NAD+

NAD+ (nicotinamide adenine dinucleotide) is a dinucleotide coenzyme found in all living cells, functioning both as an electron carrier in redox reactions (NAD+/NADH) and as a signalling molecule consumed by longevity enzymes. Unlike most coenzymes, NAD+ is not merely a cofactor — it is stoichiometrically consumed by sirtuins and PARPs, meaning its cellular availability directly limits the activity of these critical pathways.

NAD+ levels decline precipitously with age. Human studies show approximately 50% reduction in tissue NAD+ between the ages of 40 and 60, with further decline thereafter. This decline is mechanistically linked to increased PARP activity (from accumulated DNA damage), increased CD38 expression (an NAD+-consuming enzyme), and reduced NMN synthesis. The resulting NAD+ deficit impairs mitochondrial function, DNA repair capacity, and sirtuin-mediated gene regulation — converging on the hallmarks of aging.

The NAD+ precursors NMN (nicotinamide mononucleotide) and NR (nicotinamide riboside) have emerged as the leading approach to restoring NAD+ levels. Both raise blood and tissue NAD+ in human trials, with NMN also shown to improve muscle NAD+ metabolism in older adults. David Sinclair's lab at Harvard has produced the most prominent research connecting NAD+, sirtuins, and longevity, though the field has grown to include hundreds of independent research groups.

// SIRTUIN ACTIVATION

Sirtuins (SIRT1–7) are NAD+-dependent deacylases that regulate gene expression, mitochondrial biogenesis, DNA repair, and metabolic homeostasis. They require NAD+ as a co-substrate — not as a cofactor — meaning each catalytic cycle consumes one NAD+ molecule. Low NAD+ directly silences sirtuins. SIRT1 deacetylates PGC-1α (activating mitochondrial biogenesis), FOXO3 (activating stress resistance), and p53 (regulating apoptosis vs. repair). SIRT3 is the primary mitochondrial sirtuin, protecting mitochondrial function. SIRT6 is critical for DNA repair and telomere maintenance.

// PARP-MEDIATED DNA REPAIR

PARP1 and PARP2 are poly(ADP-ribose) polymerases that detect and signal single-strand DNA breaks. They consume NAD+ extensively to synthesise poly(ADP-ribose) chains — signalling molecules that recruit repair machinery. With ageing, accumulated DNA damage chronically activates PARP, depleting NAD+ and starving sirtuins. Restoring NAD+ resolves this competition, enabling both DNA repair and sirtuin activity to proceed at capacity.

// MITOCHONDRIAL FUNCTION

In mitochondria, NAD+ is the primary electron acceptor in the TCA cycle and beta-oxidation. NADH produced by these cycles donates electrons to Complex I of the electron transport chain, driving ATP synthesis. NAD+ depletion impairs mitochondrial respiration, reduces ATP output, and increases reactive oxygen species — a central driver of metabolic decline in ageing tissues.

// CD38 AND NAD+ HOMEOSTASIS

CD38 is a NAD+-consuming enzyme expressed on immune cells and other tissues. CD38 expression increases dramatically with age and inflammation (inflammaging). It is estimated to consume the majority of NAD+ in aged tissues. Inhibiting CD38 (e.g., with apigenin or other flavonoids) is a complementary strategy to precursor supplementation for raising NAD+ levels.

The NAD+ field has grown rapidly since Guarente and Sinclair's foundational work on sirtuins in the early 2000s. Key human trials: Elhassan et al. (2019) confirmed NMN raises blood NAD+ in healthy older adults; Yoshino et al. (2021, Cell Metabolism) showed NMN supplementation improved muscle NAD+ metabolism and insulin signalling in postmenopausal women. Igarashi et al. (2022) showed NMN raises NAD+ in older men in a dose-dependent manner. Direct IV NAD+ is used clinically but its superiority to oral precursors for tissue NAD+ is debated. The field's primary debate centres on which precursor (NMN vs NR) most efficiently raises tissue NAD+ and whether systemic NAD+ elevation has the same benefits as mitochondria-targeted strategies.