in the 'nicotinamide core recycling pathway,' the enzyme, NMNAT, converts NMN to NAD+. Proteins called 'sirtuins' utilize NAD+ to maintain cellular health and turn NAD+ to nicotinamide (NAM).  via the enzyme NAMPT.

Foods from a standard diet contain B3 vitamins, which are nicotinamide (NAM), nicotinic acid (NA), and nicotinamide riboside (NR). NAD+ gets synthesized from B3 vitamins or tryptophan from the diet. Humans cannot synthesize NAD+ without B3 vitamins or tryptophan from the diet or NMN supplements.

A study from 2007 demonstrates  levels in yeast. Further studies on the topic reveal  in mammalian cells up to 270%. these studies imply a special role for the salvage pathway of NAD+ biosynthesis involving NR and NMN in increasing NAD+ levels. The actual mechanism through which this happens .

Scientists continue to debate which of the two biosynthetic precursors, NMN or NR, is superior regarding safety and efficacy. Due to the size of NMN, some scientists believe it must convert to NR before crossing the cell membrane to enter cells. Once inside cells, NR would convert to NMN. If this scenario provides the full story, the more efficient precursor would be NR.