Codeage · DNA Repair · NAD+ · NMN · Cellular Longevity

DNA Repair · PARP · NAD+ · NMN · Cellular Aging · Longevity

DNA damage is constant —
and the repair system
runs on NAD+.

Every cell in the human body sustains thousands of DNA lesions every day. Oxidative stress, replication errors, ultraviolet exposure, metabolic byproducts — the sources of damage are many and continuous. What determines whether this damage accumulates into biological dysfunction is not whether it occurs, but whether the cellular repair response can keep pace. And the enzymes that coordinate that response consume NAD+ to do their work.

By Codeage✦ 8 min read✦ DNA Repair · PARP · NAD+ · NMN · Nicotinamide Mononucleotide · Cellular Aging

The constant work of
keeping the genome intact.

The human genome — approximately three billion base pairs of DNA packed into the nucleus of every cell — is under continuous assault. This is not a pathological situation. It is a biological constant. Reactive oxygen species generated by normal metabolic activity attack DNA bases. Replication errors occur when DNA is copied. Spontaneous hydrolysis breaks chemical bonds in the DNA backbone. Ultraviolet radiation induces pyrimidine dimers. Environmental chemicals react with nucleotides. By some estimates, a single human cell sustains between 10,000 and 100,000 DNA lesions every day.

The cell survives this because it has evolved an elaborate, multi-system DNA damage response — a network of detection, signaling, and repair mechanisms that identifies lesions, pauses the cell cycle if necessary, and executes the appropriate repair pathway depending on the type of damage. Base excision repair, nucleotide excision repair, mismatch repair, homologous recombination, non-homologous end joining — each pathway handles a different category of lesion with a different set of enzymes and a different cellular cost.

Central to many of these pathways — and the first responder to the most common and most acute form of DNA damage, strand breaks — is a family of enzymes called PARPs: poly(ADP-ribose) polymerases. PARPs detect DNA damage within seconds of its occurrence, bind to the break site, and begin a signaling cascade that recruits the repair machinery. They do this by consuming NAD+ — specifically, by using it as a substrate to synthesize poly(ADP-ribose) chains that serve as molecular scaffolds for the repair response. And the rate at which they consume NAD+ during active repair is substantial.

The question is never
whether DNA damage occurs.
It is whether the repair
response can keep pace —
and whether there is enough NAD+
to run it.

The Repair Response

What happens in the minutes
after a DNA strand break.

This is not a rare event. Strand breaks — both single-strand and double-strand — occur continuously in every dividing and non-dividing cell. The response sequence described here runs thousands of times per cell per day, and its NAD+ cost accumulates with every cycle.

Seconds PARP detection PARP1 · PARP2

PARP enzymes bind the damage site within seconds — and immediately begin consuming NAD+

PARP1 has an extraordinarily high affinity for DNA strand breaks. Within seconds of a break occurring, PARP1 binds to the site through its zinc-finger DNA-binding domains and undergoes a conformational change that activates its catalytic domain. It then begins synthesizing poly(ADP-ribose) — PAR chains — using NAD+ as the substrate for each ADP-ribose unit added to the chain. This PAR synthesis is the molecular signal that recruits the downstream repair machinery. PARP1 can consume hundreds to thousands of NAD+ molecules per strand break event, depending on the severity of the damage and the length of the PAR chains synthesized.

Minutes Repair recruitment Repair enzymes · scaffolding

PAR chains scaffold the repair machinery — drawing in the proteins that will fix the break

The PAR chains synthesized by PARP1 act as molecular landing pads, attracting DNA repair factors through their PAR-binding motifs. XRCC1 — a scaffold protein central to base excision repair — is one of the first recruited. Histones are displaced from the chromatin around the break site, making the DNA accessible for repair. The cell cycle checkpoint machinery is signaled. In parallel, PARP-mediated chromatin relaxation allows the repair enzymes physical access to the damaged region. This entire scaffolding process is dependent on the continued availability of NAD+ for PARP catalytic activity.

Minutes–hours Repair execution Pathway-specific enzymes

The appropriate repair pathway executes — base excision, end joining, or homologous recombination

The specific repair pathway activated depends on the type of lesion: single-strand breaks and base damage are handled primarily by base excision repair; double-strand breaks by non-homologous end joining (faster, error-prone) or homologous recombination (slower, more precise, requires a template). Each pathway uses its own set of enzymes, but all of them work in a chromatin environment that has been prepared by PARP activity — making the efficiency of the initial PARP response a determinant of how well the downstream repair executes.

Post-repair PAR degradation PARG enzyme · nicotinamide release

PAR chains are degraded — and nicotinamide is released back into the NAD+ recycling system

Once the repair is complete, the PAR chains synthesized by PARP are degraded by PARG — poly(ADP-ribose) glycohydrolase — releasing free ADP-ribose and ultimately nicotinamide as byproducts. That nicotinamide re-enters the Salvage Pathway, where NAMPT converts it back to NMN and NMNAT converts NMN to NAD+ — restoring a portion of the NAD+ consumed in the repair cycle. The efficiency of this recycling is dependent on NAMPT activity — which declines with age, meaning that aged cells are both consuming more NAD+ per repair event due to accumulated damage and less efficiently recycling the nicotinamide that repair releases.

Why DNA damage and NAD+ decline
compound each other in the aging cell.

The relationship between DNA repair and NAD+ is not simply that repair consumes NAD+. It is that the consumption pattern changes with age in a way that compounds the NAD+ decline already driven by NAMPT reduction and CD38 elevation — creating a three-front depletion dynamic whose combined effect on the NAD+ pool is greater than any single driver alone.

In a young cell, the balance between DNA damage rate, repair demand, and NAD+ availability is roughly maintained. Damage occurs at a rate that PARP enzymes can address without catastrophically depleting the NAD+ pool. NAMPT runs efficiently enough to recycle the nicotinamide released by repair back into NAD+ at a pace that keeps the pool replenished. The system is in dynamic equilibrium — NAD+ is consumed by repair, recycled by the Salvage Pathway, and maintained at levels adequate for sirtuins and mitochondrial function.

In an aging cell, this equilibrium breaks down from two directions simultaneously. First, the accumulated damage load increases: decades of oxidative stress, replication errors, and environmental insults leave more unrepaired or incompletely repaired lesions in the genome, meaning PARP activation is more frequent and the NAD+ demand of each repair cycle draws on a pool that was already declining. Second, NAMPT's declining activity means the nicotinamide released by repair is recycled back to NAD+ more slowly — so each PARP activation cycle extracts a larger net cost from the NAD+ pool than the same activation would have in a younger cell. The two dynamics reinforce each other, accelerating the NAD+ decline that declining NAMPT had already set in motion.

The PARP Family

The NAD+-consuming enzymes
at the center of the DNA damage response.

The PARP family has seventeen members, with very different levels of activity and biological roles. The three most relevant to NAD+ biology and DNA repair are described here. None of these descriptions constitute a claim about any supplement or product.

PARP1

The primary DNA damage sensor

PARP1 accounts for the vast majority of total cellular PARP activity — estimated at 85–90% of all PAR synthesis in response to DNA damage. It binds with high affinity to both single-strand and double-strand DNA breaks through its zinc-finger domains, and its PAR synthesis activity is dramatically accelerated by DNA binding. PARP1 is the enzyme most responsible for the acute NAD+ demand of the DNA damage response, and its activity has been most extensively studied in the context of both repair biology and NAD+ consumption in aging.

NAD+ relationship: primary consumer in DNA damage response — catalytic activity directly proportional to damage load

PARP2

The complementary damage responder

PARP2 shares structural similarity with PARP1 and responds to similar DNA lesions, though with lower overall activity and a somewhat distinct binding preference for specific DNA structures. PARP1 and PARP2 have partially overlapping roles in the DNA damage response — PARP2 becomes more relevant in contexts where PARP1 activity is limited. Both are activated by DNA breaks, both consume NAD+, and both contribute to the total PARP-mediated NAD+ demand that rises with accumulating DNA damage in aging tissue.

NAD+ relationship: secondary consumer — contributes to total PARP-mediated NAD+ demand alongside PARP1

PARP3

The double-strand break specialist

PARP3 has a more restricted role than PARP1 or PARP2, functioning primarily in the response to double-strand breaks — the most severe form of DNA damage. It interacts with the non-homologous end joining machinery and has roles in the stabilization of broken chromosome ends during repair. PARP3 has lower overall PAR synthesis activity than PARP1 but is specifically activated by the type of damage whose accumulation in aged tissue — particularly in post-mitotic cells like neurons and cardiomyocytes — is most associated with age-related cellular dysfunction.

NAD+ relationship: activated specifically by double-strand breaks — the lesion type whose accumulation increases most significantly with age

The Repair–NAD+ Balance

How the relationship between
DNA damage, PARP, and NAD+ shifts with age.

The Young Cell · Repair in Balance

Adequate NAD+. Efficient recycling. Damage managed.

DNA damage occurs — but at a rate PARP response can address without catastrophic NAD+ depletion

PARP1 activates, consumes NAD+, synthesizes PAR, recruits repair machinery

Repair executes completely — lesion resolved, chromosome integrity maintained

PAR degraded, nicotinamide released — re-enters Salvage Pathway

NAMPT efficiently recycles nicotinamide back to NMN and NAD+

NAD+ pool restored — sirtuins and mitochondrial function not significantly impacted

The Aging Cell · Balance Disrupted

Reduced NAD+. Slower recycling. Damage accumulating.

Accumulated damage load increases — PARP activation more frequent, total NAD+ demand rises

Each PARP activation draws on a NAD+ pool already depleted by NAMPT decline and CD38 elevation

Repair may be slower or less complete — some lesions persist longer before resolution

Nicotinamide released by PAR degradation — but NAMPT recycles it more slowly

Net NAD+ cost of each repair cycle is higher — less restored than consumed

Sirtuin activity and mitochondrial function compete for a NAD+ pool further stressed by repair demand

The Scale of the Biology

What DNA repair and NAD+
look like in numbers.

10,000–
100,000

Estimated DNA lesions per cell per day — the continuous damage load the repair system addresses

The range of estimates for daily DNA damage per cell reflects the different types of lesions and detection methods involved, but even the lower end of this range describes an extraordinarily active and continuous repair demand. Across a human body of approximately 37 trillion cells, the daily scale of DNA damage and repair is one of the most impressive biological operations in nature — and its NAD+ cost is distributed continuously across every tissue. Studies were conducted independently and did not involve any specific Codeage product.

Members of the PARP family — with PARP1 accounting for an estimated 85–90% of damage-response PAR synthesis

The PARP family is larger than most people interested in NAD+ biology realize. While PARP1 dominates the DNA damage response in terms of NAD+ consumption, the seventeen-member family has roles spanning DNA repair, chromatin regulation, telomere maintenance, mitosis, and cell death. The collective NAD+ demand of active PARP biology represents one of the three major drains on cellular NAD+ alongside sirtuin activity and CD38-mediated degradation.

Converging NAD+ depletion forces in aging — NAMPT decline, CD38 elevation, and rising PARP demand

The three-force model of NAD+ decline — documented in the NAD+ aging article — includes rising PARP demand as one of the compounding dynamics. The interaction between increased damage load driving more PARP activation and declining NAMPT reducing recycling efficiency means that the PARP contribution to NAD+ decline is not fixed but grows as both damage accumulates and the recycling system slows. The three forces together produce a deficit greater than any single driver would generate alone.

III

What the PARP–NAD+ relationship
means in the context of aging.

The connection between DNA repair and NAD+ is one of the more underappreciated dimensions of why NAD+ decline has such wide-ranging consequences in the aging cell. It is not simply that sirtuins are constrained, or that mitochondrial function declines, or that the NAD+/NADH ratio shifts. It is that the cellular repair system — the mechanism through which the genome is maintained against continuous daily damage — simultaneously draws on the same NAD+ pool that all these other systems depend on, and draws on it more heavily as age increases the damage load and slows the recycling system.

This is a compounding story, and one whose full implications for how the aging cell's genomic integrity deteriorates over decades continue to be characterized by the research community. The connections between PARP biology, NAD+ availability, and the accumulation of genomic instability in aged tissue are a developing area where new findings continue to refine the picture — what is described here reflects the current mechanistic understanding, which is still being extended by ongoing work.

For the full architecture of how NAD+ declines across the aging cell — and the roles of NAMPT, CD38, and the Salvage Pathway in that decline — the NAD+ aging article and the NAMPT article cover the upstream biology this repair story depends on. Together they form part of the mechanistic foundation of Cellular Longevity — Pillar 03 of The Longevity Code.

The repair system draws on the
same NAD+ pool that sirtuins
and mitochondria depend on.
And it draws on it more heavily
as age increases the damage load.

Codeage · Pillar 03 · Cellular Longevity

Built for the
cellular long game.

Cellular Longevity is Pillar 03 of The Longevity Code — the dimension of the system built around NAD+ biology, mitochondrial health, and the science of cellular aging.

Explore Cellular Longevity →