NAD+ Biology: The Cell's Central Metabolic Currency

What NAD+ actually is

NAD+ stands for nicotinamide adenine dinucleotide. Structurally it is two nucleotides joined tail to tail: one built around the vitamin-B3 derivative nicotinamide, the other around adenine. That nicotinamide ring is the business end of the molecule — it can accept and donate a pair of electrons, which is what makes NAD+ a coenzyme for hundreds of enzymatic reactions. The "+" in NAD+ simply denotes the oxidized, electron-accepting form.

Every cell in the body keeps a working pool of NAD+, and it is not optional: without it, the reactions that extract energy from carbohydrates, fats, and proteins grind to a halt. Because it is used and regenerated so rapidly, the cell does not store large amounts — it maintains a modest pool and recycles it constantly, which is why the synthesis and salvage machinery matters so much.

The redox role: NAD+ / NADH

The most familiar job of NAD+ is in redox chemistry — reduction and oxidation. During glycolysis, the citric acid cycle, and fatty-acid breakdown, dehydrogenase enzymes strip electrons from fuel molecules and hand them to NAD+, converting it to its reduced form, NADH. NADH then carries those high-energy electrons to the mitochondrial electron transport chain, where they drive the pumping of protons and, ultimately, the synthesis of ATP.

In this role NAD+ is a true coenzyme: it is not consumed but cycled back and forth between the NAD+ and NADH states. The NAD+/NADH ratio is therefore a readout of the cell's energy state. A high ratio (plenty of oxidized NAD+) signals that the cell has capacity to burn fuel; a low ratio signals a reduced, energy-replete or hypoxic state. This ratio is one of the inputs that tie NAD+ to energy-sensing pathways like AMPK.

Beyond energy: NAD+ as a consumable substrate

NAD+ has a second, very different life. A group of enzymes do not merely borrow its electrons — they cleave the molecule, consuming it in the process and releasing free nicotinamide. This is why NAD+ has to be continually resynthesized rather than only recycled. The three major NAD+-consuming families are the sirtuins, the PARPs, and CD38.

• [Sirtuins](/sirtuins-explained) (SIRT1–7) — NAD+-dependent deacetylases that remove acetyl groups from proteins to regulate metabolism, mitochondrial biogenesis, and DNA repair. Because they need NAD+ as a substrate, their activity tracks the cell's NAD+ supply.
• PARPs (poly-ADP-ribose polymerases) — DNA-damage-response enzymes that consume large amounts of NAD+ when repairing breaks. Heavy DNA damage can drain the NAD+ pool.
• CD38 — an NAD+-consuming enzyme involved in immune signaling and calcium regulation whose activity rises with age and inflammation, making it a major driver of age-related NAD+ decline.

The key idea is competition: sirtuins, PARPs, and CD38 all draw from the same limited NAD+ pool. When DNA damage spikes PARP activity, or when CD38 climbs with aging and inflammation, less NAD+ is left over for the sirtuins — one proposed link between falling NAD+ and the loss of sirtuin-driven maintenance functions over a lifespan.

How cells make and recycle NAD+: the salvage pathway

Cells build NAD+ by three routes: de novo synthesis from the amino acid tryptophan, the Preiss-Handler pathway from nicotinic acid, and — most importantly for day-to-day maintenance — the salvage pathway. Because sirtuins, PARPs, and CD38 constantly release nicotinamide as they consume NAD+, the salvage pathway exists to grab that nicotinamide and rebuild it back into NAD+.

The rate-limiting step of salvage is carried out by the enzyme NAMPT (nicotinamide phosphoribosyltransferase), which converts nicotinamide into nicotinamide mononucleotide (NMN). A second enzyme (NMNAT) then converts NMN into NAD+. Because NAMPT sets the pace, its expression and activity strongly influence how well a tissue can keep its NAD+ pool topped up — and NAMPT itself is regulated by the cell's circadian and energy-sensing machinery.

Age-related decline and precursors (NR, NMN)

A recurring observation across many tissues and species is that NAD+ levels tend to fall with age. The proposed drivers include rising CD38 activity (which accelerates NAD+ breakdown), increased PARP demand from accumulating DNA damage, and possibly reduced salvage capacity. Because so many maintenance programs depend on NAD+, this decline has become a central hypothesis in aging research — the idea being that restoring NAD+ might support sirtuin activity and mitochondrial function.

This is where precursors enter. Nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) are forms of vitamin B3 that feed directly into the salvage pathway, and supplementing them has been shown in studies to raise NAD+ levels in various tissues. Whether raising NAD+ this way produces meaningful longevity or healthspan benefits in humans is still an open, actively studied question — the biochemistry of precursor uptake is clearer than the clinical outcomes.

NAD+ also connects to the peptide research world through metabolism. Mitochondrial-derived peptides such as MOTS-c intersect with the same energy-sensing and metabolic networks that NAD+ helps regulate, and NAD+ metabolism is frequently discussed alongside sirtuins and mitochondrial biogenesis in the broader longevity literature.