NAD+

NAD+ (nicotinamide adenine dinucleotide) is a coenzyme present in every living cell and one of the most heavily studied molecules in metabolic and aging research. It is required for fundamental biological processes including energy metabolism, DNA repair, cellular signaling, and the regulation of gene expression. Tissue NAD+ concentrations decline with age in multiple species, and this decline is a central hypothesis in current investigations of age-related metabolic and mitochondrial dysfunction.

The molecule cycles between an oxidized form (NAD+) and a reduced form (NADH), participating in hundreds of enzymatic reactions. In the electron transport chain this redox cycling underpins ATP production. Beyond redox chemistry, NAD+ is consumed as a substrate — not merely used as a carrier — by three enzyme families studied intensively in aging research: the sirtuins (SIRT1–7), the poly(ADP-ribose) polymerases (PARPs), and the ectoenzyme CD38.

NAD+ operates through two functionally distinct roles that are frequently conflated. The first is redox cycling, in which NAD+ accepts electrons during nutrient catabolism to become NADH, which then donates those electrons to complex I of the electron transport chain. In this role the molecule is recycled rather than consumed, and the cellular NAD+/NADH ratio acts as a readout of metabolic state that influences glycolysis, the citric acid cycle, and fatty acid oxidation.

The second role is as a consumed substrate. Here NAD+ is cleaved, its nicotinamide moiety released, and the molecule must be resynthesized. This consumption is what links NAD+ availability to the signaling pathways described below, and it is why NAD+ pools can be depleted under conditions of genotoxic or inflammatory stress.

Sirtuin Activation

NAD+ is an obligate co-substrate for the sirtuins, a family of NAD+-dependent deacylases that remove acetyl and other acyl groups from histones and non-histone proteins. Through these reactions sirtuins influence gene expression, mitochondrial biogenesis via PGC-1α, circadian transcriptional feedback loops, and cellular stress responses. Because the reaction consumes NAD+ stoichiometrically, sirtuin activity is directly sensitive to NAD+ availability — the mechanistic basis for most NAD+ restoration hypotheses in aging research.

DNA Repair and PARP Activity

PARPs, particularly PARP1, detect DNA strand breaks and consume NAD+ to build poly(ADP-ribose) chains that recruit repair machinery. Under heavy DNA damage this activity can consume NAD+ rapidly enough to compete with sirtuins for the shared pool, a competition documented across multiple cellular models and one explanation for the observed link between genomic stress and metabolic decline.

CD38 and Age-Related Decline

CD38 is an NAD+-consuming glycohydrolase whose expression rises with age and with inflammatory signaling. Its increase is one of the better-characterized contributors to age-associated NAD+ depletion, and CD38 inhibition has become a research strategy in its own right, distinct from precursor supplementation.

Calcium Signaling and Salvage Synthesis

NAD+ is also converted to cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate, second messengers that mobilize intracellular calcium stores. Resynthesis proceeds primarily through the salvage pathway, in which nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme. NAMPT expression is itself circadian and declines in several aged tissues, so the balance between synthesis capacity and consumption demand — rather than either factor alone — determines the steady-state pool.

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