Especially, precursors like NMN and nicotinamide riboside have shown to work as supplements for NAD+ production.
Harvard NAD+ researcher, David Sinclair, says that NAD+ can’t be used as a supplement. Taking NAD+ directly does not produce the desired effect of increasing intracellular NAD+ levels; as the molecule can’t pass through the cell membranes. So, to make up for the shortage, NAD+ precursor molecules such as NMN supplements can be administered to increase the concentration of NAD+ in the cells. These are readily absorbed by the cells.