NAD+ (Nicotinamide Adenine Dinucleotide) is a pyridine dinucleotide coenzyme central to redox biology and cellular energy metabolism. It functions as an electron carrier in oxidative phosphorylation and as a co-substrate for the sirtuin family of NAD-dependent deacetylases, the poly-ADP-ribose polymerases (PARPs) involved in DNA repair, and the CD38 ectoenzyme involved in calcium signalling. Intracellular NAD+ levels decline with age in human tissues, a finding documented across multiple published longevity research lines and the central rationale for NAD+-restoration research as a translational longevity strategy.
NAD+ participates in two distinct categories of cellular biochemistry. As a redox cofactor it cycles between NAD+ and NADH across glycolysis, the TCA cycle and the mitochondrial electron transport chain — the central machinery of ATP production. As a co-substrate, NAD+ is consumed (not recycled) by the sirtuin deacetylases that regulate metabolic gene expression, by the PARPs that signal and repair DNA damage, and by CD38 in calcium-signalling pathways.
Intracellular NAD+ pools decline substantially with age across multiple human tissues including muscle, liver and brain, as documented in published research (Massudi et al. PLoS One 2012; Yoshino et al. Cell Metabolism 2018). The proximate causes include increased CD38 expression (the dominant NAD+-consuming ectoenzyme of ageing) and decreased de novo and salvage pathway flux.
NAD+ restoration research has therefore focused on either direct administration of NAD+, dosing of precursors (NMN, NR), or CD38 inhibition (5-Amino-1MQ inhibits the upstream NNMT enzyme that regulates methylation balance and indirectly impacts NAD+ availability).
Multiple human tissue studies have documented age-associated decline in intracellular NAD+. Massudi et al. (PLoS One, 2012) reported a 50% drop in dermal NAD+ between young (20s) and older (60+) subjects. Yoshino et al. (Cell Metabolism, 2018) found parallel declines in muscle. The proximate cause across these studies is increased CD38 expression with ageing — CD38 alone accounts for the majority of NAD+ consumption in older tissues.
This decline is the central rationale for the broader NAD+ precursor research field (NMN, NR) and for direct NAD+ administration research lines.
Sirtuin research is the primary translational interest. SIRT1 activity controls many of the longevity-associated transcription factors including PGC-1α (mitochondrial biogenesis) and FOXO transcription factors (cellular stress response). PARP1 activity, while critical for DNA repair, competes with sirtuins for NAD+ substrate.
Bender's group and others have demonstrated in preclinical models that restoring NAD+ levels can reactivate sirtuin-dependent transcriptional programmes in aged tissues — the experimental basis for the field.