What Is The Difference Between the NAD Precursors?
NAD cannot enter cells directly. Also, you already get plenty of NAD in your diet. The challenge is not getting NAD into your body, but getting it into your cells. That's why you need a precursor, which is a smaller piece of NAD than can enter the cell and help build intracellular NAD.
Niacin (NA) only gets turned into NAD if an enzyme called NAPRT is present. NAPRT is abundant in some cell-types, but it is poorly expressed in others, so NA won't replenish NAD very well in nerve or muscle cells, for example, or under conditions where NAPRT is depressed, like viral infection. Niacin also causes problems at higher doses.
Nicotinamide (NAM) can be used by every cell, but only gets turned into NAD if an enzyme called NAMPT is present. NAMPT is present in every cell, but there isn't enough of it under some circumstances, including old age and some types of metabolic stress. So NAM may not work well sometimes. NAM may also inhibit sirtuins. Finally, high doses of NAM can have toxic side effects.
Nicotinamide Riboside (NR) is available to every cell, and bypasses rate-limiting steps that can prevent NA and NAM from creating NAD. NR has no known significant negative side effects, even at doses as high as 2,000 mg/day.
Nicotinamide Mononucleotide (NMN), like NAD cannot enter cells directly -- at least not most cells -- and must be broken down into other precursors like NR and NAM before replenishing intracellular NAD. Therefore, it is more efficient, and often less expensive, to supplement with NR directly than to use NMN.