Editor, Science of NAD
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What are "Metabolic Pathways" and why do they matter for NAD?
Depending on where, when, and how the NAD will be biosynthesized, different precursors will be needed and different limits encountered...
The four main pathways for intracellular biosynthesis of NAD correspond to four different NAD precursors: Trp (De Novo Pathway), NA (Preiss-Handler Pathway), NAM (Salvage Pathway), and NR (Brenner Pathway).
The De Novo Pathway is not an important source of intracellular NAD because your body mostly uses tryptophan for protein synthesis instead of for NAD synthesis.
The Preiss-Handler Pathway for Niacin is poorly expressed in numerous cell types and is down-regulated by viral infection.
The Salvage Pathway for Nicotinamide is rate-limited under some circumstances (especially age and stress).
The Brenner Pathway is well-expressed in cells and can bypass rate-limiting steps in the other pathways that prevent the other pathways from adequately replenishing NAD. That is the reason NR gets so much attention. NMN might have its own pathways in some cell types, rather than first getting degraded to NR and NAM, but it's not clear yet.