Nicotinamide adenine dinucleotide (NAD+) and its related coenzyme NADP+ are vital within cells for oxidative energy production and resisting stress from reactive oxygen species. The ability to produce NAD+ via NAD metabolism (a.k.a. the NAD metabolome) declines with age. Therefore, supplementing with NAD+ precursors can help maintain or increase NAD+ levels if/when necessary.
Cells have 4 precursors from which NAD+ can be produced via the NAD metabolome: tryptophan (amino acid), niacin/nicotinic acid (vitamin), nicotinamide (vitamin), and nicotinamide riboside (vitamin). Nicotinamide riboside (NR) appears to be the best choice for an NAD+ precursor supplement because the genes and enzymes required to covert it to NAD+ are expressed in virtually all cells and tissues. This is not the case with tryptophan or niacin/nicotinic acid.